An electric continuity tester for checking the electrical connections, cables can be designed using this circuit diagram. This electric continuity tester produces a beep when there is a short and does not emit any sound when the circuit is interrupted or when the resistance along the circuit exceeds 1ohm. To prevent failure of sensible components, and for a long battery life, tester injects a weak signal in the circuit is checked.
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).