Motorola suggests this circuit in an 80's documentation. The circuit uses an inverter to produce a higher voltage for 9 V battery charge from a single cell.
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...