How to make a Continuity tester circuit

A continuity tester is a device used to determine the existence of an electrical connection between two points. It consists of two testing probes which will be connected to end contacts in the circuit. It can indicate whether a wire, cable or circuit is electrically closed or not between two test points. The circuit considers the points as a connected path when the resistance between them is less than a preset threshold value. If the tester detects the presence of an electrical contact between two points, the circuit indicates with an audible or light indication. The indicator of the continuity tester may be either buzzer or LED or both.

Simple Continuity tester using 555 timer IC

Here circuit is an audible continuity tester with a beep indicator using 555 tone generator circuit.

Normally continuity testers will have a preset threshold resistance value to determine whether the points are electrically closed or open. If the resistance between the probe is below the threshold, the circuit indicates the existence of a connection with a beep sound. The circuit detects a continuity if the resistance between the point is below the threshold resistance value. So on testing the null indication of a continuity does not mean that the circuit is completely open. Because it means it can be either open or with a resistance above the threshold value.

continuity tester circuit

The threshold resistance value of the circuit can preset by adjusting the potentiometer R4. To set a threshold value, connect a resistance equal to the required threshold value between the probes. Then adjust the pot to a point that just stops the tone. That is, the voltage at pin4 is below enough to reset the 555 IC; IC 555 has an active low reset. The circuit will remain in reset as long as the voltage at the reset pin 4 is below 1-0.4 V. So whenever the resistance between the test probe has value just below the threshold resistance, the voltage at reset pin reaches above than its minimum rest value. At this state, the circuit enables and generates the tone. That is the reset pin will enable and disable the tone generator with respect to the continuity between the probes.

components required

IC – NE555

Resistor – R1 – 1k, R2 – 4.7k, R3 – 220, R4 – 10K pot

Capacitor – C1,C2 – 0.1uf

LS – Loudspeaker

Supply – 9V battery

Op – amp Continuity tester with buzzer

Here the circuit for a simple audible continuity tester with a buzzer. The circuit consists of an op-amp comparator which compares the voltage values at the inverting and non-inverting terminals.continuity-tester-circuit-schematics, continuity tester

The voltage at the inverting terminal (pin2) has adjusted with a potentiometer. So the output switches to an active high state when the resistance between the probe reaches a lower value, that gives a voltage at the non-inverting terminal which is just above than at the inverting terminal.

The circuit is a variable continuity tester so the threshold value can set by adjusting the potentiometer R1.

Calculations to set the threshold value,

Supply Voltage = 6V

By voltage division, V * R2 / (R2 + R

V * R2 / (R2 + Rthreshold) = Voltage across R2

For desired threshold values, substitute the values in the equation on variable Rthreshold.

Calculation for a threshold value, Rthreshold = 100ohms

6 * 1000 / (1000+100) =  6 * 0.909 =  5.454V

The potentiometer should set so as to obtain a voltage just above 5.454 volts at the inverting terminal (pin 2).  Thus the circuit triggers or indicates as the non-inverting pin obtains a voltage above 5.454V. That is whenever the resistance between the probe reaches a value below 100ohms.

components required

IC – LM741

Resistor – R1 – 10k pot, R2 – 1k

B1- Buzzer

Supply – 6V battery

Most continuity testers are with a sound indication only. For a visual indication, an additional LED with appropriate resistance can be added in parallel to the buzzer.

The circuit will be useful during electrical maintenance to spot break in lines without a multimeter. But while using in high voltage lines ensure the circuit has no charge. If the circuit contacts directly to a live line, it can cause electrical shock, damage to the circuit or any other accidents.

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