# Schmitt Triggers

R

#### RobH

##### Guest
I have been looking at schmitt trigger circuits and didn't realise how
many variations there are.

Anyway I found this one:

The author here says that by adding a 22k resistor to the bottom diagram
turns into a schmitt trigger circuit as the top diagram.

From other schmitt trigger circuits I have see on youtube and other
sites, this one is so simple, but works. The switching from off to on is
almoast instant for the 2 leds.
But I have a question here, it says this:

Two LEDs are needed because, when the Op Amp pin 6 voltage is low, there
may still be as much as two volts present.
This could be enough to light a single LED. With two LEDs, four volts
are needed.
The Op Amp pin 6 voltage will be well below 4V so the LEDs will be off.

WHat afctually happens is both leds come on at the same time and the
voltage is 2.18v when on.

I am using a 33k resistor instead of a 22k which I don't have, if that
makes the difference.

A

#### amdx

##### Guest
On 2/9/2020 5:11 PM, RobH wrote:
I have been lookingÂ  at schmitt trigger circuits and didn't realise how
many variations there are.

Anyway I found this one:

The author here says that by adding a 22k resistor to the bottom diagram
turns into a schmitt trigger circuit as the top diagram.

From other schmitt trigger circuits I have see on youtube and other
sites, this one is so simple, but works. The switching from off to on is
almost instant for the 2 leds.
It's faster than the persistence of your eye!

But I have a question here, it says this:

Two LEDs are needed because, when the Op Amp pin 6 voltage is low, there
may still be as much as two volts present.
This could be enough to light a single LED. With two LEDs, four volts
are needed.
The Op Amp pin 6 voltage will be well below 4V so the LEDs will be off.
What are the high and low voltages you get on Pin 6?

What actually happens is both leds come on at the same time and the
voltage is 2.18v when on.
Usually they will come on at very close to same time, there will be a
slight difference of the voltage across them, but they will be pretty
similar.
If you are using a common red LED they will have a voltage drop of about
1.8v, so 3.6V for to with the rest dropped across your 560 ohm resistor.
The voltage drop is depending on the current through the LED, but the
1.8V for a red LED is close.

I am using a 33k resistor instead of a 22k which I don't have, if that
makes the difference.
You tell us, record the voltage on Pin 3 and Pin 6 when Pin 6
switches, both high and low. Then put another 33k in parallel with the
one you have and repeat the voltage records as above. Then you can tell
me what difference changing the value of the feedback (your 33k)
resistor causes.
I like this you are one upping me and removing even more parts.
You have been thinking.
Mikek

R

#### RobH

##### Guest
On 10/02/2020 00:38, amdx wrote:
On 2/9/2020 5:11 PM, RobH wrote:
I have been lookingÂ  at schmitt trigger circuits and didn't realise
how many variations there are.

Anyway I found this one:

The author here says that by adding a 22k resistor to the bottom
diagram turns into a schmitt trigger circuit as the top diagram.

Â From other schmitt trigger circuits I have see on youtube and other
sites, this one is so simple, but works. The switching from off to on
is almost instant for the 2 leds.

Â It's faster than the persistence of your eye!

But I have a question here, it says this:

Two LEDs are needed because, when the Op Amp pin 6 voltage is low,
there may still be as much as two volts present.
This could be enough to light a single LED. With two LEDs, four volts
are needed.
The Op Amp pin 6 voltage will be well below 4V so the LEDs will be off.

Â What are the high and low voltages you get on Pin 6?

What actually happens is both leds come on at the same time and the
voltage is 2.18v when on.

Â Usually they will come on at very close to same time, there will be a
slight difference of the voltage across them, but they will be pretty
similar.
If you are using a common red LED they will have a voltage drop of about
1.8v, so 3.6V for to with the rest dropped across your 560 ohm resistor.
Â The voltage drop is depending on the current through the LED, but the
1.8V for a red LED is close.

I am using a 33k resistor instead of a 22k which I don't have, if that
makes the difference.

Â You tell us, record the voltage on Pin 3 and Pin 6 when Pin 6
switches, both high and low. Then put another 33k in parallel with the
one you have and repeat the voltage records as above. Then you can tell
me what difference changing the value of the feedback (your 33k)
resistor causes.
Â I like this you are one upping me and removing even more parts.
You have been thinking.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek
The voltage on pin 6 , as the circuit is now, or per the
diagram/schematic, is 2.0v when Lo, and 4.0v when Hi

The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate upto
10.9v.

Another 33k resistor was placed in parallel with the one across pins 3
and 6. The Lo voltage was 2.0v and the Hi voltage was 3.2v on pin 6.
On pin 3, the voltage when Lo was 10.2v and when Hi the voltage was
10.2v, and both leds were flickering rapidly.

E

#### Evgeniy

##### Guest
Hello, RobH!

On Mon, 10 Feb 2020 09:26:22 +0000
RobH &lt;rob@despammer.com&gt; wrote:

On 10/02/2020 00:38, amdx wrote:
On 2/9/2020 5:11 PM, RobH wrote:
I have been lookingÂ  at schmitt trigger circuits and didn't realise
how many variations there are.

Anyway I found this one:

The author here says that by adding a 22k resistor to the bottom
diagram turns into a schmitt trigger circuit as the top diagram.

Â From other schmitt trigger circuits I have see on youtube and other
sites, this one is so simple, but works. The switching from off to on
is almost instant for the 2 leds.

Â It's faster than the persistence of your eye!

But I have a question here, it says this:

Two LEDs are needed because, when the Op Amp pin 6 voltage is low,
there may still be as much as two volts present.
This could be enough to light a single LED. With two LEDs, four volts
are needed.
The Op Amp pin 6 voltage will be well below 4V so the LEDs will be off.

Â What are the high and low voltages you get on Pin 6?

What actually happens is both leds come on at the same time and the
voltage is 2.18v when on.

Â Usually they will come on at very close to same time, there will be a
slight difference of the voltage across them, but they will be pretty
similar.
If you are using a common red LED they will have a voltage drop of about
1.8v, so 3.6V for to with the rest dropped across your 560 ohm resistor.
Â The voltage drop is depending on the current through the LED, but the
1.8V for a red LED is close.

I am using a 33k resistor instead of a 22k which I don't have, if that
makes the difference.

Â You tell us, record the voltage on Pin 3 and Pin 6 when Pin 6
switches, both high and low. Then put another 33k in parallel with the
one you have and repeat the voltage records as above. Then you can tell
me what difference changing the value of the feedback (your 33k)
resistor causes.
Â I like this you are one upping me and removing even more parts.
You have been thinking.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

The voltage on pin 6 , as the circuit is now, or per the
diagram/schematic, is 2.0v when Lo, and 4.0v when Hi

The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate upto
10.9v.

Another 33k resistor was placed in parallel with the one across pins 3
and 6. The Lo voltage was 2.0v and the Hi voltage was 3.2v on pin 6.
On pin 3, the voltage when Lo was 10.2v and when Hi the voltage was
10.2v, and both leds were flickering rapidly.
Just want to say some words. Schematic is strange, and description
is strange too. Author said:
"Adding the 22K resistor adds some positive feedback and the on and off
threshold voltages will now differ slightly. Adding this resistor
converts the circuit from a Comparator to a Schmitt Trigger."

That's not true, because threshold voltage will be the same: it will be
half of power voltage, value at point A. Feedback does not change
threshold voltage, it just changes voltage/current through LDR, so
behaviour of schematic depends on Volt-Ampere characteristics of LDR. If
characteristic is like classical resistor (resistance does not depend
on voltage/current, just light; I have not work with LDR), it will make
LDR resistance thresholds, which makes lighting thresholds.

The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate upto
10.9v.
There is something wrong: with schematic on board or OP AMP or while
measuring. Check voltage at point A, directly on pin 2, may be R2 is
not connected properly. Voltage on pin 2 should be ~6V. Schematic should
turn turn on/off leds when Voltage on point B is near voltage on point
A. You may measure resistance of LDR when it is "in dark" and when it
is "in light". Do not forget to remove it from schematic for measuring.

In addition there is a mistake on the schematic of board: there is no
22k resistor.

------
With Best Regards,
Evgeniy

R

#### RobH

##### Guest
On 10/02/2020 12:04, Evgeniy wrote:
Hello, RobH!

On Mon, 10 Feb 2020 09:26:22 +0000
RobH &lt;rob@despammer.com&gt; wrote:

On 10/02/2020 00:38, amdx wrote:
On 2/9/2020 5:11 PM, RobH wrote:
I have been lookingÂ  at schmitt trigger circuits and didn't realise
how many variations there are.

Anyway I found this one:

The author here says that by adding a 22k resistor to the bottom
diagram turns into a schmitt trigger circuit as the top diagram.

Â From other schmitt trigger circuits I have see on youtube and other
sites, this one is so simple, but works. The switching from off to on
is almost instant for the 2 leds.

Â It's faster than the persistence of your eye!

But I have a question here, it says this:

Two LEDs are needed because, when the Op Amp pin 6 voltage is low,
there may still be as much as two volts present.
This could be enough to light a single LED. With two LEDs, four volts
are needed.
The Op Amp pin 6 voltage will be well below 4V so the LEDs will be off.

Â What are the high and low voltages you get on Pin 6?

What actually happens is both leds come on at the same time and the
voltage is 2.18v when on.

Â Usually they will come on at very close to same time, there will be a
slight difference of the voltage across them, but they will be pretty
similar.
If you are using a common red LED they will have a voltage drop of about
1.8v, so 3.6V for to with the rest dropped across your 560 ohm resistor.
Â The voltage drop is depending on the current through the LED, but the
1.8V for a red LED is close.

I am using a 33k resistor instead of a 22k which I don't have, if that
makes the difference.

Â You tell us, record the voltage on Pin 3 and Pin 6 when Pin 6
switches, both high and low. Then put another 33k in parallel with the
one you have and repeat the voltage records as above. Then you can tell
me what difference changing the value of the feedback (your 33k)
resistor causes.
Â I like this you are one upping me and removing even more parts.
You have been thinking.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

The voltage on pin 6 , as the circuit is now, or per the
diagram/schematic, is 2.0v when Lo, and 4.0v when Hi

The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate upto
10.9v.

Another 33k resistor was placed in parallel with the one across pins 3
and 6. The Lo voltage was 2.0v and the Hi voltage was 3.2v on pin 6.
On pin 3, the voltage when Lo was 10.2v and when Hi the voltage was
10.2v, and both leds were flickering rapidly.

Just want to say some words. Schematic is strange, and description
is strange too. Author said:
"Adding the 22K resistor adds some positive feedback and the on and off
threshold voltages will now differ slightly. Adding this resistor
converts the circuit from a Comparator to a Schmitt Trigger."

That's not true, because threshold voltage will be the same: it will be
half of power voltage, value at point A. Feedback does not change
threshold voltage, it just changes voltage/current through LDR, so
behaviour of schematic depends on Volt-Ampere characteristics of LDR. If
characteristic is like classical resistor (resistance does not depend
on voltage/current, just light; I have not work with LDR), it will make
LDR resistance thresholds, which makes lighting thresholds.

The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate upto
10.9v.

There is something wrong: with schematic on board or OP AMP or while
measuring. Check voltage at point A, directly on pin 2, may be R2 is
not connected properly. Voltage on pin 2 should be ~6V. Schematic should
turn turn on/off leds when Voltage on point B is near voltage on point
A. You may measure resistance of LDR when it is "in dark" and when it
is "in light". Do not forget to remove it from schematic for measuring.

In addition there is a mistake on the schematic of board: there is no
22k resistor.

------
With Best Regards,
Evgeniy
Ok, I boobed by putting x2 incorrect resistors. Doh!!!
I used x2 47k resistors in error and have changed them to the 10k resistors.

At pin 2 the voltage is 6.1v.
The voltage at pin 6 is 10.5v when both leds are on using x2 10k
resistors.Putting light onto the ldr turns the leds off and the voltage
at pin 6 is 1.98v
Where it should be x2 10k ones I used a 10k potentiometer, and gives the
same results

A

#### amdx

##### Guest
On 2/10/2020 8:01 AM, RobH wrote:
On 10/02/2020 12:04, Evgeniy wrote:
Hello, RobH!

On Mon, 10 Feb 2020 09:26:22 +0000
RobH &lt;rob@despammer.com&gt; wrote:

On 10/02/2020 00:38, amdx wrote:
On 2/9/2020 5:11 PM, RobH wrote:
I have been lookingÂ  at schmitt trigger circuits and didn't realise
how many variations there are.

Anyway I found this one:

The author here says that by adding a 22k resistor to the bottom
diagram turns into a schmitt trigger circuit as the top diagram.

Â Â From other schmitt trigger circuits I have see on youtube and other
sites, this one is so simple, but works. The switching from off to on
is almost instant for the 2 leds.

Â  Â It's faster than the persistence of your eye!
But I have a question here, it says this:

Two LEDs are needed because, when the Op Amp pin 6 voltage is low,
there may still be as much as two volts present.
This could be enough to light a single LED. With two LEDs, four volts
are needed.
The Op Amp pin 6 voltage will be well below 4V so the LEDs will be
off.

Â  Â What are the high and low voltages you get on Pin 6?

What actually happens is both leds come on at the same time and the
voltage is 2.18v when on.

Â  Â Usually they will come on at very close to same time, there will
be a
slight difference of the voltage across them, but they will be pretty
similar.
If you are using a common red LED they will have a voltage drop of
1.8v, so 3.6V for to with the rest dropped across your 560 ohm
resistor.
Â  Â The voltage drop is depending on the current through the LED, but
the
1.8V for a red LED is close.

I am using a 33k resistor instead of a 22k which I don't have, if that
makes the difference.

Â  Â You tell us, record the voltage on Pin 3 and Pin 6 when Pin 6
switches, both high and low. Then put another 33k in parallel with the
one you have and repeat the voltage records as above. Then you can tell
me what difference changing the value of the feedback (your 33k)
resistor causes.
Â  Â I like this you are one upping me and removing even more parts.
You have been thinking.
Â  Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

The voltage on pin 6 , as the circuit is now, or per the
diagram/schematic, is 2.0v when Lo, and 4.0v when Hi

The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate upto
10.9v.

Another 33k resistor was placed in parallel with the one across pins 3
and 6. The Lo voltage was 2.0v and the Hi voltage was 3.2v on pin 6.
On pin 3, the voltage when Lo was 10.2v and when Hi the voltage was
10.2v, and both leds were flickering rapidly.

Just want to say some words. Schematic is strange, and description
is strange too. Author said:
"Adding the 22K resistor adds some positive feedback and the on and off
threshold voltages will now differ slightly. Adding this resistor
converts the circuit from a Comparator to a Schmitt Trigger."

That's not true, because threshold voltage will be the same: it will be
half of power voltage, value at point A. Feedback does not change
threshold voltage, it just changes voltage/current through LDR, so
behaviour of schematic depends on Volt-Ampere characteristics of LDR. If
characteristic is like classical resistor (resistance does not depend
on voltage/current, just light; I have not work with LDR), it will make
LDR resistance thresholds, which makes lighting thresholds.

The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate upto
10.9v.

There is something wrong: with schematic on board or OP AMP or while
measuring. Check voltage at point A, directly on pin 2, may be R2 is
not connected properly. Voltage on pin 2 should be ~6V. Schematic should
turn turn on/off leds when Voltage on point B is near voltage on point
A. You may measure resistance of LDR when it is "in dark" and when it
is "in light". Do not forget to remove it from schematic for measuring.

In addition there is a mistake on the schematic of board: there is no
22k resistor.

------
With Best Regards,
Evgeniy

Ok, I boobed by putting x2 incorrect resistors. Doh!!!
I used x2 47k resistors in error and have changed them to the 10k
resistors.
Those are just a voltage divider, setting the voltage on Pin 2. I
suspect you could you could use a resistor value up to 500K and have
little change in the voltage on Pin 2.
I found the input resistance of a 741 is 2Mohm, so, I did the
calculations, if you use two 10k resistors and the voltage at Pin 2 is
4.5v, changing them to 500k will only shift that voltage by 0.5v.
It would raise Pin 2 to 5v.
(Side no, there may be different style 741s, with different input
impedance, I don't know.)

At pin 2 the voltage is 6.1v.
The voltage at pin 6 is 10.5v when both leds are on using x2 10k
resistors.Putting light onto the ldr turns the leds off and the voltage
at pin 6 is 1.98v
Where it should be x2 10k ones I used a 10k potentiometer, and gives the
same results
Do you need to know how to shift the output level ( 6.1v to 1.98v) so
you can put a buzzer on the output?
Mikek

R

#### RobH

##### Guest
On 10/02/2020 15:39, amdx wrote:
On 2/10/2020 8:01 AM, RobH wrote:
On 10/02/2020 12:04, Evgeniy wrote:
Hello, RobH!

On Mon, 10 Feb 2020 09:26:22 +0000
RobH &lt;rob@despammer.com&gt; wrote:

On 10/02/2020 00:38, amdx wrote:
On 2/9/2020 5:11 PM, RobH wrote:
I have been lookingÂ  at schmitt trigger circuits and didn't realise
how many variations there are.

Anyway I found this one:

The author here says that by adding a 22k resistor to the bottom
diagram turns into a schmitt trigger circuit as the top diagram.

Â Â From other schmitt trigger circuits I have see on youtube and other
sites, this one is so simple, but works. The switching from off to on
is almost instant for the 2 leds.

Â  Â It's faster than the persistence of your eye!
But I have a question here, it says this:

Two LEDs are needed because, when the Op Amp pin 6 voltage is low,
there may still be as much as two volts present.
This could be enough to light a single LED. With two LEDs, four volts
are needed.
The Op Amp pin 6 voltage will be well below 4V so the LEDs will be
off.

Â  Â What are the high and low voltages you get on Pin 6?

What actually happens is both leds come on at the same time and the
voltage is 2.18v when on.

Â  Â Usually they will come on at very close to same time, there will
be a
slight difference of the voltage across them, but they will be pretty
similar.
If you are using a common red LED they will have a voltage drop of
1.8v, so 3.6V for to with the rest dropped across your 560 ohm
resistor.
Â  Â The voltage drop is depending on the current through the LED,
but the
1.8V for a red LED is close.

I am using a 33k resistor instead of a 22k which I don't have, if
that
makes the difference.

Â  Â You tell us, record the voltage on Pin 3 and Pin 6 when Pin 6
switches, both high and low. Then put another 33k in parallel with the
one you have and repeat the voltage records as above. Then you can
tell
me what difference changing the value of the feedback (your 33k)
resistor causes.
Â  Â I like this you are one upping me and removing even more parts.
You have been thinking.
Â  Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

The voltage on pin 6 , as the circuit is now, or per the
diagram/schematic, is 2.0v when Lo, and 4.0v when Hi

The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate upto
10.9v.

Another 33k resistor was placed in parallel with the one across pins 3
and 6. The Lo voltage was 2.0v and the Hi voltage was 3.2v on pin 6.
On pin 3, the voltage when Lo was 10.2v and when Hi the voltage was
10.2v, and both leds were flickering rapidly.

Just want to say some words. Schematic is strange, and description
is strange too. Author said:
"Adding the 22K resistor adds some positive feedback and the on and off
threshold voltages will now differ slightly. Adding this resistor
converts the circuit from a Comparator to a Schmitt Trigger."

That's not true, because threshold voltage will be the same: it will be
half of power voltage, value at point A. Feedback does not change
threshold voltage, it just changes voltage/current through LDR, so
behaviour of schematic depends on Volt-Ampere characteristics of LDR. If
characteristic is like classical resistor (resistance does not depend
on voltage/current, just light; I have not work with LDR), it will make
LDR resistance thresholds, which makes lighting thresholds.

The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate upto
10.9v.

There is something wrong: with schematic on board or OP AMP or while
measuring. Check voltage at point A, directly on pin 2, may be R2 is
not connected properly. Voltage on pin 2 should be ~6V. Schematic should
turn turn on/off leds when Voltage on point B is near voltage on point
A. You may measure resistance of LDR when it is "in dark" and when it
is "in light". Do not forget to remove it from schematic for measuring.

In addition there is a mistake on the schematic of board: there is no
22k resistor.

------
With Best Regards,
Evgeniy

Ok, I boobed by putting x2 incorrect resistors. Doh!!!
I used x2 47k resistors in error and have changed them to the 10k
resistors.

Â Those are just a voltage divider, setting the voltage on Pin 2. I
suspect you could you could use a resistor value up to 500K and have
little change in the voltage on Pin 2.
Â  I found the input resistance of a 741 is 2Mohm, so, I did the
calculations, if you use two 10k resistors and the voltage at Pin 2 is
4.5v, changing them to 500k will only shift that voltage by 0.5v.
It would raise Pin 2 to 5v.
Â (Side no, there may be different style 741s, with different input
impedance, I don't know.)

At pin 2 the voltage is 6.1v.
The voltage at pin 6 is 10.5v when both leds are on using x2 10k
resistors.Putting light onto the ldr turns the leds off and the
voltage at pin 6 is 1.98v
Where it should be x2 10k ones I used a 10k potentiometer, and gives
the same results

Â Do you need to know how to shift the output level ( 6.1v to 1.98v) so
you can put a buzzer on the output?
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek
Err, yes I would like to know that.
When the 12v is applied the leds are both on, and go off when light is
shone on to the ldr, so I'd like the buzzer to sound when the leds go off.

Thanks

E

#### Evgeniy

##### Guest
Hello, amdx!

On Mon, 10 Feb 2020 09:39:54 -0600
amdx &lt;nojunk@knology.net&gt; wrote:

Ok, I boobed by putting x2 incorrect resistors. Doh!!!
I used x2 47k resistors in error and have changed them to the 10k
resistors.

Those are just a voltage divider, setting the voltage on Pin 2. I
suspect you could you could use a resistor value up to 500K and have
little change in the voltage on Pin 2.
I found the input resistance of a 741 is 2Mohm, so, I did the
calculations, if you use two 10k resistors and the voltage at Pin 2 is
4.5v, changing them to 500k will only shift that voltage by 0.5v.
It would raise Pin 2 to 5v.
(Side no, there may be different style 741s, with different input
impedance, I don't know.)
It's not a good idea to set huge resistance on high-impedance imput:
AMP can receive radio noise on input. 10K is enough normal resistance, I
think. With R1 and R2 47K it should work too, but I do not prefer such
resistance: too high in high-impedance input. It looks like that there
was a problem with badly connected R2, because when leds are off voltage
on pin 3 should be below ~6V (voltage on point A), but he had:
The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate upto
10.9v.

------
With Best Regards,
Evgeniy Shtrenyov

A

#### amdx

##### Guest
On 2/10/2020 9:51 AM, RobH wrote:
On 10/02/2020 15:39, amdx wrote:
On 2/10/2020 8:01 AM, RobH wrote:
On 10/02/2020 12:04, Evgeniy wrote:
Hello, RobH!

On Mon, 10 Feb 2020 09:26:22 +0000
RobH &lt;rob@despammer.com&gt; wrote:

On 10/02/2020 00:38, amdx wrote:
On 2/9/2020 5:11 PM, RobH wrote:
I have been lookingÂ  at schmitt trigger circuits and didn't realise
how many variations there are.

Anyway I found this one:

The author here says that by adding a 22k resistor to the bottom
diagram turns into a schmitt trigger circuit as the top diagram.

Â Â From other schmitt trigger circuits I have see on youtube and
other
sites, this one is so simple, but works. The switching from off
to on
is almost instant for the 2 leds.

Â  Â It's faster than the persistence of your eye!
But I have a question here, it says this:

Two LEDs are needed because, when the Op Amp pin 6 voltage is low,
there may still be as much as two volts present.
This could be enough to light a single LED. With two LEDs, four
volts
are needed.
The Op Amp pin 6 voltage will be well below 4V so the LEDs will
be off.

Â  Â What are the high and low voltages you get on Pin 6?

What actually happens is both leds come on at the same time and the
voltage is 2.18v when on.

Â  Â Usually they will come on at very close to same time, there
will be a
slight difference of the voltage across them, but they will be pretty
similar.
If you are using a common red LED they will have a voltage drop of
1.8v, so 3.6V for to with the rest dropped across your 560 ohm
resistor.
Â  Â The voltage drop is depending on the current through the LED,
but the
1.8V for a red LED is close.

I am using a 33k resistor instead of a 22k which I don't have, if
that
makes the difference.

Â  Â You tell us, record the voltage on Pin 3 and Pin 6 when Pin 6
switches, both high and low. Then put another 33k in parallel with
the
one you have and repeat the voltage records as above. Then you can
tell
me what difference changing the value of the feedback (your 33k)
resistor causes.
Â  Â I like this you are one upping me and removing even more parts.
You have been thinking.
Â  Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

The voltage on pin 6 , as the circuit is now, or per the
diagram/schematic, is 2.0v when Lo, and 4.0v when Hi

The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate
upto
10.9v.

Another 33k resistor was placed in parallel with the one across pins 3
and 6. The Lo voltage was 2.0v and the Hi voltage was 3.2v on pin 6.
On pin 3, the voltage when Lo was 10.2v and when Hi the voltage was
10.2v, and both leds were flickering rapidly.

Just want to say some words. Schematic is strange, and description
is strange too. Author said:
"Adding the 22K resistor adds some positive feedback and the on and off
threshold voltages will now differ slightly. Adding this resistor
converts the circuit from a Comparator to a Schmitt Trigger."

That's not true, because threshold voltage will be the same: it will be
half of power voltage, value at point A. Feedback does not change
threshold voltage, it just changes voltage/current through LDR, so
behaviour of schematic depends on Volt-Ampere characteristics of
LDR. If
characteristic is like classical resistor (resistance does not depend
on voltage/current, just light; I have not work with LDR), it will make
LDR resistance thresholds, which makes lighting thresholds.

The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate
upto
10.9v.

There is something wrong: with schematic on board or OP AMP or while
measuring. Check voltage at point A, directly on pin 2, may be R2 is
not connected properly. Voltage on pin 2 should be ~6V. Schematic
should
turn turn on/off leds when Voltage on point B is near voltage on point
A. You may measure resistance of LDR when it is "in dark" and when it
is "in light". Do not forget to remove it from schematic for measuring.

In addition there is a mistake on the schematic of board: there is no
22k resistor.

------
With Best Regards,
Evgeniy

Ok, I boobed by putting x2 incorrect resistors. Doh!!!
I used x2 47k resistors in error and have changed them to the 10k
resistors.

Â Â Those are just a voltage divider, setting the voltage on Pin 2. I
suspect you could you could use a resistor value up to 500K and have
little change in the voltage on Pin 2.
Â Â  I found the input resistance of a 741 is 2Mohm, so, I did the
calculations, if you use two 10k resistors and the voltage at Pin 2 is
4.5v, changing them to 500k will only shift that voltage by 0.5v.
It would raise Pin 2 to 5v.
Â Â (Side no, there may be different style 741s, with different input
impedance, I don't know.)

At pin 2 the voltage is 6.1v.
The voltage at pin 6 is 10.5v when both leds are on using x2 10k
resistors.Putting light onto the ldr turns the leds off and the
voltage at pin 6 is 1.98v
Where it should be x2 10k ones I used a 10k potentiometer, and gives
the same results

Â Â Do you need to know how to shift the output level ( 6.1v to 1.98v)
so you can put a buzzer on the output?
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

Err, yes I would like to know that.
So would I! I might, from what you said below, it seems you need to
voltage shift and invert, I'm not sure that can be done with just one
transistor. ( but you know with what you have you could trigger a 555
and have clean 0 to 12v swing. (I haven't thought if it needs an
inversion.) But the 555 does do your level shift.

When the 12v is applied the leds are both on, and go off when light is
shone on to the ldr, so I'd like the buzzer to sound when the leds go off.
Let me restate that as a question. Do you want the the buzzer to buzz
when light shines on the ldr?
Oh, you are now using 12V? That shifts the numbers for that voltage
divider up to 6v and without redoing my calculation, maybe to 6.7 with
the 500k.
Mikek

A

#### amdx

##### Guest
On 2/10/2020 10:02 AM, Evgeniy wrote:
Hello, amdx!

On Mon, 10 Feb 2020 09:39:54 -0600
amdx &lt;nojunk@knology.net&gt; wrote:

Ok, I boobed by putting x2 incorrect resistors. Doh!!!
I used x2 47k resistors in error and have changed them to the 10k
resistors.

Those are just a voltage divider, setting the voltage on Pin 2. I
suspect you could you could use a resistor value up to 500K and have
little change in the voltage on Pin 2.
I found the input resistance of a 741 is 2Mohm, so, I did the
calculations, if you use two 10k resistors and the voltage at Pin 2 is
4.5v, changing them to 500k will only shift that voltage by 0.5v.
It would raise Pin 2 to 5v.
(Side no, there may be different style 741s, with different input
impedance, I don't know.)

It's not a good idea to set huge resistance on high-impedance imput:
AMP can receive radio noise on input. 10K is enough normal resistance, I
think. With R1 and R2 47K it should work too, but I do not prefer such
resistance: too high in high-impedance input.
All that is possible, 500k is not huge, It would actually be 250k to
ground, but, I was trying to teach him that, he is just setting the
voltage on pin 2 with those resistors. The ratio is important not the
absolute value.

It looks like that there
was a problem with badly connected R2, because when leds are off voltage
on pin 3 should be below ~6V (voltage on point A), but he had:
The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate upto
10.9v.
I think he is just letting light leak into his ldr giving him that
fluctuation.
Mikek

------
With Best Regards,
Evgeniy Shtrenyov

R

#### RobH

##### Guest
On 10/02/2020 16:13, amdx wrote:
On 2/10/2020 9:51 AM, RobH wrote:
On 10/02/2020 15:39, amdx wrote:
On 2/10/2020 8:01 AM, RobH wrote:
On 10/02/2020 12:04, Evgeniy wrote:
Hello, RobH!

On Mon, 10 Feb 2020 09:26:22 +0000
RobH &lt;rob@despammer.com&gt; wrote:

On 10/02/2020 00:38, amdx wrote:
On 2/9/2020 5:11 PM, RobH wrote:
I have been lookingÂ  at schmitt trigger circuits and didn't realise
how many variations there are.

Anyway I found this one:

The author here says that by adding a 22k resistor to the bottom
diagram turns into a schmitt trigger circuit as the top diagram.

Â Â From other schmitt trigger circuits I have see on youtube and
other
sites, this one is so simple, but works. The switching from off
to on
is almost instant for the 2 leds.

Â  Â It's faster than the persistence of your eye!
But I have a question here, it says this:

Two LEDs are needed because, when the Op Amp pin 6 voltage is low,
there may still be as much as two volts present.
This could be enough to light a single LED. With two LEDs, four
volts
are needed.
The Op Amp pin 6 voltage will be well below 4V so the LEDs will
be off.

Â  Â What are the high and low voltages you get on Pin 6?

What actually happens is both leds come on at the same time and the
voltage is 2.18v when on.

Â  Â Usually they will come on at very close to same time, there
will be a
slight difference of the voltage across them, but they will be
pretty
similar.
If you are using a common red LED they will have a voltage drop
1.8v, so 3.6V for to with the rest dropped across your 560 ohm
resistor.
Â  Â The voltage drop is depending on the current through the LED,
but the
1.8V for a red LED is close.

I am using a 33k resistor instead of a 22k which I don't have,
if that
makes the difference.

Â  Â You tell us, record the voltage on Pin 3 and Pin 6 when Pin 6
switches, both high and low. Then put another 33k in parallel
with the
one you have and repeat the voltage records as above. Then you
can tell
me what difference changing the value of the feedback (your 33k)
resistor causes.
Â  Â I like this you are one upping me and removing even more parts.
You have been thinking.
Â  Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

The voltage on pin 6 , as the circuit is now, or per the
diagram/schematic, is 2.0v when Lo, and 4.0v when Hi

The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate
upto
10.9v.

Another 33k resistor was placed in parallel with the one across
pins 3
and 6. The Lo voltage was 2.0v and the Hi voltage was 3.2v on pin 6.
On pin 3, the voltage when Lo was 10.2v and when Hi the voltage was
10.2v, and both leds were flickering rapidly.

Just want to say some words. Schematic is strange, and description
is strange too. Author said:
"Adding the 22K resistor adds some positive feedback and the on and
off
threshold voltages will now differ slightly. Adding this resistor
converts the circuit from a Comparator to a Schmitt Trigger."

That's not true, because threshold voltage will be the same: it
will be
half of power voltage, value at point A. Feedback does not change
threshold voltage, it just changes voltage/current through LDR, so
behaviour of schematic depends on Volt-Ampere characteristics of
LDR. If
characteristic is like classical resistor (resistance does not depend
on voltage/current, just light; I have not work with LDR), it will
make
LDR resistance thresholds, which makes lighting thresholds.

The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate
upto
10.9v.

There is something wrong: with schematic on board or OP AMP or while
measuring. Check voltage at point A, directly on pin 2, may be R2 is
not connected properly. Voltage on pin 2 should be ~6V. Schematic
should
turn turn on/off leds when Voltage on point B is near voltage on point
A. You may measure resistance of LDR when it is "in dark" and when it
is "in light". Do not forget to remove it from schematic for
measuring.

In addition there is a mistake on the schematic of board: there is no
22k resistor.

------
With Best Regards,
Evgeniy

Ok, I boobed by putting x2 incorrect resistors. Doh!!!
I used x2 47k resistors in error and have changed them to the 10k
resistors.

Â Â Those are just a voltage divider, setting the voltage on Pin 2. I
suspect you could you could use a resistor value up to 500K and have
little change in the voltage on Pin 2.
Â Â  I found the input resistance of a 741 is 2Mohm, so, I did the
calculations, if you use two 10k resistors and the voltage at Pin 2
is 4.5v, changing them to 500k will only shift that voltage by 0.5v.
It would raise Pin 2 to 5v.
Â Â (Side no, there may be different style 741s, with different input
impedance, I don't know.)

At pin 2 the voltage is 6.1v.
The voltage at pin 6 is 10.5v when both leds are on using x2 10k
resistors.Putting light onto the ldr turns the leds off and the
voltage at pin 6 is 1.98v
Where it should be x2 10k ones I used a 10k potentiometer, and gives
the same results

Â Â Do you need to know how to shift the output level ( 6.1v to 1.98v)
so you can put a buzzer on the output?
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

Err, yes I would like to know that.

So would I! I might, from what you said below, it seems you need to
voltage shift and invert, I'm not sure that can be done with just one
transistor. ( but you know with what you have you could trigger a 555
and have clean 0 to 12v swing. (I haven't thought if it needs an
inversion.) But the 555 does do your level shift.

When the 12v is applied the leds are both on, and go off when light is
shone on to the ldr, so I'd like the buzzer to sound when the leds go
off.

Â Let me restate that as a question. Do you want the the buzzer to buzz
when light shines on the ldr?
Â Your previous circuit was opposite.

Thanks

Oh, you are now using 12V? That shifts the numbers for that voltage
divider up to 6v and without redoing my calculation, maybe to 6.7 with
the 500k.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek
I was only using a 12v supply because that was what was shown on the
circuit on the website. Actually, I have now used a 9v battery supply
and the readings are now different.

On Pin 2 the voltage is 6.63v
On Pin 6 the voltage is 7.36v.
As before, putting light onto the ldr to make the leds go ff, the
voltage is 1.9v

Yes I would want the buzzer to buzz when light shines onto the ldr.

Thanks

A

#### amdx

##### Guest
On 2/10/2020 11:15 AM, RobH wrote:
On 10/02/2020 16:13, amdx wrote:
On 2/10/2020 9:51 AM, RobH wrote:
On 10/02/2020 15:39, amdx wrote:
On 2/10/2020 8:01 AM, RobH wrote:
On 10/02/2020 12:04, Evgeniy wrote:
Hello, RobH!

On Mon, 10 Feb 2020 09:26:22 +0000
RobH &lt;rob@despammer.com&gt; wrote:

On 10/02/2020 00:38, amdx wrote:
On 2/9/2020 5:11 PM, RobH wrote:
I have been lookingÂ  at schmitt trigger circuits and didn't
realise
how many variations there are.

Anyway I found this one:

The author here says that by adding a 22k resistor to the bottom
diagram turns into a schmitt trigger circuit as the top diagram.

Â Â From other schmitt trigger circuits I have see on youtube and
other
sites, this one is so simple, but works. The switching from off
to on
is almost instant for the 2 leds.

Â  Â It's faster than the persistence of your eye!
But I have a question here, it says this:

Two LEDs are needed because, when the Op Amp pin 6 voltage is low,
there may still be as much as two volts present.
This could be enough to light a single LED. With two LEDs, four
volts
are needed.
The Op Amp pin 6 voltage will be well below 4V so the LEDs will
be off.

Â  Â What are the high and low voltages you get on Pin 6?

What actually happens is both leds come on at the same time and
the
voltage is 2.18v when on.

Â  Â Usually they will come on at very close to same time, there
will be a
slight difference of the voltage across them, but they will be
pretty
similar.
If you are using a common red LED they will have a voltage drop
1.8v, so 3.6V for to with the rest dropped across your 560 ohm
resistor.
Â  Â The voltage drop is depending on the current through the LED,
but the
1.8V for a red LED is close.

I am using a 33k resistor instead of a 22k which I don't have,
if that
makes the difference.

Â  Â You tell us, record the voltage on Pin 3 and Pin 6 when Pin 6
switches, both high and low. Then put another 33k in parallel
with the
one you have and repeat the voltage records as above. Then you
can tell
me what difference changing the value of the feedback (your 33k)
resistor causes.
Â  Â I like this you are one upping me and removing even more parts.
You have been thinking.
Â  Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

The voltage on pin 6 , as the circuit is now, or per the
diagram/schematic, is 2.0v when Lo, and 4.0v when Hi

The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does
fluctuate upto
10.9v.

Another 33k resistor was placed in parallel with the one across
pins 3
and 6. The Lo voltage was 2.0v and the Hi voltage was 3.2v on pin 6.
On pin 3, the voltage when Lo was 10.2v and when Hi the voltage was
10.2v, and both leds were flickering rapidly.

Just want to say some words. Schematic is strange, and description
is strange too. Author said:
and off
threshold voltages will now differ slightly. Adding this resistor
converts the circuit from a Comparator to a Schmitt Trigger."

That's not true, because threshold voltage will be the same: it
will be
half of power voltage, value at point A. Feedback does not change
threshold voltage, it just changes voltage/current through LDR, so
behaviour of schematic depends on Volt-Ampere characteristics of
LDR. If
characteristic is like classical resistor (resistance does not depend
on voltage/current, just light; I have not work with LDR), it will
make
LDR resistance thresholds, which makes lighting thresholds.

The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does
fluctuate upto
10.9v.

There is something wrong: with schematic on board or OP AMP or while
measuring. Check voltage at point A, directly on pin 2, may be R2 is
not connected properly. Voltage on pin 2 should be ~6V. Schematic
should
turn turn on/off leds when Voltage on point B is near voltage on
point
A. You may measure resistance of LDR when it is "in dark" and when it
is "in light". Do not forget to remove it from schematic for
measuring.

In addition there is a mistake on the schematic of board: there is no
22k resistor.

------
With Best Regards,
Evgeniy

Ok, I boobed by putting x2 incorrect resistors. Doh!!!
I used x2 47k resistors in error and have changed them to the 10k
resistors.

Â Â Those are just a voltage divider, setting the voltage on Pin 2. I
suspect you could you could use a resistor value up to 500K and have
little change in the voltage on Pin 2.
Â Â  I found the input resistance of a 741 is 2Mohm, so, I did the
calculations, if you use two 10k resistors and the voltage at Pin 2
is 4.5v, changing them to 500k will only shift that voltage by 0.5v.
It would raise Pin 2 to 5v.
Â Â (Side no, there may be different style 741s, with different input
impedance, I don't know.)

At pin 2 the voltage is 6.1v.
The voltage at pin 6 is 10.5v when both leds are on using x2 10k
resistors.Putting light onto the ldr turns the leds off and the
voltage at pin 6 is 1.98v
Where it should be x2 10k ones I used a 10k potentiometer, and
gives the same results

Â Â Do you need to know how to shift the output level ( 6.1v to 1.98v)
so you can put a buzzer on the output?
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

Err, yes I would like to know that.

So would I! I might, from what you said below, it seems you need to
voltage shift and invert, I'm not sure that can be done with just one
transistor. ( but you know with what you have you could trigger a 555
and have clean 0 to 12v swing. (I haven't thought if it needs an
inversion.) But the 555 does do your level shift.

When the 12v is applied the leds are both on, and go off when light
is shone on to the ldr, so I'd like the buzzer to sound when the leds
go off.

Â Â Let me restate that as a question. Do you want the the buzzer to
buzz when light shines on the ldr?
Â Â Your previous circuit was opposite.

Thanks

Oh, you are now using 12V? That shifts the numbers for that voltage
divider up to 6v and without redoing my calculation, maybe to 6.7 with
the 500k.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

I was onlyÂ  using a 12v supply because that was what was shown on the
circuit on the website. Actually, I have now used a 9v battery supply
and the readings are now different.

On Pin 2 the voltage is 6.63v
If you are using two 10k resistors, that should split your supply
voltage in half, or 4.5V. Are you sure it's Pin 2 and are you sure you
have 2- 10k resistors. Also what it the exact part number and
manufacturer of the 741.

On Pin 6 the voltage is 7.36v.
As before, putting light onto the ldr to make the leds go ff, the
voltage is 1.9v
Don't know why you got more output range with lower Vc.

&gt; Yes I would want the buzzer to buzz when light shines onto the ldr.

I would just use the 555, but If if I took the time I think I could
make it work with two transistors. But I would need to experiment, not
design.
Mikek

R

#### RobH

##### Guest
On 10/02/2020 18:38, amdx wrote:
On 2/10/2020 11:15 AM, RobH wrote:
On 10/02/2020 16:13, amdx wrote:
On 2/10/2020 9:51 AM, RobH wrote:
On 10/02/2020 15:39, amdx wrote:
On 2/10/2020 8:01 AM, RobH wrote:
On 10/02/2020 12:04, Evgeniy wrote:
Hello, RobH!

On Mon, 10 Feb 2020 09:26:22 +0000
RobH &lt;rob@despammer.com&gt; wrote:

On 10/02/2020 00:38, amdx wrote:
On 2/9/2020 5:11 PM, RobH wrote:
I have been lookingÂ  at schmitt trigger circuits and didn't
realise
how many variations there are.

Anyway I found this one:

The author here says that by adding a 22k resistor to the bottom
diagram turns into a schmitt trigger circuit as the top diagram.

Â Â From other schmitt trigger circuits I have see on youtube
and other
sites, this one is so simple, but works. The switching from
off to on
is almost instant for the 2 leds.

Â  Â It's faster than the persistence of your eye!
But I have a question here, it says this:

Two LEDs are needed because, when the Op Amp pin 6 voltage is
low,
there may still be as much as two volts present.
This could be enough to light a single LED. With two LEDs,
four volts
are needed.
The Op Amp pin 6 voltage will be well below 4V so the LEDs
will be off.

Â  Â What are the high and low voltages you get on Pin 6?

What actually happens is both leds come on at the same time
and the
voltage is 2.18v when on.

Â  Â Usually they will come on at very close to same time, there
will be a
slight difference of the voltage across them, but they will be
pretty
similar.
If you are using a common red LED they will have a voltage drop
1.8v, so 3.6V for to with the rest dropped across your 560 ohm
resistor.
Â  Â The voltage drop is depending on the current through the
LED, but the
1.8V for a red LED is close.

I am using a 33k resistor instead of a 22k which I don't have,
if that
makes the difference.

Â  Â You tell us, record the voltage on Pin 3 and Pin 6 when Pin 6
switches, both high and low. Then put another 33k in parallel
with the
one you have and repeat the voltage records as above. Then you
can tell
me what difference changing the value of the feedback (your 33k)
resistor causes.
Â  Â I like this you are one upping me and removing even more parts.
You have been thinking.
Â  Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

The voltage on pin 6 , as the circuit is now, or per the
diagram/schematic, is 2.0v when Lo, and 4.0v when Hi

The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does
fluctuate upto
10.9v.

Another 33k resistor was placed in parallel with the one across
pins 3
and 6. The Lo voltage was 2.0v and the Hi voltage was 3.2v on
pin 6.
On pin 3, the voltage when Lo was 10.2v and when Hi the voltage was
10.2v, and both leds were flickering rapidly.

Just want to say some words. Schematic is strange, and description
is strange too. Author said:
and off
threshold voltages will now differ slightly. Adding this resistor
converts the circuit from a Comparator to a Schmitt Trigger."

That's not true, because threshold voltage will be the same: it
will be
half of power voltage, value at point A. Feedback does not change
threshold voltage, it just changes voltage/current through LDR, so
behaviour of schematic depends on Volt-Ampere characteristics of
LDR. If
characteristic is like classical resistor (resistance does not
depend
on voltage/current, just light; I have not work with LDR), it
will make
LDR resistance thresholds, which makes lighting thresholds.

The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does
fluctuate upto
10.9v.

There is something wrong: with schematic on board or OP AMP or while
measuring. Check voltage at point A, directly on pin 2, may be R2 is
not connected properly. Voltage on pin 2 should be ~6V. Schematic
should
turn turn on/off leds when Voltage on point B is near voltage on
point
A. You may measure resistance of LDR when it is "in dark" and
when it
is "in light". Do not forget to remove it from schematic for
measuring.

In addition there is a mistake on the schematic of board: there
is no
22k resistor.

------
With Best Regards,
Evgeniy

Ok, I boobed by putting x2 incorrect resistors. Doh!!!
I used x2 47k resistors in error and have changed them to the 10k
resistors.

Â Â Those are just a voltage divider, setting the voltage on Pin 2. I
suspect you could you could use a resistor value up to 500K and
have little change in the voltage on Pin 2.
Â Â  I found the input resistance of a 741 is 2Mohm, so, I did the
calculations, if you use two 10k resistors and the voltage at Pin 2
is 4.5v, changing them to 500k will only shift that voltage by 0.5v.
It would raise Pin 2 to 5v.
Â Â (Side no, there may be different style 741s, with different input
impedance, I don't know.)

At pin 2 the voltage is 6.1v.
The voltage at pin 6 is 10.5v when both leds are on using x2 10k
resistors.Putting light onto the ldr turns the leds off and the
voltage at pin 6 is 1.98v
Where it should be x2 10k ones I used a 10k potentiometer, and
gives the same results

Â Â Do you need to know how to shift the output level ( 6.1v to
1.98v) so you can put a buzzer on the output?
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

Err, yes I would like to know that.

So would I! I might, from what you said below, it seems you need to
voltage shift and invert, I'm not sure that can be done with just one
transistor. ( but you know with what you have you could trigger a 555
and have clean 0 to 12v swing. (I haven't thought if it needs an
inversion.) But the 555 does do your level shift.

When the 12v is applied the leds are both on, and go off when light
is shone on to the ldr, so I'd like the buzzer to sound when the
leds go off.

Â Â Let me restate that as a question. Do you want the the buzzer to
buzz when light shines on the ldr?
Â Â Your previous circuit was opposite.

Thanks

Oh, you are now using 12V? That shifts the numbers for that voltage
divider up to 6v and without redoing my calculation, maybe to 6.7
with the 500k.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

I was onlyÂ  using a 12v supply because that was what was shown on the
circuit on the website. Actually, I have now used a 9v battery supply
and the readings are now different.

On Pin 2 the voltage is 6.63v

If you are using two 10k resistors, that should split your supply
voltage in half, or 4.5V. Are you sure it's Pin 2 and are you sure you
have 2- 10k resistors. Also what it the exact part number and
manufacturer of the 741.

On Pin 6 the voltage is 7.36v.
As before, putting light onto the ldr to make the leds go ff, the
voltage is 1.9v

Â  Don't know why you got more output range with lower Vc.

Yes I would want the buzzer to buzz when light shines onto the ldr.

Â I would just use the 555, but If if I took the time I think I could
make it work with two transistors. But I would need to experiment, not
design.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

Thanks
Yes by using x2 10k resistors on pin 2 the voltage now is 4.15v from a
8.33v source. I don't know why it was 6.63v before.

A

#### amdx

##### Guest
On 2/10/2020 10:19 AM, amdx wrote:
On 2/10/2020 10:02 AM, Evgeniy wrote:
Hello, amdx!

On Mon, 10 Feb 2020 09:39:54 -0600
amdx &lt;nojunk@knology.net&gt; wrote:

Ok, I boobed by putting x2 incorrect resistors. Doh!!!
I used x2 47k resistors in error and have changed them to the 10k
resistors.

Â Â  Those are just a voltage divider, setting the voltage on Pin 2. I
suspect you could you could use a resistor value up to 500K and have
little change in the voltage on Pin 2.
Â Â Â  I found the input resistance of a 741 is 2Mohm, so, I did the
calculations, if you use two 10k resistors and the voltage at Pin 2 is
4.5v, changing them to 500k will only shift that voltage by 0.5v.
It would raise Pin 2 to 5v.
Â Â  (Side no, there may be different style 741s, with different input
impedance, I don't know.)

It's not a good idea to set huge resistance on high-impedance imput:
AMP can receive radio noise on input. 10K is enough normal resistance, I
think. With R1 and R2 47K it should work too, but I do not prefer such
resistance: too high in high-impedance input.

All that is possible, 500k is not huge, It would actually be 250k to
ground, but, I was trying to teach him that, he is just setting the
voltage on pin 2 with those resistors. The ratio is important not the
absolute value.

Â It looks like that there
was a problem with badly connected R2, because when leds are off voltage
on pin 3 should be below ~6V (voltage on point A), but he had:
The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate upto
10.9v.

Â I think he is just letting light leak into his ldr giving him that
fluctuation.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

------
With Best Regards,
Evgeniy Shtrenyov
Rob, I still wonder,
Do you want the the buzzer to buzz when light shines on the ldr?
Mikek

R

#### RobH

##### Guest
On 11/02/2020 14:04, amdx wrote:
On 2/10/2020 10:19 AM, amdx wrote:
On 2/10/2020 10:02 AM, Evgeniy wrote:
Hello, amdx!

On Mon, 10 Feb 2020 09:39:54 -0600
amdx &lt;nojunk@knology.net&gt; wrote:

Ok, I boobed by putting x2 incorrect resistors. Doh!!!
I used x2 47k resistors in error and have changed them to the 10k
resistors.

Â Â  Those are just a voltage divider, setting the voltage on Pin 2. I
suspect you could you could use a resistor value up to 500K and have
little change in the voltage on Pin 2.
Â Â Â  I found the input resistance of a 741 is 2Mohm, so, I did the
calculations, if you use two 10k resistors and the voltage at Pin 2 is
4.5v, changing them to 500k will only shift that voltage by 0.5v.
It would raise Pin 2 to 5v.
Â Â  (Side no, there may be different style 741s, with different input
impedance, I don't know.)

It's not a good idea to set huge resistance on high-impedance imput:
AMP can receive radio noise on input. 10K is enough normal resistance, I
think. With R1 and R2 47K it should work too, but I do not prefer such
resistance: too high in high-impedance input.

All that is possible, 500k is not huge, It would actually be 250k to
ground, but, I was trying to teach him that, he is just setting the
voltage on pin 2 with those resistors. The ratio is important not the
absolute value.

Â Â It looks like that there
was a problem with badly connected R2, because when leds are off voltage
on pin 3 should be below ~6V (voltage on point A), but he had:
The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate
upto
10.9v.

Â Â I think he is just letting light leak into his ldr giving him that
fluctuation.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

------
With Best Regards,
Evgeniy Shtrenyov

Â Â  Rob, I still wonder,
Â Do you want the the buzzer to buzz when light shines on the ldr?
Â Your previous circuit was opposite.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek
Mmm, strange that, but yes when the light shines on the ldr.
I have the led lighting up when light shines on the ldr, but I am not
sure where to connect the buzzer to. Connecting it to pin 3 on the 555
it buzzes with a low pitch and the led lights dimly, then when I shine
light on the ldr the buzzer really buzzes and the led is in full brightness.

R

#### RobH

##### Guest
On 11/02/2020 15:17, RobH wrote:
On 11/02/2020 14:04, amdx wrote:
On 2/10/2020 10:19 AM, amdx wrote:
On 2/10/2020 10:02 AM, Evgeniy wrote:
Hello, amdx!

On Mon, 10 Feb 2020 09:39:54 -0600
amdx &lt;nojunk@knology.net&gt; wrote:

Ok, I boobed by putting x2 incorrect resistors. Doh!!!
I used x2 47k resistors in error and have changed them to the 10k
resistors.

Â Â  Those are just a voltage divider, setting the voltage on Pin 2. I
suspect you could you could use a resistor value up to 500K and have
little change in the voltage on Pin 2.
Â Â Â  I found the input resistance of a 741 is 2Mohm, so, I did the
calculations, if you use two 10k resistors and the voltage at Pin 2 is
4.5v, changing them to 500k will only shift that voltage by 0.5v.
It would raise Pin 2 to 5v.
Â Â  (Side no, there may be different style 741s, with different input
impedance, I don't know.)

It's not a good idea to set huge resistance on high-impedance imput:
resistance, I
think. With R1 and R2 47K it should work too, but I do not prefer such
resistance: too high in high-impedance input.

All that is possible, 500k is not huge, It would actually be 250k to
ground, but, I was trying to teach him that, he is just setting the
voltage on pin 2 with those resistors. The ratio is important not the
absolute value.

Â Â It looks like that there
was a problem with badly connected R2, because when leds are off
voltage
on pin 3 should be below ~6V (voltage on point A), but he had:
The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate
upto
10.9v.

Â Â I think he is just letting light leak into his ldr giving him that
fluctuation.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

th

------
With Best Regards,
Evgeniy Shtrenyov

Â Â Â  Rob, I still wonder,
Â Â Do you want the the buzzer to buzz when light shines on the ldr?
Â Â Your previous circuit was opposite.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

Mmm, strange that, but yes when the light shines on the ldr.
Â I have the led lighting up when light shines on the ldr, but I am not
sure where to connect the buzzer to. Connecting it to pin 3 on the 555
it buzzes with a low pitch and the led lights dimly, then when I shine
light on the ldr the buzzer really buzzes and the led is in full
brightness.
Oops, wrong circuit above, please ignore as that is the schmitt trigger
circuit you sent me.

R

#### RobH

##### Guest
On 11/02/2020 15:32, RobH wrote:
On 11/02/2020 15:17, RobH wrote:
On 11/02/2020 14:04, amdx wrote:
On 2/10/2020 10:19 AM, amdx wrote:
On 2/10/2020 10:02 AM, Evgeniy wrote:
Hello, amdx!

On Mon, 10 Feb 2020 09:39:54 -0600
amdx &lt;nojunk@knology.net&gt; wrote:

Ok, I boobed by putting x2 incorrect resistors. Doh!!!
I used x2 47k resistors in error and have changed them to the 10k
resistors.

Â Â  Those are just a voltage divider, setting the voltage on Pin 2. I
suspect you could you could use a resistor value up to 500K and have
little change in the voltage on Pin 2.
Â Â Â  I found the input resistance of a 741 is 2Mohm, so, I did the
calculations, if you use two 10k resistors and the voltage at Pin
2 is
4.5v, changing them to 500k will only shift that voltage by 0.5v.
It would raise Pin 2 to 5v.
Â Â  (Side no, there may be different style 741s, with different input
impedance, I don't know.)

It's not a good idea to set huge resistance on high-impedance imput:
resistance, I
think. With R1 and R2 47K it should work too, but I do not prefer such
resistance: too high in high-impedance input.

All that is possible, 500k is not huge, It would actually be 250k to
ground, but, I was trying to teach him that, he is just setting the
voltage on pin 2 with those resistors. The ratio is important not
the absolute value.

Â Â It looks like that there
was a problem with badly connected R2, because when leds are off
voltage
on pin 3 should be below ~6V (voltage on point A), but he had:
The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does
fluctuate upto
10.9v.

Â Â I think he is just letting light leak into his ldr giving him that
fluctuation.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

th

------
With Best Regards,
Evgeniy Shtrenyov

Â Â Â  Rob, I still wonder,
Â Â Do you want the the buzzer to buzz when light shines on the ldr?
Â Â Your previous circuit was opposite.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

Mmm, strange that, but yes when the light shines on the ldr.
Â Â I have the led lighting up when light shines on the ldr, but I am
not sure where to connect the buzzer to. Connecting it to pin 3 on the
555 it buzzes with a low pitch and the led lights dimly, then when I
shine light on the ldr the buzzer really buzzes and the led is in full
brightness.

Oops, wrong circuit above, please ignore as that is the schmitt trigger
circuit you sent me.
Yes if it is possible.
This circuit using the 741 op amp has the led shining when light hits
the ldr, and this is the one where iI don't know which pins or where to
connect the buzzer. If it can't be done, then it can't be done, and if
it needs a 555 to make the buzzer work, then I may as well just use the
schmitt trigger circuit you sent me.

Thanks

A

#### amdx

##### Guest
On 2/11/2020 9:17 AM, RobH wrote:
On 11/02/2020 14:04, amdx wrote:
On 2/10/2020 10:19 AM, amdx wrote:
On 2/10/2020 10:02 AM, Evgeniy wrote:
Hello, amdx!

On Mon, 10 Feb 2020 09:39:54 -0600
amdx &lt;nojunk@knology.net&gt; wrote:

Ok, I boobed by putting x2 incorrect resistors. Doh!!!
I used x2 47k resistors in error and have changed them to the 10k
resistors.

Â Â  Those are just a voltage divider, setting the voltage on Pin 2. I
suspect you could you could use a resistor value up to 500K and have
little change in the voltage on Pin 2.
Â Â Â  I found the input resistance of a 741 is 2Mohm, so, I did the
calculations, if you use two 10k resistors and the voltage at Pin 2 is
4.5v, changing them to 500k will only shift that voltage by 0.5v.
It would raise Pin 2 to 5v.
Â Â  (Side no, there may be different style 741s, with different input
impedance, I don't know.)

It's not a good idea to set huge resistance on high-impedance imput:
resistance, I
think. With R1 and R2 47K it should work too, but I do not prefer such
resistance: too high in high-impedance input.

All that is possible, 500k is not huge, It would actually be 250k to
ground, but, I was trying to teach him that, he is just setting the
voltage on pin 2 with those resistors. The ratio is important not the
absolute value.

Â Â It looks like that there
was a problem with badly connected R2, because when leds are off
voltage
on pin 3 should be below ~6V (voltage on point A), but he had:
The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does fluctuate
upto
10.9v.

Â Â I think he is just letting light leak into his ldr giving him that
fluctuation.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

------
With Best Regards,
Evgeniy Shtrenyov

Â Â Â  Rob, I still wonder,
Â Â Do you want the the buzzer to buzz when light shines on the ldr?
Â Â Your previous circuit was opposite.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

Mmm, strange that, but yes when the light shines on the ldr.
I was confused, back on track now.

&gt; Â I have the led lighting up when light shines on the ldr,

Ok, when the ldr goes dark, Pin 6 of the 741 goes low.
Does it go below 1/3rd Vcc?

Remember you need the signal to go low to trigger the Pin 2 of the 555.

but I am not
sure where to connect the buzzer to. Connecting it to pin 3 on the 555
it buzzes with a low pitch and the led lights dimly, then when I shine
light on the ldr the buzzer really buzzes and the led is in full
brightness.
Did you remember to install the differentiator, The RC between Pin 6 of
the 741 and Pin 2 of the 555?
The 555 normally has a nice 0v or Vcc output.

If you add the RC and you still have a small voltage on the 555 Pin 3
when it should be 0V, try another 555.

Mikek

R

#### RobH

##### Guest
On 11/02/2020 15:55, amdx wrote:
On 2/11/2020 9:17 AM, RobH wrote:
On 11/02/2020 14:04, amdx wrote:
On 2/10/2020 10:19 AM, amdx wrote:
On 2/10/2020 10:02 AM, Evgeniy wrote:
Hello, amdx!

On Mon, 10 Feb 2020 09:39:54 -0600
amdx &lt;nojunk@knology.net&gt; wrote:

Ok, I boobed by putting x2 incorrect resistors. Doh!!!
I used x2 47k resistors in error and have changed them to the 10k
resistors.

Â Â  Those are just a voltage divider, setting the voltage on Pin 2. I
suspect you could you could use a resistor value up to 500K and have
little change in the voltage on Pin 2.
Â Â Â  I found the input resistance of a 741 is 2Mohm, so, I did the
calculations, if you use two 10k resistors and the voltage at Pin
2 is
4.5v, changing them to 500k will only shift that voltage by 0.5v.
It would raise Pin 2 to 5v.
Â Â  (Side no, there may be different style 741s, with different input
impedance, I don't know.)

It's not a good idea to set huge resistance on high-impedance imput:
resistance, I
think. With R1 and R2 47K it should work too, but I do not prefer such
resistance: too high in high-impedance input.

All that is possible, 500k is not huge, It would actually be 250k to
ground, but, I was trying to teach him that, he is just setting the
voltage on pin 2 with those resistors. The ratio is important not
the absolute value.

Â Â It looks like that there
was a problem with badly connected R2, because when leds are off
voltage
on pin 3 should be below ~6V (voltage on point A), but he had:
The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does
fluctuate upto
10.9v.

Â Â I think he is just letting light leak into his ldr giving him that
fluctuation.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

------
With Best Regards,
Evgeniy Shtrenyov

Â Â Â  Rob, I still wonder,
Â Â Do you want the the buzzer to buzz when light shines on the ldr?
Â Â Your previous circuit was opposite.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

Mmm, strange that, but yes when the light shines on the ldr.

I was confused, back on track now.

Â Â I have the led lighting up when light shines on the ldr,

Ok, when the ldr goes dark, Pin 6 of the 741 goes low.
Does it go below 1/3rd Vcc?

Remember you need the signal to go low to trigger the Pin 2 of the 555.

but I am not sure where to connect the buzzer to. Connecting it to pin
3 on the 555 it buzzes with a low pitch and the led lights dimly, then
when I shine light on the ldr the buzzer really buzzes and the led is
in full brightness.

Did you remember to install the differentiator, The RC between Pin 6 of
the 741 and Pin 2 of the 555?
Â  The 555 normally has a nice 0v or Vcc output.

If you add the RC and you still have a small voltage on the 555 Pin 3
when it should be 0V, try another 555.

Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek
I've got a weird situation here??? Vcc is 8.19v

When the ldr is on the positive rail and the is is in the negative rail,
the voltage on pin 6 is as follows:
when the ldr is unshaded the voltage is 1.93v
when the ldr is shaded the voltage is 2.5v

When the ldr is in the negative rail and led in the positive rail
The voltage on pin 6 is as follows:
When the ldr is unshaded the volatge is 4.97v
When the ldr has light on it, the voltage is 7.46v

A

#### amdx

##### Guest
On 2/11/2020 1:59 PM, RobH wrote:
On 11/02/2020 15:55, amdx wrote:
On 2/11/2020 9:17 AM, RobH wrote:
On 11/02/2020 14:04, amdx wrote:
On 2/10/2020 10:19 AM, amdx wrote:
On 2/10/2020 10:02 AM, Evgeniy wrote:
Hello, amdx!

On Mon, 10 Feb 2020 09:39:54 -0600
amdx &lt;nojunk@knology.net&gt; wrote:

Ok, I boobed by putting x2 incorrect resistors. Doh!!!
I used x2 47k resistors in error and have changed them to the 10k
resistors.

Â Â  Those are just a voltage divider, setting the voltage on Pin 2. I
suspect you could you could use a resistor value up to 500K and have
little change in the voltage on Pin 2.
Â Â Â  I found the input resistance of a 741 is 2Mohm, so, I did the
calculations, if you use two 10k resistors and the voltage at Pin
2 is
4.5v, changing them to 500k will only shift that voltage by 0.5v.
It would raise Pin 2 to 5v.
Â Â  (Side no, there may be different style 741s, with different input
impedance, I don't know.)

It's not a good idea to set huge resistance on high-impedance imput:
resistance, I
think. With R1 and R2 47K it should work too, but I do not prefer
such
resistance: too high in high-impedance input.

All that is possible, 500k is not huge, It would actually be 250k
to ground, but, I was trying to teach him that, he is just setting
the voltage on pin 2 with those resistors. The ratio is important
not the absolute value.

Â Â It looks like that there
was a problem with badly connected R2, because when leds are off
voltage
on pin 3 should be below ~6V (voltage on point A), but he had:
The voltage on pin 3 is 10.2 when Lo, but does fluctuate upto 10.5v
approx, and when Hi the voltage is 10.6v, and again does
fluctuate upto
10.9v.

Â Â I think he is just letting light leak into his ldr giving him
that fluctuation.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

------
With Best Regards,
Evgeniy Shtrenyov

Â Â Â  Rob, I still wonder,
Â Â Do you want the the buzzer to buzz when light shines on the ldr?
Â Â Your previous circuit was opposite.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

Mmm, strange that, but yes when the light shines on the ldr.

I was confused, back on track now.

Â Â I have the led lighting up when light shines on the ldr,

Ok, when the ldr goes dark, Pin 6 of the 741 goes low.
Does it go below 1/3rd Vcc?

Remember you need the signal to go low to trigger the Pin 2 of the 555.

but I am not sure where to connect the buzzer to. Connecting it to
pin 3 on the 555 it buzzes with a low pitch and the led lights dimly,
then when I shine light on the ldr the buzzer really buzzes and the
led is in full brightness.

Did you remember to install the differentiator, The RC between Pin 6
of the 741 and Pin 2 of the 555?
Â Â  The 555 normally has a nice 0v or Vcc output.

If you add the RC and you still have a small voltage on the 555 Pin 3
when it should be 0V, try another 555.

Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Mikek

I've got a weird situation here??? Vcc is 8.19v

When the ldr is on the positive rail and the is is in the negative rail,
the voltage on pin 6 is as follows:
when the ldr is unshaded the voltage is 1.93v
when the ldr is shaded the voltage is 2.5v

When the ldr is in the negative rail and led in the positive rail
The voltage on pin 6 is as follows:
When the ldr is unshaded the volatge is 4.97v
When the ldr has light on it, the voltage is 7.46v
What part of that are you uncomfortable with?

Are you sure you used the same resistor in both situations?

circuit from 100k to 2.2k. That's a big change.
You/I/we need a little better understanding of the ldr you have.
In other words, what is the resistance in complete dark and what is the
resistance with the amount of light you expect to hit it in a normal
situation. I think the can do that with your DVM. Measure it in both
light and dark.
Give me those numbers.

Now set up a circuit Vc--33k resistor--ldr--ground.
Now give me VC, and the the ldr/resistor voltage in the dark.
and
Now give me VC, and the the ldr/resistor voltage in the expected light.

I expect after doing the calculations I will find they are the same
resistances. Just making sure there is no current related characteristic.

Mikek