The circuit is from a June 1995 Electronics Now. The relay is power-dependent and both R1 and R2 should have close resistances in operation.
In this 2002 circuit, we have the activation when the reservoir is empty and shutdown when full....
This circuit was found in a 2001 documentation. It is a test of zener diodes that uses a voltmeter or...
The output current is of the order of 100 mA maximum, but the transistor used can be changed for...