Build An Everlasting Continuity Tester

When you need a continuity tester at the bench, what do you reach for? Probably your multimeter, right? It may surprise you to know that the continuity tester in the meter isn’t all that sensitive, even if it’s the yellow expensive kind. [Leo]’s will beep even if there is 50Ω of resistance in the line.

Disgusted by modern commercial testers, [Leo] set out to make the ideal continuity tester in the spirit of old school tools that do one thing and do it really well. It had to be simple to use, always ready to go, and capable of measuring continuity at 5Ω or less resistance (video, embedded below).

There’s no power switch or even labels, because it doesn’t need any. Just put the probes where you want ’em, and it either beeps and lights the LED or it doesn’t. It looks simple, but inside that blast-resistant enclosure are lots of cool features that certainly make it seem like the ideal tester to us.

Our favorite has to be the transient blocking unit that works like a little circuit breaker. They’re used to protect circuits from lighting and electrostatic discharge by way of depletion-mode MOSFETs and switches to protected mode in under a microsecond. Watch [Leo] build this workbench necessity and then abuse test it with mains power after the break.

Making your own tools, however simple or complex is a great experience. If you want to up your speedy prototyping game, [Leo]’s got you covered there with a special scratching tool for hand-scribing copper PCBs.

28 thoughts on “Build An Everlasting Continuity Tester

  1. “It may surprise you to know that the continuity tester in the meter isn’t all that sensitive, even if it’s the yellow expensive kind. [Leo]’s will beep even if there is 50Ω of resistance in the line.”

    I mean, it *is* specified straight in the manual. For a Fluke 73, for instance, “If continuity exists (resistance < 210 ohms), the beeper sounds continuously." It'd be really nice if that was range-selectable on most meters, though.

    1. I always thought the beep meant to look at the screen.

      I can understand if you mostly use it for AC, but I like to still have the beep when my connections are terminated. And if I’m testing motor windings or something, 1 ohm might be continuous, and 4 ohms might not be.

      Configurable would be nice, but I’d still be looking at the screen. I still need to differentiate 4 ohms from “infinite.” And if it is configurable then a beep can mean it is continuous, or that the setting is wrong, so I’m still looking at the screen.

      1. This statement is confusing in the blog post – most Fluke meters will beep if continuity is between 1Ω and 120Ω…I fail to see how this project is better. Granted, it is cool to develop a device that does one thing only and kick ass at it!

  2. Had a Radio Shack continuity tester about 20 years ago. It had a nice feature that latched an LED when continuity was interrupted. Found a ton on intermittent cables with that device, but passed it on to the team where I worked that supported cables thinking I’d get another later. I’ve never seen a low cost instrument since with that feature, but it sure would be a great addition to this continuity tester.

      1. in the automotive world you have a tool called a power probe II , it’s basically a very simple voltage meter, it beeps on 0v or +12v(different tones) if I’m hunting down a potentially broken wire I’ll put a current limitedvlimited of 5 or 12v at one end and the probe can be set to either display current voltage , peak high peak low or peak difference through its single button interface. wiggle wire and watch for the peak low to drop down. voilà. same could be done with any decent meter that has resettable high and low peak values and to add for anyone reading, there are newer power probe variants that can do this but they are all very slow, the pp2 will catch very fast spikes on the max and min so imho don’t waste your money.

    1. You can build one with a 555 wired as a one-shot; we have an 8-channel version that we designed to test one of our products for intermittent connections. Ground one end, use a pull-up resistor to supply voltage, toggle the 555’s reset line to put it in the ready state, and as soon as the ground connection breaks for even a moment, the 555 will be on.

  3. It’d be lovely to shrink this and cram it inside a DMM that lacks a fast continuity circuit. Maybe it could sense the LCD pin for the continuity indicator and then shunt the probes and hijack the buzzer with micro relays. Maybe add latched / astable modes. Thoughts?

  4. If you happen to not install the batteries and connect to a high voltage (over 40 V), D3 will route that to the + supply, possibly damaging the system. Best to connect D3 to GND (anti-parallel to D2), since you only need 50 mV there.

  5. I do not agree with the maker that the tool should only do one thing. There are many great tools out there that we use on a daily basis that has multipurpose. A hammer that can also pull nails. A painter tool that can scrap and also ply open cans. A plier that can cut wire. A screwdriver that has a reversible tip to switch between flat and philips. A socket tester that also tests ground fault. I can go on and on. If there is room in the design to accommodate an extra functionality without compromising the main use, why not add it? A multimeter is very useful because it can be used for so many things. In this case, all you have to do is look at the meter when it beeps, many meters will tell you the resistance while notifying continuity. What if down the road you want continuity to mean less than 1 ohm?

  6. This got me thinking. I have no idea what resistance turns on my continuity tester. Maybe what I really need is an audible ohm meter.

    First it made me think, at what amount of resistance would I want it to no longer register? Well, what do I usually use a continuity tester for? Mostly it’s just for checking connections, do I have a short where I want a short and an open where I want that.

    If two adjacent pins are connected via 50, 100 or even 1kohm resistance when I didn’t intend them to be I would want to know that. On the other hand I don’t want it telling me I have a connection where a sloppy solder joint is acting more like a resistor. And I am not going to want have to read the resistance or even listen to an automated voice speak it for every test of every connection.

    How about something which starts to sound somewhere up in the 100s of kilo-ohms? But it starts with a very low pitched tone and increases to a higher pitch (or vice versa) as the resistance drops. It wouldn’t be meant to be super accurate as I would never have the ear for that anyway. But with some practice one could easily hear the difference between a good connection and a bad one.

    Maybe cold solder joints which act as diodes or capacitor would introduce some sort of distortion making that known too.

    1. If you are handy, you can modify a meter with a known chipset, you only need to change a resistance.
      I did this to my UT210E to get less delay and and beep with a higher resistance (the leads are not that great). But you can also go the other way.
      Most of the multimeter ICs expose the sense configuration for continuity and is documented in the manual (for example DM1106EN)

  7. I anyways roll my eyes when someone makes a DIY continuity tester. It’s either a 9v battery, resistor and an LED or 3v and an inverter to make a tester capable of picking up a humid day with the probes 10 feet apart

    This on the other hand is very well thought out and well engineered, good work

  8. Turn this low ohms meter into pitch (555) and not have need to look at anything but the stuff under test, wiggle and shake included. Sound can translate a poor connection into something easy to quantify. Pitch proportionally to resistance, high pitch of beepers is not for hearing impaired. Easy to discern lowest reading with a pleasant mid to lower pitch instead of another “mosquito” noise. I have seen something like this before in one of the Popular zines back in the day. Probably not power-proof though.

    With a good DVM I always have to see a short period of a stable reading to believe it’s real. Yet I have little data of intermittent stuff when the numbers are changing all over the place. Though my Yellow Brand meter is gray it’s analogue bar display is faster reading but it is not fast enough to catch the “only when you press here or do that” flaw that will fail in the field.

    When onstage and a mic is suspect I ask for a rapid monologue or a steady humming tone whilst shaking and wiggling the mic and cable. Please no test…….test……..test………… Data must flow in a steady stream, not come in bursts while bugs still hide within.

  9. While knowing if there is continuity is a lot of the time important, knowing “how much” continuity is next in line of importance and that’s where digital meters do it right after the beep.

  10. It seems that what a lot of the posters here actually want is an old fashioned analog meter with a moving needle. They are still available, and for this kind of work you don’t even need a “nice” one. 2k/V is probably fine, although I am _very_ happy with my old Dick Smith one with a sensitivity of 100k/V.

  11. …Or you could add a variable resistor inline with your meter to adjust the sensitivity. I have done that before to be able to quickly check low resistance circuits that all sound shorted with the default setting of my Fluke meter.

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