This very simple circuit uses a phototransistor and a very sensitive relay (reed type) being used over short distances with a lens system. The circuit is from a 1967 Japanese publication.
With the original transistor, this circuit can supply a current of up to 30 A. However, we can use...
When the sensor is wetted by a spray caused by someone falling into a pool or even by a wave...
This circuit was found in a Popular Electronics from the 90's. It triggers an audible warning at...