Audible Continuity Tester With Op Amp

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I built this continuity tester to replace my silly battery with a light bulb circuit tracer. If you have ever used one of those, you know that they give false readings at low resistance. Also, hammering your circuit with a DC voltage can damage sensitive components.

By using an operational amp, the input resistance will be very high, and +V will be isolated from the circuit under test with 100K resistors.

The housing used to be a plug-in ultrasonic pest repeller (Goodwill, 99¢). It works great for this project.

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Step 1: Assembly

Since I'm deaf in one ear (and don't hear so good out of the other) I chose to use a speaker taken from a pair of headphones. I set the circuit's frequency and volume to produce a tone that I can hear easily. I just hope the neighbors don't complain about the noise.

The probes are made from cheap ballpoint pen barrels. Probe tips are made from short pieces of 14g copper wire. Use test lead wires that are flexible. Headphone wire is a good choice. (Hey, I got some of that right here!)

Drill out the end caps and insert the probe leads. Drill out the blue plastic nubbins and press in the probe tips. File the tips to a point, and solder the lead wires directly to the tips.

Step 2: The Circuit

For IC1 I used an LM358N dual operational amp. I prototyped the circuit using different op amps. They all worked, so use whatever you have in the parts bin.

IC1A is configured as a comparator. If the +input goes lower than the -input, the output goes low, which will turn on Q1. That is, if the resistance through the probes is less than 10 ohms (or any other value of R3) the output will go low.

IC1B is square wave oscillator. I got the circuit out of the LM358 app notes. As shown, the output frequency is around 1KHz. This can be varied by replacing R4 with a potentiometer.

When the transistor Q1 switches on, the output of IC1B (a 1KHz square wave) passes through the emitter, and switches Q2 on and off at 1KHz. The current from the collector of Q2 drives the speaker.

The emitter of Q1 originally drove a piezo transducer, but the results were not so good. Adding the power transistor Q2 gives enough current to drive a small speaker. The volume can be adjusted by replacing R9 with a potentiometer.

Build it. Use it. It makes a fine addition to any test bench.

Seeya
wotboa

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7 Discussions

The left hand edge of the circuit diagram is missing but this is just what I've been looking for to try and locate a car electrical fault.
I want to plug this in and then jiggle each connector in turn until I find where its wobbly on a CANBUS circuit, watching a meter while I'm standing on my head under the dashboard is not easy.

Nice instructable, with good explanation about how the cicuit works, i´ve made on continuity tester from a 555 because it was the only option i found. I´ll try this.

Thanks

2 replies

Thanks lean04
After building the tester, I wondered why the design (or the concept, really) didn't exist.
All other testers I've seen (including the 555) inject a voltage, then rely on current flow to activate a lamp or buzzer. This greatly increases the possibility of damaging sensitive components.
This tester measures the circuit at the input of the op amp, and uses ground as a reference. There is very little danger of damaging installed components.

Recently i saw a video tutorial from Dave Jones about opamps, and he said something that relates to this. When talking about the "rules" of opamps he mentioned that no current flows from or through the inputs. So the damaging components risk might be none...