Testing continuity - whether or not there is a continuous path for electricity to flow - is an essential part of electronics testing. It can help us find broken wires, cold solder joints, and failed components. However, the normal methods leave a lot to be desired in some cases. The simplest continuity testing circuit we can make for this is just a battery, a buzzer or LED (and its current-limiting resistor) and some wires.

Many makers test for continuity with a multimeter. All multimeters are capable of measuring resistance. The idea is that if there is a low enough resistance between two points, there is continuity.

Having the value of resistance on the display is helpful, especially if you are testing for something that may be connected, but not very well. Looking at the screen all the time is difficult though, as you usually need to look where you are putting the probes.

To overcome this, many multimeters have a buzzer so that under a certain resistance value, the buzzer sounds. You can look at the job and not the screen, just listening for the sound. There are a few problems with this approach, however, and with the battery/buzzer simple circuit.

In both cases, the point at which the buzzer sounds cannot be controlled. So, for example, if you are looking for cold joins which may have some contact, but not very much, you may want to change the value of resistance that the circuit sounds at so that you can detect these joins that do pass current, but have just enough resistance to pass very much current, or pass it reliably.

The opposite might be true of long wires. The voltage drop that occurs may mean you need to raise the threshold to get the buzzer to trigger, as there is enough natural resistance just in the wire to stop the preset ones from working.

By far a bigger problem, however, is the levels of voltage and current that these testers use. The battery and buzzer simple circuit would pass 9V at 15 to 30mA, and the LED around the same. That's ok for testing a wire, but not so good for sensitive circuits, or anything with a current limit. You might damage sensitive components this way. Our trusty workbench multimeter has a test voltage of 7.6V, and a current of 0.11mA. The current is not bad really, but some cheaper multimeters have a test current of well over that of the battery.

The answer to all of this is to build a circuit which can have an adjustable threshold, and has a very low voltage and current from the test probes. The circuit we present here has a test voltage of around 0.5V at a current of 0.2mA, both of which you can adapt quite easily.

Its threshold is adjustable, and none of these figures change with input voltage so the tester is stable across a wide state of battery charge or even different power supply options.

SOME HELPFUL HINTS

We encourage you to read all the way to the end of the article before you build. Not only will you then have a better feel for the overall picture as you build, but we sometimes discuss options or alternatives that you will need to have decided on.

You will need some basic hand tools for most builds. Small long-nosed pliers and flush-cut side cutters meant for electronics are the main ones. Materials like tape or glue are mentioned in the steps, too. We always produce a tools materials list if you have to go shopping, but anything that is lying around in most homes is just stated in the steps.

As always with Kids' Basics, we avoid soldering to make the build more accessible to more people, but having an adult around can still be helpful. You won't need any particular skills besides being able to identify components at a basic level, and even then, we help as you go along. If, for example, you don't already know what a resistor is, you'll probably be able to work it out from the photos and description in each step.

We do provide a schematic or circuit diagram but this is just helpful if you already know how to read one. Don’t stress if you have never learned, but take the chance to compare the digital drawing of the breadboard layout (which we call a 'Fritzing' after the company that makes the software) to the schematic and see if you can work some things out.

You can make this project from the Fritzing and photos alone. You might also like to check out our Breadboarding Basics from Issue 15.

The Build:

Parts Required:	ID	Jaycar
1 x Solderless Breadboard	-	PB8820
1 x Packet Breadboard Wire Links	-	PB8850
4 x Plug-to-plug Jumper Wires	-	WC6024
1 x 1kΩ Resistor*	R1	RR0572
3 x 1.5kΩ Resistors*	R3, R4, R5	RR0576
1 x 150kΩ Resistor*	R2	RR0624
1 x 300kΩ Resistor*	R7	RR0631
1 x 470kΩ Resistor*	R9	RR636
1 x 620kΩ Resistor*	R6	RR0639
1 x 1MΩ Resistor*	R8	RR0644
1 x 4.7nF Capacitor*	C1	RM7047
4 x 100nF MKT Capacitors*	C2, C3, C4, C5	RM7125
1 x 2.2µF Electrolytic Capacitor	C6	RE6042
1 x NE555 Timer IC	IC1	ZL3555
1 x LM324 Quad Op Amp IC	IC2	ZL3324
1 x 9V Battery	-	SB2423
1 x 9V Battery Snap	-	PH9232

Step 1:

Place the breadboard in front of you, with the outer red (+) rail facing away from you and the outer blue (-) rail closest to you. Install two wire links, one joining the two red (+) rails and one joining the two blue (-) rails.

Step 2:

Insert the LM324 IC into the middle of the board. Be careful that pin 1 is to the left. Pin 1 on any IC is usually marked with a notch in the middle the pin 1 end like this one, or a dot impressed into the case above pin 1.

Be careful though. See the dot at the other end of ours? This is a moulding mark, and can be confused for the pin 1 dot when there is no notch, but moulding marks are always in the middle, while pin 1 dots are on the side next to the pin.

Step 3:

Add a wire link from the upper blue (-) rail to pin 11 of the LM324, and one from the lower red (+) rail to pin 4. Also place a wire link from pin 2 towards the left, and another from where that link ends, across the gap in the breadboard. Count the number of rows these links cover but if you are using the packets of pre-made links, the colours and lengths should be the same as ours.

Step 4:

Insert a 10MΩ resistor (BROWN BLACK BLACK GREEN SPACE BROWN) from pin 3, off to the left of the IC, and a wire link from the end of the resistor back to pin 1. Also place a 1V4148 or 1N914 small signal diode with its anode (+, unmarked) end in the same row as the two wire links, and its cathode (-, striped end) to the lower blue (-) rail.

Step 5:

Install a wire link beside the diode, from the lower blue (-) rail to the row. Place a BC547 NPN transistor with its flat side facing you and its right-hand (emitter) leg to the wire link. Add a 1kΩ resistor (BROWN BLACK BLACK BROWN SPACE BROWN) between the middle (base) leg of the transistor and pin 1 of the LM324.

Step 6:

Place a 100nF (104 or 100n) capacitor from pin 2 of the LM324 to the lower blue (-) rail, and a 1kΩ resistor (BROWN BLACK BLACK BROWN SPACE BROWN)from the row next to the end of the LM324, to the lower red (+) rail. Also place a 2kΩ resistor (RED BLACK BLACK BROWN SPACE BROWN) from pin 3 to the next row along from where the 1kΩ is.

Step 7:

Insert a wire link from the 1kΩ resistor, crossing one row where the 2kΩ resistor ends, into the next row. Install a 5.1V Zener diode (1N4733A) with its anode (unmarked) end to the lower blue (-) rail and its cathode (striped) end to the wire link. Add a 1MΩ resistor (BROWN BLACK BLACK YELLOW SPACE BROWN) from the wire link off to the right.

Step 8:

Push the sockets of three plug-to-socket jumper wires onto a 100kΩ 16mm potentiometer. Use a 16mm, because the pins on 9mm potentiometers are too small and 24mm ones are too big. Plug one of the outer terminals into the lower blue (-) rail and the other outer one into the end of the 1MΩ resistor. The middle pin, the wiper, goes to the row with the end of the 2kΩ resistor.

Step 9:

Install a BC557 PNP transistor with its left-hand (collector) leg in the same row as the wire link that crosses the gap in the breadboard. Place a 1kΩ resistor (BROWN BLACK BLACK BROWN SPACE BROWN) from the right-hand (emitter) leg of the transistor to the upper red (+) rail, and a 1.5kΩ resistor (BROWN GREEN BLACK BROWN SPACE BROWN) from the upper red (+) rail to the middle (base) leg of the transistor.

Step 10:

Add a 15kΩ resistor (BROWN GREEN BLACK RED SPACE BROWN) from the middle (base) leg of the BC557 to the left, and a wire link between the end of the resistor and the upper blue (-) rail. Install two plug-to-plug jumper wires, one into the lower blue (-) rail and one into the row with the signal diode and two wire links. These are the test probes. Finally, plug the red wire of a 9V battery snap into the upper red (+) rail and the black wire into the lower blue (-) rail.

TESTING AND ADJUSTMENT

Connect a 9V battery to the snap and touch the probes together. The buzzer should sound immediately. If it does not, first turn the potentiometer. If there is still no sound, disconnect the battery, and check connections. There is no easy way to predict where the fault will be by breaking the circuit into sections, so just start from left to right, top to bottom, and check the wire links and components. Make sure everything starts and ends in the right place and meets all of the other components that it should.

To adjust the tester, hold the probes together and turn the potentiometer until the buzzer stops. Then, gently turn it back into the buzzer sounds. Releasing the probes should stop the sound. The potentiometer is the sensitivity control, and for most circuits, you can leave it where it is now, set to the least sensitive that it can be. There may be times, such as when testing noisy circuits or in the presence of strong sources of interference, when the sensitivity needs adjusting, otherwise, stray currents may trigger the tester when there is not in fact any continuity.

Likewise, it may need adjusting the other way when testing long lengths of wire or the like, where there is naturally some losses and the circuit may not trigger. The best way to tell is to test the unit against a few known good conductors, like a long bit of wire you know is not broken; a simple circuit with short paths; and a bigger circuit will longer current paths; and develop a sense of where the dial needs to be.

MAKING BETTER PROBES

The plug-to-plug jumper wires are passable but for long term use, you really need something better.

Start by cutting off the ends of the wires, leaving the other ends plugged into the board. Bare the wires. Split the cores about 5cm from the end of a length of figure-8 or twin-core cable between 50cm and around 1.2m, whatever is comfortable.

Bare the ends and twist onto the jumper wires. After making sure the joints are firm, tape over them to prevent shorting or loosening.

The wire we used here is light-duty speaker wire. On the other end, split about 15cm of the cores for now, and bare around 4cm to 6cm to wrap around a nail or similar, or 1cm for the screw-on probes.

At the other end, you have several options, depending on what is available to you. Some of the simplest probes are two nails, with the wire wrapped around the ends. The challenge here is they are harder to hold and if your skin is in contact with the probes, that will affect the way the device responds: at least some current will flow through your body and give a false current path. It shouldn't affect this circuit because it's looking for low resistance, but it's best avoided. The bigger problem is if you accidentally touch something live! You should never be working on live circuits of any voltage that could hurt you, but it's still better to have safe probes. One of our favourites is to mount the nails in the end of cheap plastic ballpoint pens. We have done that here with clear ones so you can see the wire wrapped around the nail and glued in place. The wire is glued at the top end, too, to stop any movement working the twisted wire free of the nail.

A far better option is dedicated probe ends. These are available from both retail and online electronics suppliers, but ours came from Jaycar because there is a store down the road from us. Twist the bared ends of the cores tightly, then feed one down the body of each probe. At the bottom, there is a cap that screws off. The wire end should, maybe with a bit of jiggling, slide through the hole. Splay the ends of the wires a bit, before screwing back the retainer to clamp down on the wire.

How It Works

THE COMPARATOR

There are several sections to the circuit, and it makes the most sense to follow along left to right. However, understanding the other bits is much easier if you understand the main block they connect to, so even though it's slightly illogical, we'll explain the op amp bit first. This is a long block of text, but you'll need to refer back to the schematic regularly. Everything you're about to read is in reference to the schematic, except for one diagram at the relevant point.

The op amp, IC1a, is one of four independent op amps in the LM324 package. They have their own input and output connections, sharing only power supply connections. You can get smaller packages for op amps: There are single and dual op amps in 8-pin packages. However, the LM324 is designed specifically for single-rail supplies, where we have a positive supply and a ground. Many op amps require a dual-rail power supply, which means a positive supply, ground in the middle, then a negative voltage on the other side. To use most op amps on a single supply, some trickery is required to give a false ground so the true ground can act as the negative voltage.

There are two problems with this: on a 9V battery, that only leaves us with 4.5V between 'ground' and the supply, which is not enough for some of the tasks we often ask of them. A bigger problem is the components we use to build the false ground. It adds a fair number of extra components in some cases though only two in our very simple example), but in this case, it would interfere with the basic operation of some of the sections and require a much more complex design. The bigger IC is not much different in price compared to many of the 8-pin dual-supply ones, and the overall component count is much lower. For Kids' Basics, that's important.

IC1a is set up as a Schmitt Trigger comparator. A comparator is an op amp set up to compare one input voltage to a fixed reference voltage. The output depends on whether the input is higher or lower than the reference. This one is an inverting comparator, which means the output is low if the input is higher than the reference. We do this by connecting the reference to the non-inverting input, marked with a +, and the source or signal to the inverting input, marked with a -. In terms of IC1a, the output is pin 1, the inverting input is pin 2, and the non-inverting input is pin 3.

The Schmitt Trigger adds something called 'hysteresis'. That term means that, instead of one point at which the signal is higher or lower than, there is an upper and lower limit. This stops unstable, rapid on/off behaviour when the input is very close to the reference. We do this via 10MΩ resistor R8. This feeds a tiny bit of the output back to the input. When the feedback connects to the non-inverting input, it's called positive feedback. R5, R6, and R7 provide the reference. We'll talk about those next but for now, just know that's where the reference comes from. When the output is high, a tiny current flows through R8 back to the input. That raises the voltage there a bit above what it was from R5, R6, and R7.

When the output goes low, some of the current flows from R5/R6/R7 back through R8 to ground, lowering the voltage at the non-inverting input. This is how we change the upper and lower trigger points. Let's say that there is 0.4V coming out of R5/R6/R7. If the output is low, then some current flows through R8 to the low output, which is sinking current to ground at this point. In our case, it was only 0.004V, but that's enough. When the probes were closed and the output went high, the current flowing back through R8 raised the voltage from R5/R6/R7 at the non-inverting input to 0.461V, so it added 0.061V. That total 0.065V is just enough to stop the output from dancing back and forth between on and off when the signal input is close to the reference input. The signal has to rise above 0.461 volts to activate a change, then fall below 0.396V to change back.

As an aside, the current at the output of the op amp also, when high, flows through 1kΩ resistor R9, through the base-emitter junction of Q2, to earth. This changes the behaviour of the circuit slightly but not enough to worry about in our case. Just know that this reduces the voltage slightly that would flow through R8 if there was no other load connected to the pin 1 output and this certainly does matter in some designs.

THE REFERENCE

The reference voltage comes from a voltage divider formed by R5 and R6. R6 is a 100kΩ potentiometer, meaning we have an adjustable voltage divider. However, R5 is ten times larger, at 1MΩ. So, R6 is only adjusting across one eleventh of the total, and it's the last eleventh, so the voltage will be small no matter what. For a detailed explanation of voltage dividers, ratios, and other maths related to them explained with M&Ms, see Classroom Issue 51. This voltage is fed to the non-inverting input of the op amp by 2kΩ resistor R7, which interacts with R8 to form another voltage divider. That way, even R6 is turned all the way to one end and connected directly to ground, there is still a voltage at the non-inverting input: It is never connected directly to ground. This works both ways. When the output is low, R7 and R8 divide the voltage coming from the R5 and R6. When the output is high, R7 and R8, with however much of R6's travel is left, divide the current from the output back to ground.

You might have already noticed ZD1 connected between one end of R5, and ground. It's a Zener diode, a very special type of diode. If current were to flow from the anode to the cathode, it would behave similar to a normal diode. The anode, which is the positive or + end, is the big end of the triangle on the circuit symbol. The cathode, which is the negative or - end, is the bar or line, marked with a stripe on the case of the physical diode. This is called 'forward biased'. All diodes have a 'voltage drop', or an amount of voltage lost as current flows through them. It's between 0.2V and 0.7V depending on the type of diode.

When a normal diode is connected the 'wrong' way around, or 'reverse biased', no current flows through it. That is, until the reverse voltage exceeds the 'breakdown voltage' rating of the diode. Then, the diode breaks down and conducts, but it gets destroyed. Most small diodes have a breakdown voltage of at least 100V. However, when you connect a Zener in reverse bias, it is not destroyed. Rather than a breakdown voltage, they have a 'Zener voltage'. This is the voltage drop in reverse bias, and any further voltage is turned into heat.

So, they are a basic voltage regulator. ZD1 has a Zener voltage of 5.1V, so if you measured the voltage between the junction of ZD1 and R4, and ground, you would see 5.1V, no matter what the input voltage is. R4 is a 1kΩ resistor and is there to limit the current through ZD1. Zener diodes have a power rating, based on that heating process. If too much current flows, they will be destroyed anyway, Zener or not. It's also really wasteful to lose that current to heat. If you want a deep dive into Zener diodes, check out Classroom Issue 46.

The upshot of all of this is that no matter what voltage there is coming through R4 from the battery, R5 and R6 are always dividing the stable, fixed voltage across ZD1, so the reference voltage sent to the non-inverting input of the comparator remains stable regardless of battery voltage and therefore, your tester will perform the same with a fresh battery as it would with an almost flat battery.

THE CURRENT SOURCE

That's the reference side of the comparator, controlling its non-inverting input. But comparators don't work on a reference alone! The point is to compare an input signal to the reference. We get that with a constant current source formed from Q1, a BC557 PNP transistor. There is a voltage divider here formed by 1.5kΩ resistor R1 and 15kΩ resistor R2. Normally in a basic PNP transistor circuit, current flows through the emitter-base junction and to ground, turning the transistor on. In this circuit, however, R1 and R2 bias the base so that it is relatively close to the supply voltage. With the values of R1 and R2, and an exactly 9V supply, the voltage at the junction of R1 and R2 is 8.18V. Refer again to Classroom issue 51 if you need more detail. Because the base-emitter voltage on the transistor is 0.7V, that means the total voltage at the emitter is 8.88V. That leaves only 0.12V to drop across R3!

To find the constant current, we take the voltage drop across R3 (0.12V) and divide it by the value of R3 (1000Ω). That gives us 0.00012A, or 0.12mA. That's a bit different to what we measured. The emitter voltage to be 8.78, leaving 0.22V, which turns into 0.22A constant current. The reasons for this are component tolerance in the resistors, and also the fact that the base-emitter voltage drop in transistors does vary. We measured ours with a proper semiconductor tester and found it to be 0.78V, but the voltage at our R1/R2 junction was 8.78V. Those numbers do not add up, but there is a reason: There is another component in play that changes how the currents and voltages stack up: D1. It's here for another reason but it influences how the current paths behave. If you take D1 out and test around the transistor with a multimeter set to voltage, everything becomes what the calculations say it should be.

Out of the collector flows the constant current of around 200µA (actual, with D1 in place). Yes, that's less than 1mA! It's a tiny current so as not to damage the circuit under test that you are probing, no matter how sensitive it is. The collector is connected to the inverting input of the op amp, one of the test probes, and diode D1. This diode is forward biased, so current flows across it the 'right' way to ground. However, as we saw in the Zener section, diodes have a forward voltage drop. For small signal silicone diodes like this, that's 0.4V to 0.5V. So, even though the constant current flows to ground, through the diode, the anode side of the diode connected to the probe and the inverting input of the op amp will always be 0.5V higher than ground. The result is that our test voltage at the probe is 0.5V.

The impedance of the op amp input is almost 1GΩ! That's so high that virtually no current flows through it, so the voltage on it is whatever the voltage drop of the diode is. That's 0.5V in our case, as we just saw. The other probe is connected to ground. When you touch the probes together, the 0.5V flows through the probes to ground, at a current of 200µA. As soon as the probes provide a path to ground, the current takes it, and the voltage, therefore, drops on the inverting input, to almost 0V. In that state, the voltage at the inverting input is lower than the reference voltage, so the output goes high. When the probes are not together, the voltage is 0.5V, higher than the reference, so the output goes low. So, when the probes are connected to something under test which has too much resistance, the buzzer stays silent. When the resistance is low enough, the buzzer sounds. There is one more component, 100nF capacitor C1, connected to the inverting input. This is just for stability, helping eliminate electronic noise that might produce false triggers.

THE OUTPUT

Speaking of the buzzer, the last and easiest section to explain is the output, taken from pin 1 of the op amp. This feeds through 1kΩ resistor R9 to the base of NPN transistor Q2. While the PNP transistor we saw earlier at Q1 works when current flows from emitter to base to ground, NPN transistors function when current can flow from the base, out the emitter, to ground. This allows a bigger current to flow from the collector to the emitter. Normally, there would be a current-limiting resistor between the supply voltage

and the transistor to stop it being overloaded: It can handle 100mA max. However, the buzzers we use draw no more than 50mA, and most draw only 15mA. They also limit their own current, unlike an LED.

So, when the output is high, current flows through the base to the emitter of Q2, allowing current to flow through the collector, from the buzzer, which sounds. When the output goes low, the base current stops and shuts off the collector-emitter current, turning off the buzzer. We use a 1kΩ resistor for R9, giving around 8.5mA of current to the base of Q2. That's because the output of the op amp is not fully at the supply rail, so it's 8.5V ÷ 1000Ω = 0.0085A or 8.5mA. That's more than enough to 'saturate', or fully turn on, the base-emitter junction, which allows maximum current and lowest resistance between the collector-emitter path.

SOUNDING BETTER

The regular piezo buzzers used are both loud and piercing. If it's too loud, you can add a series resistor. The buzzers generally work on 5 to 15V, so reducing the voltage at the current required for the buzzer provides some volume reduction. Ours works at 15mA, and on 9V, we decided to drop 4V to take the buzzer to its minimum voltage. According to Ohm's Law, that's 4V ÷ 0.015A = 266.6Ω. Remember, always convert values to base units. So, 1mA = 0.001A. 266.6 is not a standard value, but 270Ω is. We tried it, connecting between the supply rail and the buzzer's red lead. It made a difference, taking the edge of the shrill sound, but not by a long way.

An alternative is an electromechanical buzzer. Instead of a piezo, these use a coil switching on and off very fast to move a diaphragm. They have a quieter sound but most importantly, they are a more pleasant sound to many, being a lower frequency that is not as sharp. In addition, they cope better with ‘undervoltaging’ to reduce the volume. We used Jaycar's AB3452, but Altrnics sell S6100 which would do the same job.

WHERE TO NEXT

To make this more rugged and usable as a piece of test equipment long-term, it really needs to be mounted on something. While we made better probes earlier, it would serve you well to mount the board on a base, like cardboard or foam core board, so the battery and potentiometer can be secured and not flop around. It also helps to put a knob on the potentiometer.

You can get solder versions of these breadboards which are hole-for-hole copies. However, you will need soldering skills or the help of an adult who can solder. This image shows one such board that we drew on with marker to show where the rails and rows are. This is the kind of thing you'll need to make this circuit permanent.