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 can be used as a shield for microcontrollers. The circuit is from American...
This car battery power supply was found in the June 1990 issue of Radio Electronics magazine....
August 2017 - STMicroelectronics' BlueCoin development environment is available at Mouser...