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 1969 EDN, this circuit uses components from that time. Modern equivalent transistors can be...
This amplifier with old germanium transistors is from a 1957 documentation. The power is a few...
We found this circuit in an American documentation from the 90s. The circuit combines functions of 4...