I WAS REMINDED RECENTLY of a tester I designed
years ago. 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 useless 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.
• Ruggedness. You can sometimes inadvertently
test charged capacitors or energized circuits,
and the tester must survive such situations.
• Safety. Even with the most sensitive electronics,
safety means low voltage and current at
the test probes.
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.
When the product of these gains exceeds unity,
oscillation occurs. With the values shown, the
oscillation condition is met when the contact
under test is less than 5 . Other values are
possible by altering the R3/R4 ratio, or both
resistors can be replaced by a trimpot, Aj1.
To maximize the drive with the low supply
voltage, the piezo-buzzer (BZ1) is connected
between antiphase outputs of the oscillator. The
resulting sound isn’t very loud with a standard
transducer, but it’s quite sufficient for a lab or
office environment.
R8, together with D1 and D2, protect the tester
in case it’s accidentally connected to a live
circuit. It’s a 1-W, preferably fusible, resistor.
For an extended and resettable protection, it can
be substituted with a positive temperature coefficient
resistor. If so, the resistor will be able
to withstand a prolonged connection directly
across the mains. Optional diodes D3 to D6 can
be installed in parallel with the battery to ensure
its protection.
The battery can be a single standard or
rechargeable AA cell, since the circuit operates
from 1 to 1.5 V. A power switch is unnecessary
because the test terminals also act as a switch.
The only quiescent current is caused by leakages
in the components—typically in the hundreds
of nanoamperes range. When the terminals are
shorted, the current rises to about 10 mA. So in
normal operation, the battery will last for years,
and it can be soldered in place.
The buzzer can be any cheap, passive type.
As noted, this circuit goes beyond the requirements of the “ultimate” continuity tester.
For example, it not only checks the
resistance, but also the modulus of
the impedance presented to the test
probes. In some cases, this can make
a big (and useful) difference.
The secondary of a 50/60-Hz transformer
will generally have a dc resistance
below the 5- threshold, but
its impedance is mainly inductive and
higher than 5 , so no oscillation will
take place. There will simply be a light
“click” at the instant the circuit is
made, because BZ1 receives a pulse
via R2 and R4. This will always be the
case when a galvanic contact is made
between the probes.
This is a useful feature because you
can differentiate between the wires of
a multitap transformer, where a more
conventional tester sees a bunch of
shorted wires. Also, when a single
diode in a bridge rectifier is shorted,
you can immediately pinpoint the faulty
one without any de-soldering.
But there is more. If the transformer
itself is faulty, with one or more turns
shorted, the tester will sound. And
if any of the windings are connected
in opposition, it will also sound. This
feature lets you determine the phase
of the windings, both primary and secondary.
Figure 2 shows a number of
situations and their consequences for
a typical dual-primary/dual-secondary
transformer. Therefore, you can comprehensively
test a transformer without
ever connecting it to the mains.
Finally, note that if you use a
rechargeable battery, there’s no need
for a specific charge connector. The
test terminals can act as an input to the
charger, via protection diode D2.