SHORT_CIRCUIT_BEEPER

Published:2009/6/22 23:33:00 Author:May | From:SeekIC

This design offers a way to trace resistance in the milliohm range, right to a short between bridged traces beneath a solder mask. It simply translates resistance into an audible tone, which in-creases in pitch as the measured value approaches zero.In the classic op-amp multivibrator (shown in the inset), oscillation frequency is determined not only by the R1/C1 time constant, but also by the hysteresis set by the R2/R3 resistance ratio. Al in the main figure, with current boosters Q1 and Q2, is this same configuration.Assuming a virtual ground at the output of A2, free-run frequency is about 1 kHz-quite audible through a tiny 8-Ω speaker. Q1 and Q2 deliver a ±10-V squarewave to R4, dumping a ±100 mA through a short circuit placed across the probe tips. R5 ensures that open circuit voltage never ex-ceeds ±0.1 V.AZ monitors the voltage between the probes. The differential input must have its own separate path to the probe tips to eliminate test lead resistance from the measurement. Miniature zip-cord sold as loudspeaker wire makes a tidy two-conductor test lead.When the probes are open, A2's gain equals the R4/R5 divider loss, and the output of both am-plifiers is identical. This has two effects: first, hysteresis is greatly increased and the frequency falls to a low growl; second, the loudspeaker that bridges the two in-phase outputs is effectively silenced.The dead short across the probe tips will return nothing to AZ and the circuit will squeal at its nominal 1-kHz rate. Anything less than a perfect short produces some output from A2, increasing multivibrator hysteresis and lowering the pitch. The circuit has so much 'leverage, and the ear is so sensitive to pitch changes in this range, that it's easy to resolve minute resistance differences.Any general-purpose op amp will suffice in this circuit-a couple of 741s or an equivalent dual.



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