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.
Found in a General Electric documentation from the 1980s, this circuit makes use of a FET...
This circuit is based on an MC1408, an integrated circuit that is not common today. The...
Another way to increase the sensitivity of the circuit with the amplification of the sensor...