Let'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, wiring harnesses or you name it. It's ideal for auto electrical work and a host of other applications.

Features

It features six preset 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. Then there is provision for presetting any value over the range of 1Ω to 100Ω. Provided the resistance you measure is then less your preset value, the buzzer sounds and the red LED lights.

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.

The circuit uses just one low-cost op amp package, a 3-terminal regulator and not much else. Fig.1 shows the block diagram of the circuit and while it shows a lot of boxes, the concept is really quite 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 full circuit diagram and as you see, it uses just one LM324 quad op amp to do most of the circuit functions. 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 buffered by IC1c, connected as a unity gain voltage follower. This is followed by IC1d which has its gain set by seven 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.

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.

Let's say that you want to check continuity (ie, resistance) of less than 5Ω, so you set that with the rotary switch. Now you connect a 4.7Ω resistor across the test terminals. 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 100W than for low resistance values". And they 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.

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.

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 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 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

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.

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.

Fig.4: check your PC board against this full-size etching pattern before installing any of the parts.

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 for each range. That's it: make up a pair of banana plug test leads and you now have a very useful Programmable Continuity Tester.

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.