This proximity relay circuit was found in the July 1990 Radio Electronics magazine. The supply voltage should be according to the relay used.
In the figure we have the circuit of a clamp-type microammeter described in the Radio Electronics...
This circuit that also serves to control LEDs and other loads is from a 1960 publication. The...
This transistor can use any general-purpose germanium transistor like CK722 or 2N107. The circuit is...