Make a Continuity Tester





Introduction: Make a Continuity Tester

Here's a fun little project I did while bored. Then I got even more bored and made up this instructable. I describe the parts needed, the schematic, and the breadboarding process. The rest is up to you.

Step 1: What You'll Need.

You'll need:
1. an LM339, or LM393 comparator IC
2. two different color LEDs
3. two LED resistors ~300ohm to 1k in value
3. piezo buzzer (not transducer/speaker)
4. 4 identical "high value" resistors, about 5-10k or so
5. 1 "low value" resistor, somewhere's between 20 and 600 ohms
6. A breadboard and some leads.
7. A power supply. AFAIK the LM339 can handle anything up to about 14V. But you will need a minimum of about 3V to drive the LEDs and the piezo good 'nuff. Too much voltage and you risk damaging the circuit you are working on. Batteries will do fine. A couple of alkaline button cells or a single lithium coin cell would do nicely.
8. About ten minutes of free time.

Step 2: Schematic

Don't forget to clik on pic 2.

Step 3: Build

Check out the pics for step by step instructions.

If your circuit has "issues" try increasing the value of the sensitivity resistor. If it's too low, it might be swamped by variations in the 10k resistors. Alternatively, swap around the 10k resistors until it works, properly.



    • Oil Contest

      Oil Contest
    • Clocks Contest

      Clocks Contest
    • Creative Misuse Contest

      Creative Misuse Contest

    16 Discussions

    This was a nice project can it be used to test for continuity of a category cable?

    hello i want to use an LM393 to make the smallest possible light, or line following robot, it's for a competition, help is much apreciated as i am only familiarized with atmels (arduino in my case) but i heard these IC are pretty usefull, and as i found one, for free(:D) i was wondering how that could happen, i have LEd's resistor cap's etc... help me please please please :)

    4 replies

    Well, a comparator compares 2 voltages. It sinks current through an open collector output when 1 input has a higher voltage than the other. So you would need a sensor that produces an analog range of voltage. The comparator would give a signal when the voltage reaches a certain threshold. IR LED/receiver is one way to do it. I never used an arduino, but most of the PICs I use have comparators built in. Arduino's certainly have ADC's. This is like a comparator on steroids. By using an ADC you can give your Arduino more numbers to play with. With the right code, you can make your sensor "smarter," using an ADC vs a comparator.

    yer but the arduino is too big for the size of the bot =S i'm just using a light detector, with a transistor and some stuff, i actually have an instructable about that...

    i'm about to start with PIC's :)

    Hmmm. I'm not quite sure of myself in this territory. I know you'll find some good ideas by googling "light following robot." I know you can do it with comparators and IR LEDs, but I just don't quite remember how. :P

    lol =| maybe its an over kill just for a light follower. i'll use an IR transistor to avoid objects... muahahahaha

    I use the electronics of a musical greeting card as a continuity tester. I soldered two wires to the contacts that connect when you open the greeting card, and on the other end of the wires I soldered pins from an IC socket as tips. The batteries last long, and you don't have to look at it as it makes music when you have continuity. It's fast enough to brush one tip over a multi-pin connector and still hear which of the pins is the one. I just put it into a small zip-lock bag to isolate it.

    it's one of the functions of a multimeter to test a connection for continuity... It's one of the best ways to check for broken wires and connections.

    Yep, that's correct. To do this, a continuity tester sends a little bit of current through the connection you are testing and checks the voltage drop at the other end. This is a "real" continuity tester that is safe to use on sensitive components. If you use 3V battery and 10k resistors, the output impedance is relatively high. The maximum current flow is about 150 microamps. If you use the simple "battery and LED" method with 3V and a 500 ohm resistor, you will be putting up to 2.6 milliamps across your connection.

    Yeah, do you like it? The ground probe is connected with a removeable plug, and it stores in the body. I cut channels on the back, and all the wiring is routed in these channels, so nothing protrudes. Then a layer of clear packing tape over top keeps the wiring from coming out. The IC and all resistors are stuffed in the square cutout in front of the button. The button is a tactile, momentary push-to-make switch. It's on the instant you press the button. There's no annoying dial or startup delay, as there is with multimeters and their sophisticated firmware. But this is just as accurate and safe to use on delicate circuitry. The automatic shutdown is brilliant... let go of the button. I figure the batteries might last 10 years. :)

    Sorry bout the schemmy. I don't have much patience in working with graphical-type programs. But I think the schemmy should be workable if you zoom in on it or print it out.

    Hmm, that's a good question. I just looked at some other instructables which have been rated, and I don't see a "rate" button anywhere. Two people have rated this, though... So someone must know, or maybe some work is currently being done on the site?