LED Polarity Tester





Introduction: LED Polarity Tester

Ever soldered a LED into your project backwards? It can really mess up a good day!

If you don't know, LED's have a positive side and a negative side, and will only light up when wired correctly. Most LED's have one longer lead to indicate the polarity, but not all. Here's a simple solution to find the correct side BEFORE installing it into your project.

Step 1: Equiptment List

1: 2AA battery pack. Can be found on eBay for about $10 for 12
2: Wire strippers/cutters. C'mon, you don't have a pair these in your back pocket already?

Step 2: What to Do

1. Strip the ends of the two leads coming from the battery holder. 2. Insert batteries.
3. MOST IMPORTANT! Reward yourself with a cold beer. (if under 21, milk will substitute)

Step 3: How to Use

1. Stick your LED in question to the wires, note which wire is + and which is -

2. Install your LED into your project correctly the first time.

Step 4: Other Uses

Testing the polarity of anything is extremly easy now! I also use mine for test hook up's, to test motors, and even to power my lasers. Life, my friends, just got a little bit easier.

Now, after I finish this case left over from step 2, number 3, I'll check the comments section and answer any questions you might have.



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    17 Discussions

    I have a LED sign which i am tring to connect it to power supply. the problem i am facing is only some of the LEDs are lighten and very dem, is this because the current is not enough or could be the wiring.

    please help me out if you know!

    Sign Back.jpgSign Front.jpg



    Go scavenge some busted electronic thing. Cost: Free

    10 to 12 $ go to radioshack or jaycar and buy one for 40c

    Nice Tread that adds to the post. Can those not particularly risk adverse presume that this methods OKish for all but those red ones. Just seen that the last comment was 4months ago oh well..


    You really should be using a current-limiting resistor. Depending on the specs of the LED being tested, this method could burn it out. A limiting resistor in series will prevent destroying or decreasing the life of your LEDs.

    7 replies

    hmm, I see. I need to try this. I haven't had any problems with it, but you're right. how about adding a 100ohm resistor to the circut?

    47 ohm 1/8 watt generally works for me when I work on 12v. or less

    use the following formula to calculate the resistance for the current-limiting resistor:
    power supply volts(3v) subtract from the forward voltage of led (1.7v)
    3v - 1.7v

    now take that answer and divide it by the current specs of led.Example: 1.30v divide by 0.01
    equals 130.00 so yeah a 100 ohm should do.

    an easier way is to use a digital multimeter to test led.Switch the meter to diode test.When the multimeters negative and positive probes are on the correct leads of the led,the led will shine. THUS the led is working.

    2 AA batteries at about 1.5 max each, 3 volts max. Most LED's are about 2. I wouldn't be too concerned about a resistor, as long as you are just using it briefly to test the LED.

    Beware of the 660nm red LEDs' exception : the cup-shaped lead is the anode (and also the longer lead). They've a lower forward voltage too (around 1.7V) and I noticed that some of them suffer from being powered directly from 3V (besides becoming hot, they turn green too !).