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Basic circuit help please

A

amdx

Guest
On 2/4/2020 2:13 PM, Andy Bennet wrote:
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
Thank you, I did find a video showing that, but this page is much better.
Mikek
 
A

amdx

Guest
On 2/4/2020 6:01 PM, amdx wrote:
On 2/4/2020 2:13 PM, Andy Bennet wrote:
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


  Thank you, I did find a video showing that, but this page is much
better.
                Mikek
I may need a little more input, I have a voltage the sets at 5V, then
drops to 2V. A negative going pulse. That is the 555 trigger on Pin 2.
I need the output to go low when the input goes low.

Mikek
 

Guest
On Tuesday, 4 February 2020 14:00:00 UTC, default wrote:
On Mon, 3 Feb 2020 21:05:07 -0800 (PST), tabbypurr 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?
I see a search was not helpful :) Biassing a tr by a single R from V+ to base, and nothing else. No thermal or gain stabilisation.

and what makes it desirable from a
bean-counter's point of view?
just one resistor.
Quiescent collector voltage can cover a fair range, so only good for low V_out & low P_diss so you don't get too much thermal drift. But it works.


NT
 
A

Andy Bennet

Guest
On 05/02/2020 00:14, amdx wrote:
On 2/4/2020 6:01 PM, amdx wrote:
On 2/4/2020 2:13 PM, Andy Bennet wrote:
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


   Thank you, I did find a video showing that, but this page is much
better.
                 Mikek

I may need a little more input, I have a voltage the sets at 5V, then
drops to 2V. A negative going pulse.  That is the 555 trigger on Pin 2.
 I need the output to go low when the input goes low.
Swop over the LDR and pot?
 
D

default

Guest
On Tue, 4 Feb 2020 23:34:19 -0800 (PST), tabbypurr@gmail.com wrote:

On Tuesday, 4 February 2020 14:00:00 UTC, default wrote:
On Mon, 3 Feb 2020 21:05:07 -0800 (PST), tabbypurr 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?

I see a search was not helpful :) Biassing a tr by a single R from V+ to base, and nothing else. No thermal or gain stabilisation.

and what makes it desirable from a
bean-counter's point of view?

just one resistor.
Quiescent collector voltage can cover a fair range, so only good for low V_out & low P_diss so you don't get too much thermal drift. But it works.


NT
Thanks. I've used it a time or two. A long time ago with germanium
transistors.
 
J

Jasen Betts

Guest
On 2020-02-04, default <default@defaulter.net> wrote:
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?
You connect a current source (a resistor) to the transistors base
the collector current then depends on whatever the transitor feels
like using for Hfe today. (warmer transitor = more current)

eg:

Dynamic microphone adaptor for devices intended for electret
microphones.

| inside the device
|

+---[1M]----+------- TIP --+---[1K]----- +5V
| / |
|| | |/ | | ||
+------||---+--------| BC547C | +-----||--- to
dynamic | || |e | || preamp
mic |O \ |
| |
+-----------------------+------- SLEEVE -------- 0V

|
|

Called suicide bias possilby because if used in power applications
you get thermal run-away very easily,

--
Jasen.
 

Guest
On Thursday, 6 February 2020 02:32:34 UTC, Jasen Betts wrote:
On 2020-02-04, default <default@defaulter.net> wrote:
On Mon, 3 Feb 2020 21:05:07 -0800 (PST), tabbypurr 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?


You connect a current source (a resistor) to the transistors base
the collector current then depends on whatever the transitor feels
like using for Hfe today. (warmer transitor = more current)

eg:

Dynamic microphone adaptor for devices intended for electret
microphones.

| inside the device
|

+---[1M]----+------- TIP --+---[1K]----- +5V
| / |
|| | |/ | | ||
+------||---+--------| BC547C | +-----||--- to
dynamic | || |e | || preamp
mic |O \ |
| |
+-----------------------+------- SLEEVE -------- 0V

|
|

Called suicide bias possilby because if used in power applications
you get thermal run-away very easily,
Ya, but that cct diag is not suicide bias.


NT
 
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