Continuity tester using 741 IC

This continuity tester using 741 an operational amplifier, operates as a differential amplifier. A good application for this circuit would be to test welds made on a printed circuit board. It could be also used to test continuity of wires, connections, etc.

How the continuity tester works?

The voltage difference between the inverting terminal (-) and the non-inverting terminal (+) of the operational amplifier is amplified with an of open loop gain (no feedback resistor).

The open loop gain of an op amp is very large and causes the operational amplifier output to go into saturation. This output voltage could be maximum (9 volts in our circuit) or minimum (0 V).

If resistors (R1, R3) and (R4, R5) have the same values, the voltage difference at the inputs of the operational amplifier is 0 volts and the output will be 0 volts.

To achieve the desired operation of the circuit, a 470 ohm resistor and a 10 kilohms potentiometer are included. This allows for a small voltage difference between the 2 inputs of the operational amplifier and causes an unbalance of the circuit.

When testing the circuit, a small resistance appears between the probes. This causes the unbalance disappears and the output of the operational amplifier turns on the LED. For testing the circuit must not be connected to the power supply.

741 Operational Amplifier Pinout

Continuity tester calibration

• Connect a resistance of 4 ohms or fewer between the probes.
• Adjust the potentiometer so that LED just lights up with the resistance that is connected between the probes.
• If we short-circuit the probes (A and B) LED should turn off.

An Off LED indicates that there is continuity and welding or track you are testing is in good condition.

Due to the very small resistance of the tracks, connections and welds, which are tested, it is desirable that the probes (A and B) are of very good quality, very clean and have virtually no resistance.

Continuity tester using 741 list of parts

• 1  741 operational amplifier (IC)
• 2  22k resistors (R1, R3)
• 3  1K resistors. (R4, R5, R7)
• 1  470K resistor. (R6)
• 1  10 ohm resistor. (R2)
• 1  10K potentiometer. (VR1)
• 1  red LED (D1)
• 1  9V battery (B)
• 2  probes (A, B)
• 1  NO (normally open) switch (SW)