Continuity tester circuit using 555

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. If the tester detects the presence of an electrical contact between two points, the circuit indicates with an audible or light indication. The indication of the continuity tester may be either buzzer or LED or both. Here circuit is an audible continuity tester with a beep indicator using 555 tone generator circuit.

continuity tester circuitNormally continuity testers will have a preset threshold resistance value to determine whether the points are electrically closed or open. The circuit detects the continuity if the resistance between the point is below the threshold resistance value. So if a continuity tester shows any break in the line, it cannot be considered as a complete open connection. Because it means it can be either open or with a higher resistance than the threshold value.

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 and 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. Hence the reset pin will enable and disable the tone generator with the continuity between the probe.

The circuit will be useful during electrical maintenance to spot break in lines. 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.

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

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