Step 2: The LED

Picture of The LED
LEDs come in different sizes, brightnesses, voltages, colors and beam patterns, but the selection at Radioshack is pretty small and so I just picked up a couple different LEDs from what they had in a few different brightnesses and voltages. I kept close track of what LED was what voltage because I didn't want to accidentally send too much current through one of the low voltage LEDs.

The first thing I did with the LEDs was figure out which wire (its called an electrode) was positive and which was negative. Generally speaking the longer wire is the positive electrode and the shorter wire is the negative electrode.

You can also take a look inside the LED itself and see whats going on. The smaller of the metal pieces inside the LED connects to the positive electrode and the bigger one is the negative electrode (see picture below). But be warned - in the LEDs I picked up I didn't always find this to be true and some of the LEDs had the longer electrode on the negative when it should be on the positive. Go figure - its OK though, if it didn't light up I just flipped it around.

Once I knew what was positive and what was negative I just had to remember what the voltage of each LED was.

All my LEDs recommended 20mA of current. 20mA is standard for most LEDs.


The two connections are called ANODE and CATHODE not positive and negative, the long wire/short wire identification is not consistent between manufacturers and once they are cut off becomes useless. The flat on the side of the moulding will indicate which lead is which (refer to the data sheet for your LEDs. Many LEDs include an integral zener diode which will limit the reverse biased voltage, should it be connected the "wrong way round", but without a series resistor this could destroy the diode). The attached pic shows how a diode's connections are identified in an electrical circuity. To make an LED illuminate it needs to be forward biased, ie the anode needs to be positive relative to the cathode. The series current-setting resistor can go on either side of the diode

mikeybee8 years ago
Here's another, pretty sure fire way to tell which side is (+) or (-): Most LEDs come with a very small raised ridge at the base of the 'bulb'. If you look closely at that ridge, it's not a complete circle -- there will be a small notche-off portion. This notch-off, from what I've found so far, is always lined up with the (-) side of the LED.
This is by far the best way to distinguish which lead is an anode and which is a cathode.
zombiefire5 years ago
mine are the same size HELP!!!!!!!
peepee45 years ago
i need leds!

freerunnin16 years ago
the flat side on the bottom ring of an LED is the negative:)
The longer lead is not always the positive (anode). Some LEDs are backwards (such as many infrared LEDs), where the short lead is the anode and the long lead is the cathode. This site has more info about designing simple LED circuits and they also offer tech support via phone or email: http://www.LunarAccents.com
downgrade8 years ago
Also, on almost all LEDs the leads will be differnt lengths, The longer one should be the positive.
ThierryElec8 years ago
On 660nm "deep red" LEDs, the reflecting cup (and also the bottom of the LED die) is the anode : this is the exception to the general rule. Sorry, I first posted this comment on the "view all steps" page the first time.