These optical relays, which can act as a shield, were found in an American documentation from the 1980s. The circuit has a LDR sensor. It can be adapted for other types of sensors.
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...