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By Trent Jackson
A programmable
continuity tester
No matter how high-falutin’ is your involvement with electronics,
one of the most common bench tests is for continuity. And sure, you
can always rake out the multimeter but this little tester does a better
job, with selectable resistances. It makes an ideal Go/No Go Tester.
L
Fig.1: the block diagram of the Programmable Continuity Tester. It feeds
a current through the device under test (DUT) and the resulting signal is
then buffered, amplified and compared with a reference voltage.
68 Silicon Chip
ET’S FACE IT, almost every analog
and digital multimeter does
have built-in capabilities for testing
continuity. However, this function is
somewhat limited. Most DMMs are
preset to beep that little miniature
buzzer inside when the continuity is
below about 40Ω or so.
Wouldn’t it be nice to have a device
that allows you to set this minimum
continuity to anywhere between 1Ω
and 100Ω? Well, that is exactly what
this project does. It is accurate, reliable
and works very well.
It can be used to check the resistance
of all sorts of low resistance devices:
lamp filaments, motor windings, relays, switches, transformers, speakers,
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Fig.2: most the circuit functions are performed by a single LM324 quad op amp IC. These initially buffer and amplify
the signal from the DUT, after which the signal is compared against a fixed voltage reference in IC1b. The output of
IC1b then drives a buzzer and indicator LED via transistor Q1.
wiring harnesses or you name it. It’s
ideal for auto electrical work and a
host of other applications.
Features
The unit features six preset resistance levels: 5Ω, 10Ω, 20Ω, 50Ω, 75Ω
and 100Ω, selected by a rotary switch.
Now if any resistance that you measure
is less than the preset value, the buzzer
sounds and a red LED lights. In addition, there is provision for presetting
any resistance value over the range of
1Ω to 100Ω. Provided the resistance
you measure is less than your preset
value, the buzzer sounds and the red
LED lights.
How it works
The circuit uses just one low-cost op
amp package, a 3-terminal regulator
and not much else. Fig.1 shows the
block diagram and while it shows a
lot of boxes, the concept is really quite
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straightforward. There is a current
source to feed the device under test
(DUT), three op amps used as buffer
and amplifier stages, a comparator and
buffer, and the LED and buzzer.
Fig.2 shows the circuit diagram and
as you see, it uses just one LM324 quad
op amp to do most of the circuit func
tions. A 3-terminal regulator (REG1)
derives a fixed 5V from the 9V battery.
The fixed 5V is required because the
current source and comparator rely on
having precise voltage levels.
Resistor R1 and trimpot VR1 set the
maximum current (into a short circuit)
for the device under test (DUT) at
16.6mA. The voltage developed across
the DUT is then fed to IC1c via a 330W
resistor which, together with ZD1,
provides transient input protection.
IC1c is connected as a unity gain
voltage follower and acts as a buffer
stage. This is followed by op amp
IC1d which has its gain set by one of
seven switched resistors (trimpot VR2
included).
The output of IC1d goes to another
unity buffer (IC1a) and is then fed to
pin 5 of IC1b which is connected (no
feedback) as a comparator. Pin 6 is
connected to a voltage divider which
means its level is +2.5V. Now if pin 5 is
less than the +2.5V at pin 6, the output
of the comparator goes low to turn on
transistor Q1, the buzzer and LED2.
Half-supply reference
The key fact about this circuit is
the +2.5V at pin 6 of IC1b; everything
relies on this.
Now we’ll backtrack a bit, to see how
the circuit functions when testing an
actual resistance. Let’s say that you
want to check continuity (ie, resist
ance) of less than 5Ω, so you set that
with the rotary switch. That done, you
connect a 4.7Ω resistor across the test
terminals.
July 2003 69
Fig.3: the assembly is straightforward but take care with the switch wiring, as it’s easy to make a mistake with the
connections. Take care also when installing the semiconductors, as these can easily be damaged if mounted the
wrong way around on the PC board.
As previously noted, VR1 is set to
provide a maximum current into the
DUT of 16.6mA. Now because the DUT
is 4.7Ω, the voltage developed across
it will be 4.7 x .0166 = 78mV.
This is passed through the unity
gain buffer unchanged (that’s what
a unity gain buffer does!) and fed to
IC1d, where it will be amplified by a
factor of 31.3, as set by resistors R11
and R10. So the voltage at the output
of IC1d will be 0.078 x 31.3 = 2.44V.
This is less than the +2.5V at pin 6 of
IC1b and so Q1 will be turned on to
sound the buzzer and light LED2.
The same process happens with
the other resistance ranges. The gain
of IC1d is changed via the switchable
resistors to suit the selected threshold
resistance.
Now some readers won’t be happy
with the above description. “Hang on
a minute” they’ll say. “The current
set by trimpot VR1 is nowhere near
constant and will be quite a bit less for
higher resistances around 100Ω than
for low resistance values”. And they
Table 1: Resistor Colour Codes
o
No.
o 2
o 1
o 1
o 1
o 3
o 1
o 1
o 1
o 3
o 1
o 1
o 1
70 Silicon Chip
Value
100kΩ
68kΩ
39kΩ
15kΩ
10kΩ
6.8kΩ
3.3kΩ
1.2kΩ
560Ω
330Ω
180Ω
100Ω
4-Band Code (1%)
brown black yellow brown
blue grey orange brown
orange white orange brown
brown green orange brown
brown black orange brown
blue grey red brown
orange orange red brown
brown red red brown
green blue brown brown
orange orange brown brown
brown grey brown brown
brown black brown brown
5-Band Code (1%)
brown black black orange brown
blue grey black red brown
orange white black red brown
brown green black red brown
brown black black red brown
blue grey black brown brown
orange orange black brown brown
brown red black brown brown
green blue black black brown
orange orange black black brown
brown grey black black brown
brown black black black brown
siliconchip.com.au
Parts List
1 PC board, 70 x 55mm, coded
04207031
1 plastic utility box, 130 x 67 x
44mm
1 label to suit box
2 knobs to suit rotary switch and
potentiometer
1 SPST toggle switch (S1)
2 5mm LED bezels
2 panel mount banana sockets,
one red, one black
1 9V battery
1 9V battery holder
4 adhesive PC board standoffs
(Jaycar HP-0760; pack 25)
1 1-pole 12-position rotary
switch (S2)
1 self-oscillating piezo buzzer;
Jaycar AB-3459 or equivalent
2 cable ties
Rainbow cable
1 200Ω horizontal mount trimpot
(VR1)
1 100kΩ linear potentiometer
(VR2)
The PC board and battery holder are mounted on the lid of the case, as shown in
this photo (see text). Use several cable ties to keep the wiring neat and tidy but
leave enough slack in the wiring so that the lid can be opened out.
Fig.4: check your PC
board against this fullsize etching pattern
before installing any
of the parts.
Semiconductors
1 LM324 quad op amp (IC1)
1 7805 3-terminal regulator
(REG1)
1 BC558 PNP transistor (Q1)
1 5mm green LED (LED1)
1 5mm red LED (LED2)
2 1N4004 silicon diodes (D1, D2)
1 4.7V 1W zener diode (ZD1)
Capacitors
1 100µF 16V PC electrolytic
1 10µF 16V PC electrolytic
2 100nF (0.1µF) MKT polyester
or monolithic
Resistors (1%, 0.25W)
2 100kΩ
1 3.3kΩ
1 68kΩ
1 1.2kΩ
1 39kΩ
3 560Ω
1 15kΩ
1 330Ω
3 10kΩ
1 180Ω
1 6.8kΩ
1 100Ω
will be right. But that does not alter
the validity of the circuit, because the
gain resistors selected by the rotary
switch have been selected with this
factor in mind.
If you have trouble accepting this,
let’s try another example, this time
using the 100Ω range. And this time,
let’s make the device under test (DUT)
a resistance of 95Ω. We said before
siliconchip.com.au
that trimpot VR1 is adjusted to give
a maximum test current (into a short
circuit) of 16.6mA. By the magic of
Ohm’s Law and the specified 5V supply, this means that the total resistance
of R1 and trimpot VR1 is 300Ω. Try it:
5V/300Ω = 16.6mA.
Therefore, when we connect 95Ω
across the DUT terminals, the total
current flowing will be 5V/395Ω =
12.7mA (we never said the test current was fixed!). The resulting voltage
across the 95Ω resistance is 1.2V and
this is amplified in IC1d by a factor of
2, giving 2.4V at pin 5 of comparator
IC1b. Once again, the output of IC1b
will be low, Q1 will turn on and the
buzzer will sound.
We’ll leave it to you to confirm the
principle on other ranges but don’t
July 2003 71
Fig.5: this full-size artwork
can be used as a drilling
template for the front panel.
Note that it’s best to make the
larger holes by drilling small
pilot holes first and then
carefully enlarging them to
size using a tapered reamer.
worry, it does. In fact, in theory, trimpot VR1 could have been omitted and
R1 specified as 300Ω and the circuit
would work identically. Trimpot VR1
is really only required to cope with
slight tolerance variations in the circuit components.
Putting it together
All the circuit components, with
the exception of the rotary switch and
potentiometer VR2, are mounted on a
PC board measuring 70 x 55mm and
coded 04207031. The parts overlay
and wiring diagram is shown in Fig.3.
Assembly is very straightforward.
Mount all the PC pins (18 required)
first, followed by the resistors and diodes. Make sure the diodes are in the
right way around and the same comment applies to the two electrolytic
capacitors. Then mount the polarised
piezo buzzer, the transistor, 3-terminal
regulator and the LM324 IC.
The finished PC board mounts on
the lid of the case using four adhesive
standoffs (Jaycar HP-0760; pack 25).
The battery holder is mounted on the
lid with a dob of hot-melt glue or you
could use double-sided foam tape. All
front panel components are mounted
on the base of the case so you can fit
the label to the case and use it as a
drilling template for the on/off switch,
two LED bezels, rotary switch, potentiometer (VR2) and the two banana
plug sockets.
Rotary switch setup
The rotary switch needs to be set
to provide seven positions before it
is mounted in the case: pull off the
indexing washer and set it back on the
threaded bush to give the right number
of positions. Try it by hand before you
mount it in position.
Once the case hardware is mounted,
complete all the wiring as shown in
Fig.3. When all is complete, carefully
check your work and then fit a 9V
battery and switch on. The green LED
should light.
Now switch your multimeter to the
200mA range and connect it across
the test terminals. Adjust VR1 for a
current of 16mA.
That done, switch down to the
20mA range and readjust VR1 to obtain
a reading of 16.6mA.
Now do a series of checks to see that
each range gives the correct buzzer
result (and with the red LED lit), using
suitable test resistors. That’s it: make
up a pair of banana plug test leads and
you now have a very useful ProgramSC
mable Continuity Tester.
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