# Continuity Tester Differentiates Resistance

This simple but clever continuity tester sports two LEDs: RED LED = circuit resistance is < approx 10K, GREEN LED = circuit resistance <10Ω. It consumes approx 8mA from a 9V battery. When not operating, battery discharge current is immeasurable so no ON/OFF switch is required.

## Schematic of the Continuity Tester Circuit

Circuit operation

The unique feature that makes this feasible is the way it applies power when the leads are connected to a resistance. When current flows through D1, transistor Q1 turns on and powers the op amp circuit. When power is applied, the non-inverting input of U1A is biased at 40mV. The circuit under is biased at approx 4mA with the current flowing through LED D1. When the circuit under test is below 10Ω, the voltage developed is below 40mV so the op amp (wired as a comparator) outputs a positive signal and turns on the Green LED. E = I*R = 4mA * 10Ω = 40mV

Op amp selection

I chose the inexpensive LM358 dual op amp—these cost only about \$0.33 each—the 2nd section is unused. A single supply (ground sensing) op amp is required. Note that the LM741 will not work. An interesting property of the PNP Darlington transistor input stage is that when it is reversed biased (@ +9V and Vcc = 0V), it conducts no current. In checking this feature, I increased the voltage to 28V and determined that it conducts no measurable current at this voltage as well.

Stretching the performance

I believe that the sensitivity can be increased to detect <1Ω, however I did not test for this condition. To do so, reduce the value of R1 & R2 to 220Ω so that the test circuit current (and LED current) is approx 18mA. Then reduce the bias voltage at the non-inverting input of U1A to 18mV by reducing R5 to 2.2K. It possibly may be reduced even further, but this may require an op amp with low input offset voltage such as the LM158 (Vos = 2mV Max). (The garden-variety LM358 has Vos = 7mV Max.) Protoboard photo

Note that I used a blue LED rather than a green one.

Enjoy…

## Related Tutorials

• gvorster

Tried a 3.7v small lipo and still works. This is perfect for checking pcb traces.

• gvorster

I built it on a breadboard and it works great! Don’t need the red led as it is way too sensitive for me, so I shorted it.

• jamesag

i love your resistor hack, i registered so i can compliment you on it, lovely XD

• Ezio

This is a clever circuit. I tried a lot to design a continuity checker which both had no on/off switch and be able to detect low ohm.
I overlooked the LM358 characteristics which you used so well: input going to GND, and high reverse voltage limit of the input PNP transistors.
Truly good. Thanks you.

• Jim Keith

The 2nd op amp section could be easily wired to provide another voltage (resistance) threshold by dividing R5 into 2 resistors (perhaps 3.3K & 1K) for a 2nd (lower) reference voltage. This would provide a 2.5Ω resistance threshold.

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