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?
Dr.Bill2 years ago
Go scavenge some busted electronic thing. Cost: Free
techxpert4 years ago
10 to 12 \$ go to radioshack or jaycar and buy one for 40c
codongolev5 years ago
I usually just look inside the LED, but that works too.
Da_huuudge6 years ago
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..
RocketPenguin6 years ago
llllllllllllllllllllllllooooooooooolllllllllllllllllllllllllllllllllllllll
inventorjack7 years ago
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.
viacin (author)  inventorjack7 years ago
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?
6 years ago
47 ohm 1/8 watt generally works for me when I work on 12v. or less
6 years ago
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
=1.30v

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.
6 years ago
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.
7 years ago
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.
7 years ago
yes but the current it a consern to
6 years ago
the current on 3volt batteries are low.
Dr.Bill6 years ago
wow
ThierryElec6 years ago
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 !).