Using this circuit diagram can be designed a very simple logic tester consists of two comparators, the system works with different reference voltages supplied by different voltage dividers. R3-R4-R5 divider provides a voltage of about 40% from Ucc voltage, pin 6 of IC1b, and an approximately 16% of Ucc, pin 3 of IC1a. When Ucc = 5 V, the voltage is exactly the threshold comparators TTL (0.8 V and 2.0 V).
Similarly, R6-R7-R8 divider voltage supply 23% and 73% of UCC's IC1a pin 3 respectively IC1b's pin 6 (the standard threshold levels correspond CMOS comparators).
Voltage to be measured, U, is applied to IC1b pin 5 and IC1a pin 2 and compared with that reference. IC1b comparator output goes into the state H when U is below the voltage at pin 3.