This circuit was found in a Popular Electronics from the 90's. Its purpose is to detect the ionization of the air. The meter can be replaced by the current scale of an ordinary multimeter.
This circuit is from the documentation of Radio Shack 1975. It can be mounted with 12 V relays,...
Using an optocoupler this circuit turns on the light in a bedroom for a set amount of time when...
This circuit is used for remote current monitoring. In the original version of the 80s a lamp is...