Using electronic diagram below can be made a very simple TTL tester. One of the three LEDs will light depending on the applied voltage at test point (TP). This voltage is transmitted first to two comparators (A1 and A2). At the other input of each comparator voltage reference is brought from the voltage divider R4/R5/R6. Values for this components gives thresholds at 0.8 and 2.4 V and the range between these two values is the TTL "prohibited area". If the voltage at TP is less than 0.8 V output of A2 swings in state "0" logic and results in ignition red LED (D6). If measured voltage is greater than 2.4 V, the output of A1 will flip the logic state "0", so the green LED (D5) will light. When the voltage will be between 0.8 and 2.4 V and then no output of A1 or A2's output will be in state "0" logic. When TP is not connected to any circuit, the same situation is generated due to the action of R1 and R3. Inverting input of A3 is in this situation, in "1" through R9 so that the yellow LED (D7) lights.