Using a dual 555 CMOS this very useful circuit on the bench was found in the July 1998 Electronics Now magazine. The circuit is battery powered.
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....