36 May/June 2018
BUILD IT YOURSELF
What is a continuity test? I say it
depends on your particular use. In
my case, it’s usually either for
beeping out cables or for beeping
out printed circuit board (PCB)
By Douglas Booth
Idesign and build functional equipment to test PCB assemblies. This involves making all sorts of custom cables and the custom board assemblies.
The latter often involve hand-soldering SMT
components, such as my favorite 80-pin fine-pitched
microcontroller. There is plenty of opportunity for
error, and a continuity tester is very handy for
detecting both shorts and opens.
Somewhere in the 25 ohm area has proven to be
a convenient trip point for my use. In other words, if
the resistance between my continuity tester probes is
less, I get a beep; if the resistance is more, there’s no
For years, I used a simple op-amp based circuit
with two AAA batteries, some resistors, and a piezo
electric beeper. It served me well, but had two
1. I’m getting old, and I work in a noisy
environment. Half the time, I can’t hear the
beeper, or worse, I imagine that I do hear it.
2. I forget to turn it off, so the batteries die.
Most multimeters have a built-in continuity tester, but
these also have problems:
1. Again, I can’t hear the beeper.
2. There is a significant voltage between the probes
(7V on my Fluke meter). That voltage can damage
some devices (not to mention polarity issues).
3. There is a slight but unacceptable delay each time
it detects continuity. Frequently, I plant a probe on
one pin and ‘rake’ the other pin across IC pins,
looking for shorts and opens. That only works when
the feedback is ‘instant.’