Audible ohmmeter cuts down on false readings

ON Semiconductor 2N6028

One of my go-to diagnostic tools is a simple home-brew audible ohmmeter (AudOhm). This circuit has served me well for many years, allowing “heads up” testing of wiring harnesses and control equipment, and various components such as transistors, relays, resistors, LEDs, and switches. I created this gadget (Figure 1) because I had been frustrated by the false readings of solid tone on resistances of several ohms or more using a standard DVM in audible mode. Bad connections can fool a DVM tester, but not my AudOhm.

This audible ohmmeter makes use of a programmable unijunction transistor.
Figure 1. This audible ohmmeter makes use of a programmable unijunction

Theory of operation

The AudOhm makes use of the unique properties of an unusual semiconductor known as a programmable unijunction transistor (PUT). These devices, such as the 2N6028 shown in Figure 1, are unique in that they only conduct after the anode terminal voltage drops below the voltage applied to the gate. Conduction ends when the anode current drops below a sustaining level, and the 22 kΩ resistor does not supply enough current to lock the PUT into steady conduction. By creating a bridge circuit to feed the gate, and including the tested circuit in that network, the PUT will produce an audible pulse at a rate directly related to the test circuit voltage drop.

When constructed as shown in Figure 1, using a 20 microfarad capacitor, and a 50 Ω pot, the range is up to about 150 Ω. Once initially constructed, the unit is calibrated just once by shorting the test leads and adjusting the trimpot to cause the tone to begin to warble, and then adjust back to solid tone. It helps to use a 10 turn pot. No power switch is needed: the batteries last the same as shelf life. I use three AAA cells in series.

The AudOhm in action

The AudOhm produces a solid tone when the leads are shorted, and breaks into oscillation as the resistance rises. Once adjusted properly, resistances on the order of a fraction of an Ohm can be heard. Rather than a steady tone for low resistances, the sound becomes modulated at a rate dependent on circuit resistance. The user can tell without looking if the path is a wire or not, preventing accidentally mistaking a component or coil as a wire connection. It can also indicate circuit capacitance, which sounds like a short “pip” that will repeat only if the leads are reversed.

If your work requires sensing resistances outside the range of operation of this circuit, you can adjust the values of the trimpot and the capacitor to suit your requirements. Don’t be afraid to experiment!

Materials on the topic

  1. Datasheet ON Semiconductor 2N6028


1-4 Layer PCBs $2

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