Found in a 90s Popular Electronics, this circuit checks if a remote-control transmitter is emitting signals. The sensor is a general-purpose type found in many electronic devices.
The frequency of this type is oscillator is given by the RC network. Calculations can be found in...
Found in old documentation about neon lamps, this circuit produces very low frequency signals that...
This circuit was obtained from an American publication of 1982, but can be assembled with ease...