Serial switch with PIN diode (CB21257E)

This circuit was found in a Popular Electronics from the 90's. The circuit is connected in series.

Liquid level sensor (CB23807E)
This sensor was published in Radio Electronics magazine in April 1962. The two-transistor circuit...
DC Motor Control 555 (CB24908E)
Another of the many similar configurations we have in this section. This one is from a Radio...
Solenoid Current Monitor 2 (CB5400E)
This circuit like the previous one is intended for 24V x 3W solenoids being connected from the...