This circuit was found in the Electronics Experimenter Handbook from 1991. The sensor is a diode and the circuit has a relay according to the voltage used in the supply.
Found in a 1980s documentation, this amplifier uses an integrated circuit of the era. The RLC...
This circuit produces a voltage drop in the first pulse, turning off the relay. On the second...
Operational amplifiers or comparators like those in the LM139 series can be used on this circuit,...