Electrical Schematic

As VD2 is reverse-biased, the circuit only produces heat.

Are you sure ?

it works with dc voltage

It looks a little odd, what is it supposed to do (what are B1 and B2)?

What is VD2 doing, it has the symbol of an LED but is connected around the wrong way for an LED and doesn't look like it is an LED being used as a light sensor?

My initial guess would be a capacitive level sensor controlling an LED, but the circuit doesn't look like it will work as it is shown.

Can you provide more information please?

It's a circuit to test the gullibility of CR4 members before the homework questions are submitted.

It is a "bullshit" detector.

1. R7 & R8 just load the DC power supply. Perhaps they should be drawn across B2 or B1 for testing purposes.
2. B1, B2 are chemical detectors. Probably their resistance depends on the quantity "sniffed"; but even more on temperature, so the B1 detector (sealed) compensates for temperature effect on B2 which is the "sniffer".
3. B1, B2 have very high resistances (mostly hot air), so the junction FET VT3 "source follower" is necessary for low input current, even a "guard ring" around the gate to deflect leakage currents. Unfortunately, its gate junction diode is forward biased, so it will not work as intended......
4. R4 adjusts the B1/B2 potentiometer level at which the VT1/VT2 level detector fires and stays on.
5. the VT1/2 "fire" level depends mostly on the voltage of zener diode VD1 - or it would if VD1 had any bias current to set its voltage [no current through VT1 till it fires]. There could be a problem with VT1 emitter-base reverse voltage rating.
6. As already noticed, the light emitting diode VD2 has reversed polarity so VT1/2 cannot conduct anyhow - could be a secret Russian avalanche device??

The whole might have been a lot of smoke (detector).....

67model

Thank You very much!!!!!!!!!!!!!

Well I guess after the previous comments by others and your lack of response to my questions I don't need to hold my tongue any longer.

Have you now got enough information to answer your pictorial homework question of "spot the circuit faults"?

Dear Dmitry2689,

It is a common habit among civilized people to say "thanks" when the answer was what you expected.

Unfortunately you are not an exception.