This circuit was found in an 80s Hands On Electronics. It senses the water level in a reservoir by driving a pump when needed.
With the circuit shown it is possible to measure very low resistances, of the order of fraction of ohm,...
This circuit triggers a Sonalarme when the LED used as a sensor receives light. The circuit was...
With a touch of switch S1 the motor starts and remains so until a touch of switch S2 turns it off....