This circuit triggers a relay when a light beam that hits the phototransistor is cut off. The circuit is from Electronics Now and the supply voltage depends on the relay voltage.
This circuit for cars with fans was found in Silicon Chips documentation from the 90s. It is used...
The frequency of this circuit, which uses one of the LM139 comparators, depends on C1. The circuit...
This interesting high-quality amplifier configuration was featured on the cover of the June 1994...