Welcome Notice

Register Log in

Basic circuit help please

J

Jasen Betts

Guest
On 2020-01-30, default <default@defaulter.net> wrote:
On Thu, 30 Jan 2020 11:56:55 -0600, amdx <nojunk@knology.net> wrote:


Look at the schematic
https://circuitdigest.com/fullimage?i=circuitdiagram/Automatic-Light-Fence-Circuit-Diagram-with-Alarm.png

That is the classic 555 astable multivibrator circuit, with the
addition of another external trigger. That is NOT what should be
there. He would want it as a retriggerable monostable multivibrator.
Stays on while light is blocked then times out when light is resumed.

That circuit will not work properly. The better idea is to use the
transistor to short the timing cap (threshold pin) to ground and
ignore the pin 2 trigger input entirely.
so, connect trigger to where?

The author of the article, Pankaj Khatri, has no clue as to what he's
doing. If he did manage to get it to work (as planned) it isn't
because it was designed to work as planned.
--
Jasen.
 

Guest
On Friday, 31 January 2020 05:32:38 UTC, Jasen Betts wrote:
On 2020-01-29, RobH <rob@despammer.com> wrote:

I have put together a circuit from here:
https://circuitdigest.com/electronic-circuits/light-fence-circuit-diagram-with-alarm

and this is the schematic:
https://circuitdigest.com/fullimage?i=circuitdiagram/Automatic-Light-Fence-Circuit-Diagram-with-Alarm.png


I have the led, ldr and the 100k pot part of the circuit working fine,

That's a surprise... there should be a resistor below R1.
I have a very vague memory of an opamp being treated like that and yet working. Looking at opamp input circuits reveals there are voltage swing limits and so-called phase inversion that complicate the picture. At this late hour I can't work out if it could actually work, but can't dismiss it out of hand.


NT
 
A

Andy Bennet

Guest
On 01/02/2020 04:03, tabbypurr@gmail.com wrote:
On Friday, 31 January 2020 05:32:38 UTC, Jasen Betts wrote:
On 2020-01-29, RobH <rob@despammer.com> wrote:

I have put together a circuit from here:
https://circuitdigest.com/electronic-circuits/light-fence-circuit-diagram-with-alarm

and this is the schematic:
https://circuitdigest.com/fullimage?i=circuitdiagram/Automatic-Light-Fence-Circuit-Diagram-with-Alarm.png


I have the led, ldr and the 100k pot part of the circuit working fine,

That's a surprise... there should be a resistor below R1.

I have a very vague memory of an opamp being treated like that and yet working. Looking at opamp input circuits reveals there are voltage swing limits and so-called phase inversion that complicate the picture. At this late hour I can't work out if it could actually work, but can't dismiss it out of hand.


NT
General opamp theory says it does not work and never could.

If it does work then that is due to parameters way off the datasheet,
therefore not guaranteed to work for any or all devices.

The circuit "designer" does not have a clue, and seems to be playing
with all combinations of all pins of random devices until something
appears to work - and then publishing it.
 

Guest
On Saturday, 1 February 2020 08:33:53 UTC, Andy Bennet wrote:
On 01/02/2020 04:03, tabbypurr wrote:
On Friday, 31 January 2020 05:32:38 UTC, Jasen Betts wrote:
On 2020-01-29, RobH <rob@despammer.com> wrote:

I have put together a circuit from here:
https://circuitdigest.com/electronic-circuits/light-fence-circuit-diagram-with-alarm

and this is the schematic:
https://circuitdigest.com/fullimage?i=circuitdiagram/Automatic-Light-Fence-Circuit-Diagram-with-Alarm.png


I have the led, ldr and the 100k pot part of the circuit working fine,

That's a surprise... there should be a resistor below R1.

I have a very vague memory of an opamp being treated like that and yet working. Looking at opamp input circuits reveals there are voltage swing limits and so-called phase inversion that complicate the picture. At this late hour I can't work out if it could actually work, but can't dismiss it out of hand.

General opamp theory says it does not work and never could.
General opamp theory doesn't come into it. General opamp theory only applies when 2 Rs are used on the input to set a known voltage.


If it does work then that is due to parameters way off the datasheet,
therefore not guaranteed to work for any or all devices.
if it works, and the op at least says it does, it'll be due to the input circuit issues of the opamp, which should be sufficiently consistent for any one type of opamp.

The circuit "designer" does not have a clue, and seems to be playing
with all combinations of all pins of random devices until something
appears to work - and then publishing it.
I really ought to lash one together & test it, but am way too occupied with other stuff atm.


NT
 
D

default

Guest
On Fri, 31 Jan 2020 22:21:56 -0000 (UTC), Jasen Betts
<jasen@xnet.co.nz> wrote:

On 2020-01-30, default <default@defaulter.net> wrote:
On Thu, 30 Jan 2020 11:56:55 -0600, amdx <nojunk@knology.net> wrote:


Look at the schematic
https://circuitdigest.com/fullimage?i=circuitdiagram/Automatic-Light-Fence-Circuit-Diagram-with-Alarm.png

That is the classic 555 astable multivibrator circuit, with the
addition of another external trigger. That is NOT what should be
there. He would want it as a retriggerable monostable multivibrator.
Stays on while light is blocked then times out when light is resumed.

That circuit will not work properly. The better idea is to use the
transistor to short the timing cap (threshold pin) to ground and
ignore the pin 2 trigger input entirely.

so, connect trigger to where?
ignore the trigger pin, no connection.
If you did wire a monostable (not the astable as shown) and use the
trigger, and hold it low longer than the time constant, it will glitch
periodically as it times out then resume timing. (assuming it is DC
coupled as this schematic shows)

Shorting the timing cap to ground insures the output changes state and
stays that way (no glitch) until the short is removed then times
normally until it times out.

The glitch is very fast, but can be noticeable in the load.
The author of the article, Pankaj Khatri, has no clue as to what he's
doing. If he did manage to get it to work (as planned) it isn't
because it was designed to work as planned.
 
R

RobH

Guest
On 01/02/2020 13:40, amdx wrote:
On 2/1/2020 4:32 AM, tabbypurr@gmail.com wrote:
On Saturday, 1 February 2020 08:33:53 UTC, Andy Bennet  wrote:
On 01/02/2020 04:03, tabbypurr wrote:
On Friday, 31 January 2020 05:32:38 UTC, Jasen Betts  wrote:
On 2020-01-29, RobH <rob@despammer.com> wrote:

I have put together a circuit from here:
https://circuitdigest.com/electronic-circuits/light-fence-circuit-diagram-with-alarm


and this is the schematic:
https://circuitdigest.com/fullimage?i=circuitdiagram/Automatic-Light-Fence-Circuit-Diagram-with-Alarm.png



I have the led, ldr and the 100k pot part of the circuit working
fine,

That's a surprise... there should be a resistor below R1.

I have a very vague memory of an opamp being treated like that and
yet working. Looking at opamp input circuits reveals there are
voltage swing limits and so-called phase inversion that complicate
the picture. At this late hour I can't work out if it could actually
work, but can't dismiss it out of hand.

General opamp theory says it does not work and never could.

General opamp theory doesn't come into it. General opamp theory only
applies when 2 Rs are used on the input to set a known voltage.


If it does work then that is due to parameters way off the datasheet,
therefore not guaranteed to work for any or all devices.

if it works, and the op at least says it does, it'll be due to the
input circuit issues of the opamp, which should be sufficiently
consistent for any one type of opamp.

The circuit "designer" does not have a clue, and seems to be playing
with all combinations of all pins of random devices until something
appears to work - and then publishing it.

I really ought to lash one together & test it, but am way too occupied
with other stuff atm.


NT

 I think we lost our OP.
               Mikek
No, I'm still here reading the posts.

As I am a hobbyist with electronics and use youtube for most of my
answers, I am very interested in why some posters says it doesn't or
shouldn't work. I know that you can get a flashing led with just a 555,
but I'm completely ignorant whether an op amp is also needed or not as
the case may be.
I'd do some reading up on the 741 and watch some videos about it.

I see the point of what some people say about the author not having a
clue etc, as I am very aware that that is also the case with other
topics on youtube.

I only posted my question because the buzzer didn't work originally in
the way it was postioned on the schematic.
I know that it does work now, but I am also looking at or for other
similarish circuits.
I have found something on the hobby-circuits site which I'll put
together when I have all the parts.
 
R

RobH

Guest
On 01/02/2020 17:28, amdx wrote:
On 2/1/2020 10:56 AM, Ralph Mowery wrote:
In article <h9llnmF6140U1@mid.individual.net>, rob@despammer.com says...

No, I'm still here reading the posts.

As I am a hobbyist with electronics and use youtube for most of my
answers, I am very interested in why some posters says it doesn't or
shouldn't work. I know that you can get a flashing led with just a 555,
but I'm completely ignorant whether an op amp is also needed or not as
the case may be.
I'd do some reading up on the 741 and watch some videos about it.

I see the point of what some people say about the author not having a
clue etc, as I am very aware that that is also the case with other
topics on youtube.

I only posted my question because the buzzer didn't work originally in
the way it was postioned on the schematic.
I know that it does work now, but I am also looking at or for other
similarish circuits.
I have found something on the hobby-circuits site which I'll put
together when I have all the parts.



The way it is suppose to work is the 741 is set up as a voltage
comparater.  When the voltage on one of the input pins is larger or
smaller than the other, the 741 output changes from low to high.  The
741 is not a good device to use and a few more resistors should be used
to get it to work consistnatly.  The transistor seems to invert the
signal and drive the led.  Then the 555 gets triggered to send an output
voltage to the buzzer for a period of time.

The 741 is usually an amplifier and there are dedicated comparators that
work much better.


 I have not put any thought into the 741, I concentrated on getting the
555 to work properly first.
 The OP pointed out the the 741 and transistor did not pull pin 2 low
enough to trigger the 555. So I don't doubt some 741 problem.
 I wonder if it's possible to trigger the 555 directly from the
resistor/ldr connection and skip the 741/transistor circuit all together.
 Hey ROBH,
 When you light the ldr, what does the voltage drop to? (at 100k at ldr
connection)
You might need to keep adding 100k resistors in series with you 100k pot
until you get it to drop to just under 3 volts.
 This might not work after you connect it to pin 2, but it is worth a try.
If it doesn't work at least you will know why, even if we have to tell
you. IF you do this give us light and no light voltages and then the
same when connected to pin 2 of the 555.
                              Mikek
Measuring the voltage between the 1ook pot and the ldr was stationary at
just under 8v, even when the ldr was shaded. The voltage at the R2 210
resistor was 6.75v with no shading and 5.98v with shading.
I have also found that the 10uf capacitor between pins 1 and 2 on the
555 does nothing. Although it might change the frequency of the buzzer??
 
J

Jasen Betts

Guest
On 2020-02-01, tabbypurr@gmail.com <tabbypurr@gmail.com> wrote:
On Friday, 31 January 2020 05:32:38 UTC, Jasen Betts wrote:
On 2020-01-29, RobH <rob@despammer.com> wrote:

I have put together a circuit from here:
https://circuitdigest.com/electronic-circuits/light-fence-circuit-diagram-with-alarm

and this is the schematic:
https://circuitdigest.com/fullimage?i=circuitdiagram/Automatic-Light-Fence-Circuit-Diagram-with-Alarm.png


I have the led, ldr and the 100k pot part of the circuit working fine,

That's a surprise... there should be a resistor below R1.

I have a very vague memory of an opamp being treated like that and
yet working. Looking at opamp input circuits reveals there are voltage
swing limits and so-called phase inversion that complicate the
picture. At this late hour I can't work out if it could actually work,
but can't dismiss it out of hand.
it can overload the one side of the differential pair and drive the
next stage directly, invoke the fault condition known as phase
inversion etc... the op-amp may still be amplifying, but it's not
running in any mode that's blessed by the data-sheet.


--
Jasen.
 
J

Jasen Betts

Guest
On 2020-02-01, Ralph Mowery <rmowery28146@earthlink.net> wrote:
In article <h9llnmF6140U1@mid.individual.net>, rob@despammer.com says...

No, I'm still here reading the posts.

As I am a hobbyist with electronics and use youtube for most of my
answers, I am very interested in why some posters says it doesn't or
shouldn't work. I know that you can get a flashing led with just a 555,
but I'm completely ignorant whether an op amp is also needed or not as
the case may be.
I'd do some reading up on the 741 and watch some videos about it.

I see the point of what some people say about the author not having a
clue etc, as I am very aware that that is also the case with other
topics on youtube.

I only posted my question because the buzzer didn't work originally in
the way it was postioned on the schematic.
I know that it does work now, but I am also looking at or for other
similarish circuits.
I have found something on the hobby-circuits site which I'll put
together when I have all the parts.



The way it is suppose to work is the 741 is set up as a voltage
comparater. When the voltage on one of the input pins is larger or
smaller than the other, the 741 output changes from low to high. The
741 is not a good device to use and a few more resistors should be used
to get it to work consistnatly. The transistor seems to invert the
signal and drive the led. Then the 555 gets triggered to send an output
voltage to the buzzer for a period of time.

The 741 is usually an amplifier and there are dedicated comparators that
work much better.
They're faster amd more efficient, but the 741 is plenty fast enough
to watch the output of a light dependant resistor. sure a LM339 (or
LM393) would out-perform it, but in this application
you wouldn't notice, other than the price.

--
Jasen.
 
A

amdx

Guest
On 2/1/2020 1:01 PM, RobH wrote:
On 01/02/2020 17:28, amdx wrote:
On 2/1/2020 10:56 AM, Ralph Mowery wrote:
In article <h9llnmF6140U1@mid.individual.net>, rob@despammer.com says...

No, I'm still here reading the posts.

As I am a hobbyist with electronics and use youtube for most of my
answers, I am very interested in why some posters says it doesn't or
shouldn't work. I know that you can get a flashing led with just a 555,
but I'm completely ignorant whether an op amp is also needed or not as
the case may be.
I'd do some reading up on the 741 and watch some videos about it.

I see the point of what some people say about the author not having a
clue etc, as I am very aware that that is also the case with other
topics on youtube.

I only posted my question because the buzzer didn't work originally in
the way it was postioned on the schematic.
I know that it does work now, but I am also looking at or for other
similarish circuits.
I have found something on the hobby-circuits site which I'll put
together when I have all the parts.



The way it is suppose to work is the 741 is set up as a voltage
comparater.  When the voltage on one of the input pins is larger or
smaller than the other, the 741 output changes from low to high.  The
741 is not a good device to use and a few more resistors should be used
to get it to work consistnatly.  The transistor seems to invert the
signal and drive the led.  Then the 555 gets triggered to send an output
voltage to the buzzer for a period of time.

The 741 is usually an amplifier and there are dedicated comparators that
work much better.


  I have not put any thought into the 741, I concentrated on getting
the 555 to work properly first.
  The OP pointed out the the 741 and transistor did not pull pin 2 low
enough to trigger the 555. So I don't doubt some 741 problem.
  I wonder if it's possible to trigger the 555 directly from the
resistor/ldr connection and skip the 741/transistor circuit all together.
  Hey ROBH,
  When you light the ldr, what does the voltage drop to? (at 100k at
ldr connection)
You might need to keep adding 100k resistors in series with you 100k pot
until you get it to drop to just under 3 volts.
  This might not work after you connect it to pin 2, but it is worth a
try.
If it doesn't work at least you will know why, even if we have to tell
you. IF you do this give us light and no light voltages and then the
same when connected to pin 2 of the 555.
                               Mikek



Measuring the voltage between the 1ook pot and the ldr was stationary at
just under 8v, even when the ldr was shaded.
Something wrong there! Unhook the 742 pin 2 from the connection and put
your meter at the connection. It should change voltage when it is dark.
What meter are you using?
You could put two more 100k in series with the 100k pot. that will
lower the 8v, but also allow a larger swing between light and dark.


The voltage at the R2 210
resistor was 6.75v with no shading and 5.98v with shading.
I have also found that the 10uf capacitor between pins 1 and 2 on the
555 does nothing.
If you will recall, there should not be any connection of the 10uf to
pin 2.
At this point I need a drawing of the 555 circuit you are using. Post it
where ever you can and send me the link. Once I see it, I will post any
corrections needed.
Mikek
 
A

amdx

Guest
On 2/1/2020 6:49 PM, amdx wrote:
On 2/1/2020 1:01 PM, RobH wrote:
On 01/02/2020 17:28, amdx wrote:
On 2/1/2020 10:56 AM, Ralph Mowery wrote:
In article <h9llnmF6140U1@mid.individual.net>, rob@despammer.com
says...

No, I'm still here reading the posts.

As I am a hobbyist with electronics and use youtube for most of my
answers, I am very interested in why some posters says it doesn't or
shouldn't work. I know that you can get a flashing led with just a
555,
but I'm completely ignorant whether an op amp is also needed or not as
the case may be.
I'd do some reading up on the 741 and watch some videos about it.

I see the point of what some people say about the author not having a
clue etc, as I am very aware that that is also the case with other
topics on youtube.

I only posted my question because the buzzer didn't work originally in
the way it was postioned on the schematic.
I know that it does work now, but I am also looking at or for other
similarish circuits.
I have found something on the hobby-circuits site which I'll put
together when I have all the parts.



The way it is suppose to work is the 741 is set up as a voltage
comparater.  When the voltage on one of the input pins is larger or
smaller than the other, the 741 output changes from low to high.  The
741 is not a good device to use and a few more resistors should be used
to get it to work consistnatly.  The transistor seems to invert the
signal and drive the led.  Then the 555 gets triggered to send an
output
voltage to the buzzer for a period of time.

The 741 is usually an amplifier and there are dedicated comparators
that
work much better.


  I have not put any thought into the 741, I concentrated on getting
the 555 to work properly first.
  The OP pointed out the the 741 and transistor did not pull pin 2
low enough to trigger the 555. So I don't doubt some 741 problem.
  I wonder if it's possible to trigger the 555 directly from the
resistor/ldr connection and skip the 741/transistor circuit all
together.
  Hey ROBH,
  When you light the ldr, what does the voltage drop to? (at 100k at
ldr connection)
You might need to keep adding 100k resistors in series with you 100k pot
until you get it to drop to just under 3 volts.
  This might not work after you connect it to pin 2, but it is worth
a try.
If it doesn't work at least you will know why, even if we have to
tell you. IF you do this give us light and no light voltages and then
the same when connected to pin 2 of the 555.
                               Mikek



Measuring the voltage between the 1ook pot and the ldr was stationary
at just under 8v, even when the ldr was shaded.
Something wrong there! Unhook the 742 pin 2 from the connection and put
your meter at the connection. It should change voltage when it is dark.
 What meter are you using?
 You could put two more 100k in series with the 100k pot. that will
lower the 8v, but also allow a larger swing between light and dark.


 The voltage at the R2 210
resistor was 6.75v with no shading and 5.98v with shading.
I have also found that the 10uf capacitor between pins 1 and 2 on the
555 does nothing.

 If you will recall, there should not be any connection of the 10uf to
pin 2.
At this point I need a drawing of the 555 circuit you are using. Post it
where ever you can and send me the link. Once I see it, I will post any
corrections needed.
                     Mikek

ROBH, this is exactly the circuit you want.
https://www.electronics-tutorials.ws/wp-content/uploads/2018/05/waveforms-tim38.gif
C1 is your 10uF, R1 is 100k resistor that will give you a 1 second buzz,
200k will be a 2 second buzz. R2 can be prety much anything but use 10k.
Pin 3 is your buzzer. For the switch if you don't have one, just use a
wire to momentarily ground pin two. If after you set this up it doesn't
work, take a picture and post it somewhere or send it to my email address.
Let me know, I want to get something working.
Mikek
 
R

RobH

Guest
On 02/02/2020 00:49, amdx wrote:
On 2/1/2020 1:01 PM, RobH wrote:
On 01/02/2020 17:28, amdx wrote:
On 2/1/2020 10:56 AM, Ralph Mowery wrote:
In article <h9llnmF6140U1@mid.individual.net>, rob@despammer.com
says...

No, I'm still here reading the posts.

As I am a hobbyist with electronics and use youtube for most of my
answers, I am very interested in why some posters says it doesn't or
shouldn't work. I know that you can get a flashing led with just a
555,
but I'm completely ignorant whether an op amp is also needed or not as
the case may be.
I'd do some reading up on the 741 and watch some videos about it.

I see the point of what some people say about the author not having a
clue etc, as I am very aware that that is also the case with other
topics on youtube.

I only posted my question because the buzzer didn't work originally in
the way it was postioned on the schematic.
I know that it does work now, but I am also looking at or for other
similarish circuits.
I have found something on the hobby-circuits site which I'll put
together when I have all the parts.



The way it is suppose to work is the 741 is set up as a voltage
comparater.  When the voltage on one of the input pins is larger or
smaller than the other, the 741 output changes from low to high.  The
741 is not a good device to use and a few more resistors should be used
to get it to work consistnatly.  The transistor seems to invert the
signal and drive the led.  Then the 555 gets triggered to send an
output
voltage to the buzzer for a period of time.

The 741 is usually an amplifier and there are dedicated comparators
that
work much better.


  I have not put any thought into the 741, I concentrated on getting
the 555 to work properly first.
  The OP pointed out the the 741 and transistor did not pull pin 2
low enough to trigger the 555. So I don't doubt some 741 problem.
  I wonder if it's possible to trigger the 555 directly from the
resistor/ldr connection and skip the 741/transistor circuit all
together.
  Hey ROBH,
  When you light the ldr, what does the voltage drop to? (at 100k at
ldr connection)
You might need to keep adding 100k resistors in series with you 100k pot
until you get it to drop to just under 3 volts.
  This might not work after you connect it to pin 2, but it is worth
a try.
If it doesn't work at least you will know why, even if we have to
tell you. IF you do this give us light and no light voltages and then
the same when connected to pin 2 of the 555.
                               Mikek



Measuring the voltage between the 1ook pot and the ldr was stationary
at just under 8v, even when the ldr was shaded.
Something wrong there! Unhook the 742 pin 2 from the connection and put
your meter at the connection. It should change voltage when it is dark.
 What meter are you using?
 You could put two more 100k in series with the 100k pot. that will
lower the 8v, but also allow a larger swing between light and dark.


 The voltage at the R2 210
resistor was 6.75v with no shading and 5.98v with shading.
I have also found that the 10uf capacitor between pins 1 and 2 on the
555 does nothing.

 If you will recall, there should not be any connection of the 10uf to
pin 2.
At this point I need a drawing of the 555 circuit you are using. Post it
where ever you can and send me the link. Once I see it, I will post any
corrections needed.
                     Mikek
There is a photo I took here:
https://www.dropbox.com/h?preview=IMG_20200202_170044.jpg
 
A

amdx

Guest
On 2/2/2020 11:08 AM, RobH wrote:
On 02/02/2020 00:49, amdx wrote:
On 2/1/2020 1:01 PM, RobH wrote:
On 01/02/2020 17:28, amdx wrote:
On 2/1/2020 10:56 AM, Ralph Mowery wrote:
In article <h9llnmF6140U1@mid.individual.net>, rob@despammer.com
says...

No, I'm still here reading the posts.

As I am a hobbyist with electronics and use youtube for most of my
answers, I am very interested in why some posters says it doesn't or
shouldn't work. I know that you can get a flashing led with just a
555,
but I'm completely ignorant whether an op amp is also needed or
not as
the case may be.
I'd do some reading up on the 741 and watch some videos about it.

I see the point of what some people say about the author not having a
clue etc, as I am very aware that that is also the case with other
topics on youtube.

I only posted my question because the buzzer didn't work
originally in
the way it was postioned on the schematic.
I know that it does work now, but I am also looking at or for other
similarish circuits.
I have found something on the hobby-circuits site which I'll put
together when I have all the parts.



The way it is suppose to work is the 741 is set up as a voltage
comparater.  When the voltage on one of the input pins is larger or
smaller than the other, the 741 output changes from low to high.  The
741 is not a good device to use and a few more resistors should be
used
to get it to work consistnatly.  The transistor seems to invert the
signal and drive the led.  Then the 555 gets triggered to send an
output
voltage to the buzzer for a period of time.

The 741 is usually an amplifier and there are dedicated comparators
that
work much better.


  I have not put any thought into the 741, I concentrated on getting
the 555 to work properly first.
  The OP pointed out the the 741 and transistor did not pull pin 2
low enough to trigger the 555. So I don't doubt some 741 problem.
  I wonder if it's possible to trigger the 555 directly from the
resistor/ldr connection and skip the 741/transistor circuit all
together.
  Hey ROBH,
  When you light the ldr, what does the voltage drop to? (at 100k at
ldr connection)
You might need to keep adding 100k resistors in series with you 100k
pot
until you get it to drop to just under 3 volts.
  This might not work after you connect it to pin 2, but it is worth
a try.
If it doesn't work at least you will know why, even if we have to
tell you. IF you do this give us light and no light voltages and
then the same when connected to pin 2 of the 555.
                               Mikek



Measuring the voltage between the 1ook pot and the ldr was stationary
at just under 8v, even when the ldr was shaded.
Something wrong there! Unhook the 742 pin 2 from the connection and
put your meter at the connection. It should change voltage when it is
dark.
  What meter are you using?
  You could put two more 100k in series with the 100k pot. that will
lower the 8v, but also allow a larger swing between light and dark.


  The voltage at the R2 210
resistor was 6.75v with no shading and 5.98v with shading.
I have also found that the 10uf capacitor between pins 1 and 2 on the
555 does nothing.

  If you will recall, there should not be any connection of the 10uf
to pin 2.
At this point I need a drawing of the 555 circuit you are using. Post
it where ever you can and send me the link. Once I see it, I will post
any corrections needed.
                      Mikek


There is a photo I took here:
https://www.dropbox.com/h?preview=IMG_20200202_170044.jpg
I don't know why, but all I can pull up from the link are my own files.
Anyone know how I can view his picture?
Mikek
 
R

RobH

Guest
On 02/02/2020 17:18, amdx wrote:
On 2/2/2020 11:08 AM, RobH wrote:
On 02/02/2020 00:49, amdx wrote:
On 2/1/2020 1:01 PM, RobH wrote:
On 01/02/2020 17:28, amdx wrote:
On 2/1/2020 10:56 AM, Ralph Mowery wrote:
In article <h9llnmF6140U1@mid.individual.net>, rob@despammer.com
says...

No, I'm still here reading the posts.

As I am a hobbyist with electronics and use youtube for most of my
answers, I am very interested in why some posters says it doesn't or
shouldn't work. I know that you can get a flashing led with just
a 555,
but I'm completely ignorant whether an op amp is also needed or
not as
the case may be.
I'd do some reading up on the 741 and watch some videos about it.

I see the point of what some people say about the author not
having a
clue etc, as I am very aware that that is also the case with other
topics on youtube.

I only posted my question because the buzzer didn't work
originally in
the way it was postioned on the schematic.
I know that it does work now, but I am also looking at or for other
similarish circuits.
I have found something on the hobby-circuits site which I'll put
together when I have all the parts.



The way it is suppose to work is the 741 is set up as a voltage
comparater.  When the voltage on one of the input pins is larger or
smaller than the other, the 741 output changes from low to high.  The
741 is not a good device to use and a few more resistors should be
used
to get it to work consistnatly.  The transistor seems to invert the
signal and drive the led.  Then the 555 gets triggered to send an
output
voltage to the buzzer for a period of time.

The 741 is usually an amplifier and there are dedicated
comparators that
work much better.


  I have not put any thought into the 741, I concentrated on
getting the 555 to work properly first.
  The OP pointed out the the 741 and transistor did not pull pin 2
low enough to trigger the 555. So I don't doubt some 741 problem.
  I wonder if it's possible to trigger the 555 directly from the
resistor/ldr connection and skip the 741/transistor circuit all
together.
  Hey ROBH,
  When you light the ldr, what does the voltage drop to? (at 100k
at ldr connection)
You might need to keep adding 100k resistors in series with you
100k pot
until you get it to drop to just under 3 volts.
  This might not work after you connect it to pin 2, but it is
worth a try.
If it doesn't work at least you will know why, even if we have to
tell you. IF you do this give us light and no light voltages and
then the same when connected to pin 2 of the 555.
                               Mikek



Measuring the voltage between the 1ook pot and the ldr was
stationary at just under 8v, even when the ldr was shaded.
Something wrong there! Unhook the 742 pin 2 from the connection and
put your meter at the connection. It should change voltage when it is
dark.
  What meter are you using?
  You could put two more 100k in series with the 100k pot. that will
lower the 8v, but also allow a larger swing between light and dark.


  The voltage at the R2 210
resistor was 6.75v with no shading and 5.98v with shading.
I have also found that the 10uf capacitor between pins 1 and 2 on
the 555 does nothing.

  If you will recall, there should not be any connection of the 10uf
to pin 2.
At this point I need a drawing of the 555 circuit you are using. Post
it where ever you can and send me the link. Once I see it, I will
post any corrections needed.
                      Mikek


There is a photo I took here:
https://www.dropbox.com/h?preview=IMG_20200202_170044.jpg

 I don't know why, but all I can pull up from the link are my own files.
Anyone know how I can view his picture?
                                Mikek
I know why, you have to log in, but I have shared it now with the email
address I see here.
I don't know how to make it public yet.
 
R

RobH

Guest
On 02/02/2020 01:05, amdx wrote:
On 2/1/2020 6:49 PM, amdx wrote:
On 2/1/2020 1:01 PM, RobH wrote:
On 01/02/2020 17:28, amdx wrote:
On 2/1/2020 10:56 AM, Ralph Mowery wrote:
In article <h9llnmF6140U1@mid.individual.net>, rob@despammer.com
says...

No, I'm still here reading the posts.

As I am a hobbyist with electronics and use youtube for most of my
answers, I am very interested in why some posters says it doesn't or
shouldn't work. I know that you can get a flashing led with just a
555,
but I'm completely ignorant whether an op amp is also needed or
not as
the case may be.
I'd do some reading up on the 741 and watch some videos about it.

I see the point of what some people say about the author not having a
clue etc, as I am very aware that that is also the case with other
topics on youtube.

I only posted my question because the buzzer didn't work
originally in
the way it was postioned on the schematic.
I know that it does work now, but I am also looking at or for other
similarish circuits.
I have found something on the hobby-circuits site which I'll put
together when I have all the parts.



The way it is suppose to work is the 741 is set up as a voltage
comparater.  When the voltage on one of the input pins is larger or
smaller than the other, the 741 output changes from low to high.  The
741 is not a good device to use and a few more resistors should be
used
to get it to work consistnatly.  The transistor seems to invert the
signal and drive the led.  Then the 555 gets triggered to send an
output
voltage to the buzzer for a period of time.

The 741 is usually an amplifier and there are dedicated comparators
that
work much better.


  I have not put any thought into the 741, I concentrated on getting
the 555 to work properly first.
  The OP pointed out the the 741 and transistor did not pull pin 2
low enough to trigger the 555. So I don't doubt some 741 problem.
  I wonder if it's possible to trigger the 555 directly from the
resistor/ldr connection and skip the 741/transistor circuit all
together.
  Hey ROBH,
  When you light the ldr, what does the voltage drop to? (at 100k at
ldr connection)
You might need to keep adding 100k resistors in series with you 100k
pot
until you get it to drop to just under 3 volts.
  This might not work after you connect it to pin 2, but it is worth
a try.
If it doesn't work at least you will know why, even if we have to
tell you. IF you do this give us light and no light voltages and
then the same when connected to pin 2 of the 555.
                               Mikek



Measuring the voltage between the 1ook pot and the ldr was stationary
at just under 8v, even when the ldr was shaded.
Something wrong there! Unhook the 742 pin 2 from the connection and
put your meter at the connection. It should change voltage when it is
dark.
  What meter are you using?
  You could put two more 100k in series with the 100k pot. that will
lower the 8v, but also allow a larger swing between light and dark.


  The voltage at the R2 210
resistor was 6.75v with no shading and 5.98v with shading.
I have also found that the 10uf capacitor between pins 1 and 2 on the
555 does nothing.

  If you will recall, there should not be any connection of the 10uf
to pin 2.
At this point I need a drawing of the 555 circuit you are using. Post
it where ever you can and send me the link. Once I see it, I will post
any corrections needed.
                      Mikek

ROBH, this is exactly the circuit you want.
https://www.electronics-tutorials.ws/wp-content/uploads/2018/05/waveforms-tim38.gif

C1 is your 10uF, R1 is 100k resistor that will give you a 1 second buzz,
200k will be a 2 second buzz. R2 can be prety much anything but use 10k.
Pin 3 is your buzzer. For the switch if you don't have one, just use a
wire to momentarily ground pin two. If after you set this up it doesn't
work, take a picture and post it somewhere or send it to my email address.
 Let me know, I want to get something working.
                                        Mikek
Ok, I put this circuit together, and what happens is that when I either
hold or put the 10k resistor from pin 2 into to the gnd rail, the buzzer
beeps continually. When I either let go or disconnect from GND it stops
beeping..
Where does the LDR go, and or the led.

Thanks
 

Guest
On Sunday, 2 February 2020 18:36:14 UTC, RobH wrote:
On 02/02/2020 17:18, amdx wrote:
On 2/2/2020 11:08 AM, RobH wrote:

There is a photo I took here:
https://www.dropbox.com/h?preview=IMG_20200202_170044.jpg

 I don't know why, but all I can pull up from the link are my own files.
Anyone know how I can view his picture?
                                Mikek

I know why, you have to log in, but I have shared it now with the email
address I see here.
I don't know how to make it public yet.
post it on a sensible image hosting site
Would be interesting to many people see a video of this thing actually working, including clear view of the input side of the 741.


NT
 

Guest
On Saturday, 1 February 2020 21:02:42 UTC, Jasen Betts wrote:
On 2020-02-01, tabbypurr wrote:
On Friday, 31 January 2020 05:32:38 UTC, Jasen Betts wrote:
On 2020-01-29, RobH <rob@despammer.com> wrote:

I have put together a circuit from here:
https://circuitdigest.com/electronic-circuits/light-fence-circuit-diagram-with-alarm

and this is the schematic:
https://circuitdigest.com/fullimage?i=circuitdiagram/Automatic-Light-Fence-Circuit-Diagram-with-Alarm.png


I have the led, ldr and the 100k pot part of the circuit working fine,

That's a surprise... there should be a resistor below R1.

I have a very vague memory of an opamp being treated like that and
yet working. Looking at opamp input circuits reveals there are voltage
swing limits and so-called phase inversion that complicate the
picture. At this late hour I can't work out if it could actually work,
but can't dismiss it out of hand.

it can overload the one side of the differential pair and drive the
next stage directly, invoke the fault condition known as phase
inversion etc... the op-amp may still be amplifying, but it's not
running in any mode that's blessed by the data-sheet.
It reminds me a lot of suicide bias. It's deprecated but still gets used in cost cutting commercial products.


NT
 
D

default

Guest
On Mon, 3 Feb 2020 21:05:07 -0800 (PST), tabbypurr@gmail.com wrote:

it can overload the one side of the differential pair and drive the
next stage directly, invoke the fault condition known as phase
inversion etc... the op-amp may still be amplifying, but it's not
running in any mode that's blessed by the data-sheet.

It reminds me a lot of suicide bias. It's deprecated but still gets used in cost cutting commercial products.
What is suicide bias? and what makes it desirable from a
bean-counter's point of view?
 
A

amdx

Guest
On 2/3/2020 11:09 PM, tabbypurr@gmail.com wrote:
On Sunday, 2 February 2020 18:36:14 UTC, RobH wrote:
On 02/02/2020 17:18, amdx wrote:
On 2/2/2020 11:08 AM, RobH wrote:

There is a photo I took here:
https://www.dropbox.com/h?preview=IMG_20200202_170044.jpg

 I don't know why, but all I can pull up from the link are my own files.
Anyone know how I can view his picture?
                                Mikek

I know why, you have to log in, but I have shared it now with the email
address I see here.
I don't know how to make it public yet.

post it on a sensible image hosting site
Would be interesting to many people see a video of this thing actually working, including clear view of the input side of the 741.


NT
I'm working with him offline.
He now has a working one shot 555, so that is solved.


For a start I'm doing just ldr-resistor with the center connection to
Pin 2. I realize there is a level transition problem with a single neg
pulse to Pin 2. However we are working towards that.


We are just now adjusting the resistor-ldr to drop below 1/3Vc to
trigger the 555. As built is only dropped to 4.3 in the dark, so he is
now adjusting the resistor value.

Next we will need to differentiate the negative going signal from the
ldr. I do see a problem with a slowly decreasing light situation and we
may end up with some fast switch between the ldr and the 555 circuit.
I think he has limited parts, so we may have to do something with the
741 or two PNPs. I think I saw he only has PNP transistors.
If you have an Idea to make cause a fast switch from a slow transition,
I'm open to ideas. He has the 741, so can you make that work, or two NPNs.
If this is daylight to sunset sensor, we will need a fast transition.

Mikek
 
A

Andy Bennet

Guest
On 04/02/2020 16:54, amdx wrote:
On 2/3/2020 11:09 PM, tabbypurr@gmail.com wrote:
On Sunday, 2 February 2020 18:36:14 UTC, RobH  wrote:
On 02/02/2020 17:18, amdx wrote:
On 2/2/2020 11:08 AM, RobH wrote:

There is a photo I took here:
https://www.dropbox.com/h?preview=IMG_20200202_170044.jpg

   I don't know why, but all I can pull up from the link are my own
files.
Anyone know how I can view his picture?
                                  Mikek

I know why, you have to log in, but I have shared it now with the email
address I see here.
I don't know how to make it public yet.

post it on a sensible image hosting site
Would be interesting to many people see a video of this thing actually
working, including clear view of the input side of the 741.


NT

 I'm working with him offline.
He now has a working one shot 555, so that is solved.


For a start I'm doing just ldr-resistor with the center connection to
Pin 2. I realize there is a level transition problem with a single neg
pulse to Pin 2. However we are working towards that.


 We are just now adjusting the resistor-ldr to drop below 1/3Vc to
trigger the 555.  As built is only dropped to 4.3 in the dark, so he is
now adjusting the resistor value.

 Next we will need to differentiate the negative going signal from the
ldr. I do see a problem with a slowly decreasing light situation and we
may end up with some fast switch between the ldr and the 555 circuit.
 I think he has limited parts, so we may have to do something with the
741 or two PNPs. I think I saw he only has PNP transistors.
 If you have an Idea to make cause a fast switch from a slow transition,
I'm open to ideas. He has the 741, so can you make that work, or two NPNs.
 If this is daylight to sunset sensor, we will need a fast transition.
                                        Mikek


Turn the 741 into a schmitt trigger, rather than trying to explain see:-
http://www.circuitstoday.com/schmitt-trigger-using-op-amp
 
Toggle Sidebar

Welcome to EDABoard.com

Sponsor

Top