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.
This circuit was found in old documentation, however the components used are common today. The...
This circuit is from a 1964 Electronic Design. The sensor is an LDR and the transistors are...
This circuit from Delta Products was published in the May 1974 Radio Electronics magazine. The...