This circuit uses two 741s
or equivalent op amp to form a beeper that is used to
detect short circuits. This design offers a way to trace
resistance in the milliohm range, right to a short
between bridged traces beneath a solder mask. A1 is
configured as a multivibrator. R1 and C1 determine the
frequency of oscillation and Q1 and Q2 boost the output.
Assuming a virtual ground at the output of A2, free-run
frequency is about 1kHz. Q1 and Q2 deliver a +/-10v
squarewave to R4, dumping a +/-100mA through a short
circuit placed across the probe tips. R5 ensures that the
open circuit voltage never exceeds +/-0.1v. A2 monitors
the voltage between the probes. When the probes are open,
A2's gain equals the R4/R5 divider loss, and the output
of both amplifiers is identical. This has two effects:
First, hysteresis is greatly increased and frequency
falls to a low growl, and secondly, the loudspeaker that
bridges the two in-phase outputs is effectively silenced.
A dead short across the probe tips will return nothing to
A2 and the circuit will squeal at its nominal 1kHz rate.
Anything less than a perfect short produces some output
from A2, increasing multivibrator hysteresis and lowering