Tweezing Diodes

Surface mount diodes are simple enough — all you need to do is make sure you have the anode and cathode in the right order when you place them on the pad when you solder them. These SMD diodes come in industry-standard packages, but do you think there’s an industry-standard way of marking the cathode? Nope, not by a long shot. To solve the problem of figuring out which way the electrons go through his LEDs, [Jesus] built a simple pair of LED tweezers.

The purpose of these tweezers is to figure out which way is up on a LED. To do this, [Jesus] picked up a pair of multimeter and power supply compatible SMD test clips that are sufficiently tweezy. These tweezers come with red and black wires coming out the back, but cutting those leads off, peeling back the insulation and adding a CR2032 battery holder and 220Ω resistor turns these tweezers from a probe into an electrified poker.

To figure out what the arcane symbols on the bottom of an SMD diode mean, all [Jesus] has to do is touch each side of the pair of tweezers to one of the contacts on a LED. If it lights up, it’s that way around. If it doesn’t light up, the battery is dead, or the diode is backwards. It’s a great project, especially since these SMD test clip tweezer things can be had from the usual online retailers for just a few bucks. We would recommend a switch and marking which tweeze is ground, though.

37 thoughts on “Tweezing Diodes

      1. I once stripped an old USB cable to power another project.
        The regulator on the new project kept getting really hot, and it took me a while to figure out why: the USB cable’s positive was black, and the negative was red.

        I don’t know how you even do that, short of plain malice.

      2. Yes, especially in the early days of LEDs. I’m not even sure I was aware that one lead was longer, but it was also a time of getting LEDs that might not be the same brightness. So early on I took a partly worn down 9v battery, the were handy, and the terminals are close together, and soldered a 1K resistor to one of the terminals. Then it was easy to find an LED of the right color (another thing that wasn’t always obvious back then, and immediately know which lead was the cathode.

        I need it less now, but I still do it, it’s one way of using up 9v batteries routinely taken out of smoke detectors.


      1. Reminds me of the VCR days. I used to call those little ‘C’ clips or ‘E’ clips by the analogy ‘thousand mile an hour clips’. Where it went – nobody knows.

      1. Diode-test is meant to return a voltage. My crappy old DMM seems to have a ~2mA current-source, and will display the diode forward-voltage so observed with that current. It will happily light a white LED while showing 3.6V on the display.

  1. I like it. I work through a microscope a lot. I would (will as soon as I make some) use this with a foot switch to test parts as I place them. Add this to the solder needles, which came Monday, and really work great.

    1. If it were only that standardised…

      See the comment up top about differing marking conventions within the same manufacturer!

      Admittedly, for a *single* model of LED, from a single mauf, the markings on the rear should be consistent….

      But… (Of course there’s a but)

      I’ve seen parts from… Osram, IIRC, that flipped the god damed package orientation, relative to the die, ON THE SAME MODEL, but a newer batch. WHY. Had to rework more 0603 reverse mount diodes than I ever want to think about, as the fab house missed it during assembly, and our documentation team were not aware of the up-revison of that part.

        1. Yes, I agree. I would have liked very much for the sub con to have done that test. It would have saved me an awful lot of hassle.

          My issue was more that a supplier changed the relative markings of the case (w.r.t) die, mid-production cycle of a product. All I saw, unfortunately, due to the relatively small volumes was an eventual update of the datasheet at Farnell! Our volumes, and purchasing structure, meant we were a long way away from any product change notification, if there was ever one issued…!

  2. I once worked as a surface mount machine operator. One feature of the machine I found interesting was a pair of jaws on the nozzle that could re-center the part or do electrical tests. I know it did resistance and capacitance but not sure about diode checking. At least normaly (but not all the time) they are all in the reel the same way

  3. You should be careful with pasting links. This link is all you needed –

    We don’t need to know all your browsing habits and perhaps you should be guarding your privacy and protecting your online transaction accounts. An ebay account is often linked to a paypal account so I would be removing the remember my paypal from ebay just in case. Perhaps you can report your own post to HAD so they can remove it or update the link.

    1. Would that happen to be the side that has a slightly wider bezel than the other? I ask because that’s what I always looked for and it was almost always how I identified the positive side. Have to emphasize slightly because it really is a very slight difference on some of the ones I’ve seen in modern devices. I have really good vision but some of those things take a bit of focus to notice it. Helps to sit it next to a ruler that has MM markings, there isn’t a whole MM of difference but it makes the difference pop.

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