In this we post we discuss an accurate continuity tester circuit whose measuring capacity could be adjusted depending on the resistance range of the load under test. The range of the continuity tester could be set from a minimum resistance between the test probes to a maximum of 50 ohm test, the output being indicated with a buzzer sound.
Whenever the resistance between the test probes is lower than 100 ohms, the continuity tester on most digital multimeters would display a closed circuit.
On the other hand t he continuity-test resistance level could be adjusted anywhere between the resistance of directly the test probes and 49 ohms with the continuity tester discussed in the article. The device is particularly good for detecting bad connections in multiconductor cables which have identical wire length and wires thickness.
How the Circuit Works
The continuity tester's schematic is shown in the diagram below. The non-inverting input pin#3 of opamp IC1 receives the voltage at the intersection of R4 and R5 (approximately 0.09 volt).
As soon as the test probes are coupled to an external resistance, this resistance gets wired in series with R5, lowering the voltage at IC1's non-inverting input pin #3.
This op-amp has a gain of 100 and is designed as a non-inverting amplifier. Its output pin #1 is wired to IC1's inverting input pin #6, which is set up as a comparator. The voltage chosen by R3 is seen by the IC's non-inverting input pin #6. The IC's output pin#7 turns high as soon as the voltage at its pin#6 input is higher than the voltage at its inverting input pin#5.
Because of this transistor Q1 gets activated, and the buzzer sounds. You can choose the top resistance value where the buzzer starts sounding by altering R3.
Only if the resistance between the test probes is lower than the predetermined level would this buzzer activate. The set point is modified by connecting the test probes to a known-value resistor. (Make sure the test probes include pointed tips that can pierce any dirt or oxide on the surfaces.)
In order to check multi-conductor wires, configure the set point with the test probes across a known good connection.
The cable's remaining wires also should be tested for continuity. Since, the current from the 9-volt source should first travel via a 1K resistor, the current through any circuit under test will not surpass 9 milliamperes (R4). R3 is a linear-taper potentiometer that may be crudely adjusted to provide a rough estimate of the resistance being monitored.
R3 could be changed with a 10-turn precision potentiometer, and the remaining resistors could be substituted with metal-film resistors with a low temperature coefficient in case you wanted a more precise device.
Make sure to check the resistance of the test probes while calibrating the adjustment dial, and make sure to incorporate the same set of calibrated leads while collecting your readings.
The device may be calibrated to work like an accurate low-level ohmmeter or to test the length of a coil of wire depending on its resistance after it is correctly calibrated.
Derek says
What would the voltage across the probes end up being? In other words what would the voltage the wire or unit being probed see?
admin says
It can calculated using a voltage divider formula.
Derek says
Ok I see it now. If I probed a 1K resistor it would be in parallel with R5 making a voltage divider with R4. So the voltage is always less than a volt with a 9V supply since your load is in parallel with R4.
Thanks for sharing the circuit.
admin says
That’s correct, thanks very much for your valuable feedback…