Mar 14

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How to identify the polarity of a LED

Here I present 3 different ways to identify any LED’s polarity. I hope this is useful for people who are starting to work with LEDs.

1st Way

The longest “leg”is the positive one.

2nd Way

If the LED legs have been cut and are equal, then see if the LED has a flat side on the head. If so, this is the negative side of the LED.

3rd Way

If you couldn’t use any of the above methods, then look directly into the LED. (You must use backlighting if the LED is not High-Brightness).
You will see two plates almost touching each other. The biggest plate is USUALLY the negaive side.

About the author

Ricardo Dias

Ricardo Dias was born in Entroncamento, Portugal and is studying Electronic and Telecommunications Engineering at University of Aveiro. He is a researcher in robotics and is currently the Team Leader of CAMBADA@MSL team. As a hobby, he is also a freelancer Stage Lighting Designer.

Permanent link to this article: http://ricardo-dias.com/2010/03/14/how-to-identify-the-polarity-of-a-led/


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  1. rasb

    regarding the last method, i came across several LED’s that were the opposite…Now thats something that surprised me, but apparently im not the first one to notice that !!
    Obrigado pelo website

    1. Ricardo Dias

      Thanks “rasb”

      Thank you for your reply.
      I’ve also noticed that some LEDs are different in the this aspect. I doubted the quallity of the LED. Apparently, it didn’t happen just to me. Thanks for sharing :)

      Best regards,
      Ricardo Dias

  2. P.rambabu

    Thanks for ur information

  3. Ben

    I’m glad you listed three methods for identifying LEDs cause I had to use them all! Very helpful! Thanks.

    1. Ricardo Dias

      You’re welcome Ben :)


  4. peter desjardins

    I have a small circuit for a flashing LED. This circuit uses the 555 timer The speed at which the LED (D1) is turned on and off is set by the values of R1 and R2.
    Regarding this cct…can I put a potentiometer in-line with the +9v rail to alter the frequency of the flashing. ? The thing is, I found this cct on the internet, but I don’t know if the values of the resistors is ok for a flash of about
    1 Hz / second. So if I build this cct, it may flash like mad, or it may flash very slowly, I have no way of knowing. The values they suggest on: http://www.555-timer-circuits.com/flashing-led.html is:
    R1 = 1k, R2 = 470k, R3 = 1k.
    Would you perhaps know perhaps if the values of the resistors in this cct is good for about 1Hz / second (ie. approx one flash per second??) or alternatively use a variable resistor (ie a pot.). If this can be done, could you recommend the best ohmic value pot. to get , and should I replace R1 with the pot ?

    Many Thanks if you can pass on your technical advice.

    Best Regards, Peter

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