0

enter image description here This is a simple sonar device using piezoelectric mic and speaker. I understand, the C2-VD1-VD2-C3 is detector circuit that is intended to enable the sound generator based on CD4001 that produces sound tone once an obstacle is detected.

I don't understand the purpose of the VD2 though. It looks like the idea was to implement a voltage doubler, however, for that to work we need to feed AC to it, and AC is not present there, so, as far as I understand, there is no chance for VD2 to become forward biased. In my opinion, a single VD1 and C3 would suffice. (w/o C2 too).

Why is VD2 there?

Jimmy Falcon
  • 372
  • 1
  • 10
  • 1
    Why do you say there is no AC present? I maintain there will be AC present across F1. – Kevin White Jan 30 '23 at 02:34
  • Because AC must go below 0 volts (below GND rail). If it would, then VD2 would make sense and the circuit would work as a voltage doubler. But it doesn't. – Jimmy Falcon Jan 30 '23 at 09:11
  • At the right end of C2 it WILL go below zero during the low going part of the cycle from the collector of the transistor. – Kevin White Jan 30 '23 at 23:45

1 Answers1

0

When VT2 turns on you get a change in the voltage on its collector towards ground.This will drive the voltage on the other side of C2 negative. VD2 clamps this voltage to (or close to) 0V.

RoyC
  • 8,418
  • 6
  • 25
  • 40