Connection tester - continuity tester with LED indication

connection tester In the April issue of Elektor, we published a circuit for a contact tester with an acoustic indication. As a result of this publication we received a number of requests from readers for a contact tester with an optical indication. The circuit described here fits that particular bill rather nicely. Like the original design this circuit has its own printed circuit, the only difference being that this one uses a LED, rather than a buzzer to denote a good contact. The theoretical aspects of this circuit were discussed in detail in the April issue so for now we will restrict ourselves to recapping the calibration procedure. Place a 1 SZ resistor be- tween the probes and adjust P1 until the LED is just about to light. Remove the resistor and create a short circuit between the probes. The LED should now light. To make sure that calibration is correct, place a resistor of only a few ohms between the probes. If the LED lights up now, the calibration procedure will have to...