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, from a 1999 publication, uses an improvised optical coupler with LED or...
This reducer was found in documentation from the 1980s, using transistors of the time. The higher...
This circuit was obtained in a 1991 Electronics Handbook. The publication no longer exists, but...