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

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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