Zener reference

A part of the RS3900 square operator is used as a comparator connection, using Zener D1 as the reference voltage. When the voltage applied by R1 to pin 13 exceeds the breakdown voltage of Zener D1, the comparison amplifier differential voltage produces a sufficiently high output voltage to turn on the LED. It can be used for teaching demonstrations of comparator actions. The Zener breakdown voltage should be less than 9V.