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I have a Raspberry Pi 1 Model A and am trying to control an LED from an app that I will be running on it. The wiring I'm planning on going with is:

enter image description here

Even though thats a RPI 3 in the diagram above, the concept is the same:

  1. Connect the RPi to a suitable power source (via Micro USB)
  2. Connect the 3.3V pin on the RPi to the power rail on the breadboard
  3. Connect the column where the LED (1.2V @ 100mA) will be placed to power through a resistor (47 ohm)
  4. Then just connect the column where the LED's 2nd leg is located back to a GPIO pin that is configured to be an output pin

So I wanted to verify (I'm afraid I'm going to fry my pi!):

  • That the wiring is correct; and
  • That I'm using the correct resistor to do the job (R = V/I = (3.3V - 1.2V)/.1A = 2.1V / .1A = 21 ohms...so I figured 47 ohms is OK); and
  • That I should be configuring my GPIO pin to be on output pin afterall

Thanks!

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  • 1
    A LED that is 1.2V/100mA, is it a special one, high intensity? Typical values I have seen is more in the range 1,8V/20mA for red LED's.
    – MatsK
    Sep 15, 2017 at 19:23
  • Hi @MatsK (+1) it might be...its all they had at my local RadioShack though. Will my resistor be enough for it? Is it somehow incompatible with the RPi? Or am I fine?
    – smeeb
    Sep 15, 2017 at 19:25
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    Possible duplicate of Why won't the LED turn on?
    – Brick
    Sep 15, 2017 at 19:26
  • Phrasing of the question is not identical to the duplicate, but the answers are directly relevant.
    – Brick
    Sep 15, 2017 at 19:27
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    The Raspberry Pi's GPIO can drive 16mA and 100mA is far above that! And please use U for voltage, V is for Velocity ;-) And if you do recalculate you will get a R = 75-100 ohm.
    – MatsK
    Sep 15, 2017 at 19:32

1 Answer 1

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There is some considerations:

  • A maximum of 16mA per GPIO with the total current from all GPIOs not exceeding 50mA.

  • Calculate the resistance, its the limiter for the current.

    Normal data for a red LED: ~1.8 volt, 16mAmpere (0,016A)

    U = 3.3V - 1.8V = 1.5V

    R = U / I

    1.5V/0.016A = 94 ohm

And regarding the schematic, this is working, your a bit turned around. enter image description here

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    You cant use that LED ! 100mA is more than the 16mA that the GPIO can be loaded with!!!!! And that I have stated several times!!!!! So buy a LED that is approx in the 16mA range.
    – MatsK
    Sep 15, 2017 at 19:53
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    Oh I can read what you've wrote perfectly fine! You've told me (like you said) several times that I can't use this LED. But you are confusing me because you're using an example that uses a 1.8V LED rated at .02A, and I'm not sure where that's coming from. Is that just an example you're providing here?
    – smeeb
    Sep 15, 2017 at 19:56
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    Sorry for being annoyed. Here is some quite common values for LED that will shed some light over the issue: upload.wikimedia.org/wikipedia/commons/d/d6/…
    – MatsK
    Sep 15, 2017 at 20:00
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    OK, I think what you're trying to say is that if I had a 1.8V LED rated at 20mA, then I would need 3.3V - 1.8V / .02A = 75 ohm. If that's the case, and you're just citing this as an example, then I follow you 100%.
    – smeeb
    Sep 15, 2017 at 20:05
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    It would not hurt to try the LED w/ 16-20 mA. LEDs simply get dimmer with less current, but it might well be bright enough. I'm dubious an "only one I could find at Radio Shack" LED is rated at 100 mA. Note that they will burn out if you overcurrent them.
    – goldilocks ♦
    Sep 21, 2017 at 16:28

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