Ultimate Continuity Tester Hides Many Tricks Up Its Sleeve

At the time, I wanted to build The Ultimate Continuity Tester,  and I established a wish list of all the features I required: A real continuity  tester. Too many multimeters and sounders react at resistances as high as hundreds or even thousands of ohms, which makes them practically us
Ultimate Continuity Tester Hides Many Tricks Up Its Sleeve - schematic

eless in many cases. Within a board or a system, there are always medium-conductivity paths everywhere, so they sound most of the time. A connector, a PCB track, or a wire, even a long one, has a resistance generally below 1. Having a threshold much higher than that generates false alarms. Speed. Many testers require a contact of tens of milliseconds or more, which makes the testing of large numbers of connections very frustrating. It is impossible to swipe quickly across a large number of pins. Cheap to make and use. That meant a very small number of dirt-cheap components and a power consumption as frugal as possible from a cheap power source. This ruled out the usual 9-V battery, one of the least efficient and most expensive sources available. No power switch. You invariably forget it`s on  the afternoon preceding your holiday leave, and timers aren`t good enough. They tend to go off unnoticed just when you reach the wanted connection. At first sight, the circuit in Figure 1 doesn`t seem very impressive, but it fulfills all of these requirements, and then some. It looks like some half-cooked multivibrator, but appearances can be deceptive. Q1 and Q2 form a two-stage, non-inverting amplifier, whose input and output are connected via C3 in order to cause oscillations. Each stage has its gain carefully defined: Q1 by the ratio of R4 to R1, and Q2 by the ratio of R2 to the sum of R8 and whatever sits between the test probes....

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