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.
This circuit appears elsewhere in this section, but taken from other sources. This is an English...
Found in a 1975 radio amateur publication, this simple oscillator is for code practice. The PNP...
This circuit for some amps was found in a 90s Radio Electronics. The voltage can be between 6 and 25 V...