This circuit uses two relays that switch when the voltage exceeds the value given by zener D2. The circuit is from a Radio Electronics magazine from April 1975.
To operate in a harmonic of the crystal frequency, tuned to a trimmer in the resonant circuit of...
This amplifier for recording heads was found in a Radio Electronics from September 1963. The...
This circuit is from the 1983 Intersil Databook. The circuit operates on 5 V power and has a...