(for surface mount work)
A kit is available from
Talking Electronics for $2.50 plus postage.
It is best to buy other kits at
the same time to save postage
HERE to buy kit.
SERVICING SURFACE MOUNT PROJECTS
2 levels of sensitivity
It squeals, so you don't have to look
This design is cheaper, simpler and smaller than our other CONTINUITY
A Continuity Tester is essential for testing digital work and when it comes
to surface-mount projects, you need a tester that beeps and has very fine
The clever design of the circuit needs no switch and it has two levels of
One probe only squeals when the resistance is very low. Anything above 300
ohms does not produce a sound.
This is called the SHORT CIRCUIT detector. It detects continuity of tracks
The other probe allows resistances up to 40k to be detected and as the
resistance increases, the sound from the buzzer decreases.
This lets you detect a resistance between two points.
When you are dealing with a surface mount project, you cannot take your
eyes off the probe tips as you might short between two components or slip
off the part you are testing. That's why you need a buzzer.
Continuity Tester Slimline Circuit
HOW THE CIRCUIT WORKS
The project uses 4 components and 2 watch cells.
The Short-Circuit probe simply connects the mechanical
buzzer to the 3v supply. The buzzer will allow a small resistance and this
will change the volume of the output.
The High Sensitivity probe includes the gain of the transistor to
allow resistances up to a few kilo ohms to be included in the circuit and
this will allow diode and transistor junctions as well as FET junctions to
be detected for shorts and leaks.
The buzzer is an electronic buzzer and has no moving parts or contacts.
There are two separate coils inside the sealed unit and when power is
connected, a transistor is turned ON via a fairly high value resistor and about 10mA flows through one of the
coils to pull the diaphragm closer to the coil. The coil has a metal rod
down the middle (called the core) and the diaphragm has a magnet in the centre of the disc.
The magnetic flux from the main coil also cuts the turns of the second coil
and and it produces voltage and current that is passed to the base of the
transistor to turn it on harder.
The action is called regenerative action and occurs very quickly and soon
the transistor is turn on almost fully. This pulls the diaphragm closer to the coil
and the magnet on the centre of the diaphragm adds to the current into the
base to turn the transistor on more.
The magnet is now close to the coil. It touches the
coil and makes a nose. But it cannot get
any closer and thus the spike produced in the coil by the magnet, ceases.
This causes the transistor turn turn OFF very slightly and the current in
the main coil reduces. This causes the magnetic flux to reduce and the
voltage and current in the coil connected to the base also reduces. This
causes the diaphragm to move away from the coil and the magnet produces a
voltage and current in the second coil that is in the opposite direction and
it has the effect of turning the transistor OFF almost completely.
Finally, the effect of the magnet moving away from the coil has less effect
on turning the transistor OFF and the resistor on the base starts to turn
the transistor ON to repeat the cycle.
The secret is the magnet on
The buzzer is called MECHANICAL BUZZER and will operate on 2v-3v
and up to 5v or even 12v.
TESTING A PROJECT
The first thing you have to do with an unknown project is test the trackwork.
This consists of testing all the tracks for continuity and all the pins for "shorts."
A multimeter can be used but it takes much longer to do the
same job. And you have to keep looking at the needle.
The two probes can be parallel or at
a slight angle so you know which probe your are using.
No "ON-OFF" switch is needed as the circuit is designed to take zero
current when not used.
Our CONTINUITY TESTER gives an audible indication of continuity
between the probes so you can keep your eyes on the
probe tip. Secondly its response-time is very short so that you can make lots
of tests very quickly while listening for the beep.
The negative lead of the Continuity Tester is placed on the first pin of the
chip and you quickly wipe the probe down the other pins on the same side then the
12 pins on the other side. The only time you will hear a beep is if a short is
present or if the two probes touch.
When you hear a beep, you should examine the track-work carefully to see if a
fault is present or if the pins are joined by a track, or some other component.
Continue this procedure with pins 2, 3, 4 etc. until all the possible
combinations have been covered.
This is repeated with all the chips in the project and any other connections
you can find on the board. This is the only way to locate a hidden short and
even though it involves thousands of tests, it will be much quicker than using
CONSTRUCTING THE TESTER
All the components are mounted on a small, slim, PC board.
Start assembly at one end of the board and fit each component as you come to
it. The transistor must be fitted around the
correct way and the holes on the board will assist you. If you are not sure,
refer to the photo above.
The probe tip is made from long machine pins and soldered to the top of the
FITTING THE 2 WATCH-CELLS
The two cells for the project are fitted into two slots in the PC board and a
piece of shim steel is soldered to the top and bottom of the board for each
cell to complete the circuit.
You must make sure the shim is kept flat so the edge of each cell does not
touch the top of the cell via the shim and create a short-circuit.
The shims must also be
soldered so you can get your fingers onto the cells and
remove them when they "run out of electrons."
FITTING THE 2 WATCH-CELLS
The wander lead can have either an alligator clip, E-Z clip attached to
When everything has been soldered in place, touch the two probes together and the
buzzer will emit a
We have used it many times for microcontroller projects sent to us for
repair. It has found cracks and dry joints the eye missed.
We keep it on the workbench and I am sure you will find it invaluable too.
Kit: $2.50 plus postage.
HERE to buy kit.
1 - 1k
1 - 1M
1 - BC 557 transistor
1 - Mechanical Buzzer
2 - watch cells AG3
2 - probes (long machine pins)
4 pieces of shim steel
30cm hook-up flex
1 - ezy clip or alligator clip
15cm fine solder
1 - CONTINUITY TESTER SLIMLINE