Found in an 80's Elektor, this circuit can use any equivalent optical coupler. The buzzer is of the Sonalert type with included oscillator.
We found this circuit in a 1967 Electronic design. However, it can be easily implemented even...
STMicroelectronics MP23AB02B MEMS Audio Sensor is a compact, low-power microphone built with a...
This circuit was found in the February 1993 Radio Electronics magazine. The frequency depends on...