I'm trying to reverse engineer a PCB and made a schematic.

The bad thing is I only have PCB images from both sides, not the actual PCB. This PCB is used to detect a very small black object spinning at the speed of 23000 RPM.

I need to know the following things about the schematic:

  1. Can any one help me to learn the working of this circuit? I know bottom one is inverting Operational amplifier but I don't know about the upper one.
  2. The opamp IC is an LM324A and they are running it on 1.65V. Is it possible to run it at 1.65V? The suggested voltage for the IC is more than 3V.

enter image description here

Here is my actual PCB on which I'm workingenter image description here

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    \$\begingroup\$ Yes i need help. Actually i don't know much about this circuit. I know bottom one is inverting Operational amplifier but I don't know about the upper one I think it's amplifier with peak detector circuit. Actually I'm looking someone to explain the upper one. \$\endgroup\$ – mastermind Oct 31 '20 at 20:36
  • \$\begingroup\$ mastermind - I was offering to help you with the edit to fix the incorrect op-amp part number you showed originally (no need to reply to that - I've done it now). My experience is that trying to reverse-engineer PCB images (without the physical PCB) is rarely an efficient use of time and is often unsuccessful, unless the PCB is very simple. (I especially can't help in this case, since from a few quick checks, I disagree with the partial schematic you have drawn already e.g. C16/R39 which you show, and I would have to start again from the beginning.) However I wish you sincere good luck! \$\endgroup\$ – SamGibson Oct 31 '20 at 20:53
  • \$\begingroup\$ Operation of LM324 on 1.65V SHOULD be impossible. Common mode voltage is 2V max and maybe 1.5V typical. U4C pin 10 is in the illegal range at 1.65V and in all cases headroom is about zero, if that. At 3V operation is possible with care. Where is the 1.65V from? \$\endgroup\$ – Russell McMahon Nov 1 '20 at 11:41
  • \$\begingroup\$ Are you SURE about the 1.65V? \$\endgroup\$ – Russell McMahon Nov 1 '20 at 11:42
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    \$\begingroup\$ It would probably be easier to find a circuit that did the same job. U4C maintains a fixed DC level and produces pulses at output. Negative edges are coupled to comparator U4d to produce positive output pulses. There will be many cc=ts available that do this. \$\endgroup\$ – Russell McMahon Nov 2 '20 at 4:46


I would go through the most common configurations.

  • Voltage follower
  • In/non-inverting
  • In/non-summing
  • Differential
  • Integrator
  • Differentiator

It could be a peak detector too.


From the datasheet:

enter image description here

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    \$\begingroup\$ That is not the correct datasheet and not the correct section of the datasheet you do link to. The correct section is the "Recommended Operating Conditions". The minimum operating voltage is 3V. And it is not going to be very good at that because of the common-mode limitations. \$\endgroup\$ – Kevin White May 12 at 17:15
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    \$\begingroup\$ Tip: the standard on SE sites is to use quotation markdown for quotes rather than bold as you have done. > or >1. for numbered list items. \$\endgroup\$ – Transistor May 12 at 17:16
  • \$\begingroup\$ I don't want to quote sources and quote the OP. I tried that with the last one, and it was just confusing. \$\endgroup\$ – adamaero May 12 at 17:24
  • \$\begingroup\$ Quoting sources is requested here whenever possible. It can be very unobtrusive. eg you can see images of me here . \$\endgroup\$ – Russell McMahon May 13 at 0:13
  • \$\begingroup\$ ?????????????????? \$\endgroup\$ – adamaero May 13 at 8:16

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