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.
The circuits shown below consist of diode protections against accidental polarity reversal in the supply...
This FM transmitter was described in the Radio Electronics magazine of June 1963. It operates in...
The current can be kept constant in the load circuit using the integrated regulator LM350. The...