This circuit that we found in a 1990 documentation uses an LED as a sensor. The operational amplifier is equivalent to the 741 and the trigger adjustment is made in the potentiometer.
This circuit is from an American 1967 documentation and can be implemented with modern equivalent...
This circuit was obtained in a 1998 publication, but it can be easily assembled. The circuit...
This passive circuit was found in the March 1995 issue of Elexctonics Now magazine. The circuit is for...