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Can I have help understanding this schematic? Answered

I am in the process of building an infrared motion detector. I've finished the infrared transmitter but I haven't been able to start on the receiver yet because the schematic has confused me. One part of the circuit, on the lower-right of the corner, appears not to be connected with the rest of the circuit at all. Until I understand how the two parts interact with each other, I can't continue building.
I am still something of an amateur when it comes to solid-state circuitry; maybe someone who's more experienced will know what this means?


Yes. All the wires in your diagram labeled VREF should be connected together.

Similarly,  all the wires labeled V+ should be connected together, and all the wires labeled GND should be connected together.

Similarly,  the two wires labeled COMP should be connected to each other.

Ironically, this circuit was drawn and labeled in this manner in order make it easy to read.  And I think if you read enough of these diagrams it will become easy for you too.

Also, all those little numbers on the wires in the set {1, 2, 3,... 14}, those refer to the 14 pins on the LM324 quad op-amp.  And it just so happens for that op-amp package, the pins on the corners {1,7,8,14} correspond to the output pin of each of these 4 op-amps.

In terms of where things are coming from and where they are going to:  This signal called VREF is a constant voltage equal to about 40% of the supply voltage.  It starts as the voltage at pin 12, which is the output of a voltage divider.  Then it gets fed to a follower (an op amp with its output tied to its inverting input)  From there it is labeled VREF, it gets fed to the non-inverting inputs on two other op-amps, and they do something with it.

Excellent, thank you. One more question though - there's a pin on IC1A that's marked with an = sign. Which pin on the IC does this denote?

It is possible that this is actually an 11 on it's side....I have never seen an = used to designate a pin....

You got it absolutely right, pin 11 is the ground pin of the LM324.   A

I concur.  Pin 11 is ground of the LM324, and Pin 4 is V+. The equals sign is an "11" turned sideways, and that diagram has a "4" turned sideways too.

BTW, I usually use alldatasheet.com for looking these things up.
They're not the only datasheet-lookup site out there, but the name is easy to remember.  If there's a better one let me know.

I pulled mine as a PDF from National Semi... a long time ago...  A

I was a bit rushed and just guessed *blush* LOL

Goodhart your guesses are better then 97% of the electronic padawans
utterances on ibles.


Thanks....I have been "fooling around with it" for awhile now (but have no desire to work with tubes ever again, unless it is just to fix someone's radio or something :-)


Sorry, sometimes my replies seem totally random too *sigh*

I read you fine.
Used to be able to bias the grids of a pentode or
make a thyratron glow red myself.


7 years ago

So, IC1A is the first AC gain stage,  IC1B is a full rectifier and gain stage,
IC1D is the buffered reference driver and IC1C is used as the reference
level comparator that feeds the T2 NPN transistor base emitter which
lights and buzzes when a pulsating IR signal is detected long enough
to charge C3 by the T1 photo transistor.


It's a way to fit all the information on easily. The two VREFs should be connected (It means Voltage Reference). By the way, there are probably easier ways of building an IR motion detector. That circuit looks over-complicated.

Yes, it means that the VREF's should be connected together. The bottom part of the schematic is generating a reference voltage for use above via the opamp configured in the voltage follower configuration. Also take note if the "comp" signal that is broken up in a similar fashion. It is fed into the bottom schematic above the VREF generator. It looks like that is the final output stage to the receiver with some sort of LED visual feedback of the output state.