This is a circuit suggested by Linear Technology in a 1980s documentation. The integrated circuit is from the era. Observe the input and output voltage.
This circuit was found in a Hands On Electronics from the 90s. The microphone is a small 75-ohm...
This circuit triggers an oscillator when any of the sensors is opened. The circuit can be powered by...
This simple earphone receiver was sold in the 1960s in Japan. The circuit uses a single germanium...