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.
The capacitors of this 80's circuit must be ceramic and the transistor can be any PNP that...
The transistors of this timing circuit may be more modern types such as BC548 and BC558. The...
The circuit shown in the figure is indicated for alternative sources of energy, such as those...