The figure shows the schematic for a versatile logic probe. The zener diode clamps the input signal just above the TTL inverter`s 2.2-V trigger voltage. Zener diode D1 can be left out if the probe is going to be used only on TTL circuits. The logic probe is based mainly on inverters IC1-a and IC1-b, which are sections of a 7404 integrated circuit. Depending..