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.
The purpose of this circuit found in a 1967 GE SCR manual is to control a heating element. The SCR...
This circuit was published in a 1981 magazine, but it can be easily assembled using a BC548 paraQ1...
This benchtop power supply with various output voltages from 1.5 to 12 V was found in Radio...