This circuit makes use of a low impedance microphone. The relay is according to the supply voltage which can be between 9 and 15 V. The operational gain is approximately 100.
We found this circuit in the Radio Electronics magazine of January 1986. The frequency is given by the...
This circuit has a type of bias called "virtual earth" providing good gain, given by R2. The...
The figure shows how to convert a rectangular signal into a triangular signal using an integrator. The...