We found this circuit in a 1996 publication. Equivalent components can be tried, especially in the case of the photodiode. The circuit is powered by a 12 V source.
This very simple circuit uses a phototransistor and a very sensitive relay (reed type) being used...
We found this circuit in Radio Electronics magazine from January 1958. It uses a neon lamp as the...
The integrated circuits in this application are not very common today. One consists of an A / D...