This powerful sound emitter used in defense to intimidate people was found in a 90s Popular Electronics. The circuit produces a strong modulated tone of 3 kHz. The modulation is 10 Hz.
With the configuration shown in the figure, we have the relay being activated when the microphone picks...
This TTL generator that can function as a control shield or external clock for microcontrollers was found in...
The OP07 operational amplifier is not common today, as the circuit is from an old European manual....