The frequency of this circuit can be varied from zero. The circuit uses the CMOS 4046 and was found in the November 1984 issue of Radio Electronics magazine. The base frequency is given by C1.
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....