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SILICON CHIP Mini Projects #023 – by Tim Blythman Continuity Tester A Continuity Tester is one of the simplest pieces of test gear out there. Still, it can perform functional tests on numerous devices such as fuses, globes, resistors and even diodes. Its simplicity means it can be assembled on a breadboard. C ontinuity Testers check for the presence of a low-resistance circuit, as found in a functional fuse or light globe. Most multimeters have a continuity mode and will make a sound when a circuit with a low enough resistance is probed. A typical threshold (based on the multimeter in front of me) is 150W. Many readers will have a multimeter, but for those who do not, you can simply assemble a handful of components on a breadboard. Even if you have a multimeter with a continuity function, you might still be interested in this circuit and how it works. For example, you could create a continuity tester that operates with a different resistance threshold. You can also use this circuit to trigger near a particular current value. It uses a Darlington transistor arrangement, which is also a handy configuration to know about. Circuit details Fig.1 shows a very basic continuity tester circuit. The LED and its ballast resistor are in a standard configuration. You can imagine that connecting a 150W (or lower) resistance would cause the LED to light up, which is what we want. However, the LED will still light up if a 1kW resistor was connected, even though the LED current is lower. It would be hard to tell the difference in brightness, and thus to tell if we truly have continuity or not. Fig.2 is an improved version. It still has the LED and resistor, but the test points are displaced by some other circuitry. The two PNP bipolar transistors 62 Silicon Chip are arranged in what is called a Darlington configuration (named after Sidney Darlington). This is not restricted to PNP transistors and will work much the same with NPN types. The two collectors are connected together, while the base of one transistor (Q2) is connected to the emitter of Q1. This effectively gives a single device with three leads, similar in function to a regular transistor. Components are even manufactured as such, with two transistors in one package, still with three external leads. This arrangement has the advantage that the gain of the transistor pair is much higher than the gain of the individual transistors. For most scenarios, multiplying the individual gains is a good approximation. There are some downsides. For example, the base current must pass through two PN junctions, so the effective base-emitter voltage drop is doubled compared to a typical single device. We’ll assume with a value of 1.2V (or about two 0.6V diode drops) for our circuit. The arrangement also means that the saturation voltage (between the collector and emitter when the transistor is on) must also be higher, by one diode-drop. If this were not the case, there would not be enough voltage to keep both transistors biased on. In the Continuity Tester, the benefit of the high gain of the Darlington pair is a sharper threshold transition. We can set a threshold current by means of the 470W resistor connected to Q1’s base. Consider a current flowing through the device under test. It will flow through the 1.5kW resistor and then can either pass through the upper 470kW resistor or from the base of Q1 and through the Darlington pair. Below about 2.5mA (1.2V ÷ 470W), all the current flows through the resistor, since there is not enough voltage developed to overcome the forward voltage of the two PN junctions. But soon, there is enough current to cause Fig.1: you might think that a circuit like this could do the job of testing for continuity, but the LED will light up even if a relatively high resistance is probed. Fig.2: this improved circuit adds two transistors in a Darlington configuration. Note the cyan rectangle outlining the two transistors; it has three wires crossing its border. They can be treated as the base, emitter and electronics collector of magazine the pair. Australia's siliconchip.com.au 3mA 2mA 1mA 0mA 100W 200W 300W 400W 500W Scope 1: the vertical axis is the LED current, while the horizontal axis is the resistance between the test probes. The green trace shows the very soft response offered by the circuit in Fig.1. The blue trace of the Fig.2 circuit has a much sharper transition. some to flow through the base of the Darlington pair. With a 5V supply and a red or yellow LED, about 6mA will flow through the LED when the pair is switched on fully. Parts like the BC557 have a gain well above 100, meaning the Darlington pair has a gain of over 10,000. For 6mA to flow through our LED, we need no more than 0.6µA to flow into the base of Q1. To turn this threshold current into a resistance, we choose the value of the second resistor to supply just over 2.5mA when a 150W resistance is placed across the test points. The resistance between the 5V rail and the base of Q1 should be about 1.5kW (3.8V ÷ 2.5mA). Just like a regular diode or transistor, the actual voltage across the PN junction is not always exactly the same, so the actual transition will not be perfectly sharp, but it will be much sharper than for the circuit shown in Fig.1. Scope 1 shows the results of a simulation comparing these two circuits, with the horizontal axis being the resistance between the test points. The Fig.1 circuit produces the green trace, while the Fig.2 circuit is the blue trace. Note that the Fig.2 circuit transitions much more sharply. It still is not a ‘brick-wall’ cutoff, but it is good enough for our purposes. Assembly We have used two of the same type of transistor in our Darlington pair, which works out neatly since they have the same pinout and we can use the layout shown in Fig.3. Note that a Darlington pair will often use a smaller transistor for Q1 and a power transistor for Q2, so that will not always be the case. The purple wires are the test leads, while the power rails on the breadboard should be connected to a suitable power supply. We’ve used a regulated 5V supply from an Arduino board, which is necessary because the supply voltage figures into the threshold calculations. A 9V supply should work just as well, although the value of the 1.5kW resistor will need to change. The threshold current (2.5mA) does not depend on the supply voltage, but the LED current does (due to the 470W resistor). Using it The first test you can do (once you have connected power) is to touch the two probes together. The LED should light up when they touch and stay off when they are not touching. If this is not the case, check your wiring before continuing. You can test out the Continuity Tester on some resistors, fuses or globes. Be sure to only use it on parts that are out of circuit, since it will interact with and possibly cause damage to other powered circuits. Touch one probe to each terminal or lead of the device. The LED will light if the fuse or globe has a low resistance. If the LED Parts List – Continuity Tester (JMP023) 1 small breadboard [Jaycar PB8820] 2 BC557 45V 100mA PNP transistors [Jaycar ZT2164] 1 yellow or red 3mm LED [Jaycar ZD0110] 2 470W ¼W axial resistors [Jaycar RR0564] 1 1.5kW ¼W axial resistor [Jaycar RR0576] 1 5V DC power supply Hookup wire or jumper wires siliconchip.com.au Australia's electronics magazine Fig.3: we laid out our circuit on a breadboard like this, since it is easy to do and you might want to assemble it in a hurry (eg, if your multimeter has a flat battery). is off or dim, then the resistance is higher and the fuse or globe is probably faulty. It is not foolproof, since it only applies a very small current. It’s not uncommon for a fuse to test OK with a continuity tester but then fail in circuit where it has to handle a higher current. On the other hand, a continuity test failure is usually definitive. Other applications A transistor circuit like this is wellsuited to driving heavier loads than just LEDs. The BC557 can handle up to 100mA through its collector, so is well-suited to driving small globes, buzzers and relays if you need a different sort of indication. The relay simply replaces the LED and its resistor. You can use such a circuit to detect a current or voltage. Keep in mind that the relay should be equipped with a diode to catch the inductive spike when it switches off. Also remember that the Darlington configuration will drop almost a volt, even when fully switched on, so your supply should have enough headroom to drive the relay with the reduced voltage. You can imagine that our original Fig.1 circuit would be quite hopeless at driving a relay and that the Darlington transistor is handy at providing the extra current needed. SC This simple circuit can be used to test if things like fuses and globes have continuity, ie, they have a low resistance and are probably operational. March 2025  63