Compact continuity tester with Darlington pair circuit - driving indicator when low impedance is connected between inputs

摘要

A sensitive continuity tester has a current source supplying a high impedance input circuit and associated amplifier and indicator. It can withstand large external voltages, and is very economical, consisting essentially of two diodes, two transistors, and a relatively simple indicator. It is also compact. The input circuit contains a high impedance resistor(3) in series with two diodes(4, 5) conducting in the same direction. One of the diodes(5) is brassed off by the supply voltage(6) via a series protection resistance pari(7, 8) whose junction controls the amplifier(9). The amplifier drives a switching transistor(11) with the indicator(12) as a load, the amplifier and switch forming a Darlington pair. If an impedance less than 200 kilhms is connected between the inputs(1, 2) the amplifier is driven and the indicator illuminated.