Logic testers are simple but very helpful devices in testing digital circuits. A logic probe can be designed in many different ways. In this particular design, a combination of discrete and TTL logic components is applied to test different logic levels. This logic tester can test and display three different logic levels: the “0” and the “1” levels including the undefined logic state also known as “never mind”.
If the tested voltage level is below 1 volt, the logic tester will recognize it as a logical “0”. In that case, the emitter-collector junction of T2 conducts and the D5 LED lights up optically displaying a “logic 0”. If the tested voltage is between 1 volt and 2 volts, both T1 and T2 does not conduct and the XOR gate U1 receives two dissimilar logic states at its inputs. The XOR output becomes positive and the D6 LED lights up signalling a “never mind” logical state. The same thing happens by a open input or when testing a blind IC pin.
When the tested point is above 2 volts, the D4 LED lights up signalling a logical “1” state. Another plus of the featured circuit is the fact that it uses a 7486 IC for the XOR gate. Since this IC has 4 gates inside, the circuit can be expanded to build a four channel logic analyzer.