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Help with LED??



  • Very new to building, please forgive my stupidity...
    I want to use the blue LED that I purchased but I am thinking the output of the Raspbery Pi 3 model B will not be sufficient as I am at an understanding the GPIO output of pin-8 "the port the tutorial I found says to use", is 3.3v; while the spec sheet shows my LED require a constant 3.5v with a possible peak of 4.0v.

    My major misunderstanding is if the Pi is outputting 5.0 v, or 3.3v; any help would be greatly appreciated... From my newbish calculations I would need an 82ohm resistor if the GPIO is outputting 5v, bu if its outputting 3.3v the Blue LED will probably not power on.

    Here is my planned wiring diagram for a LED, Fan, and a simple "power on" button. I am basically following youtube videos, so if I can change my hot to the 5v in the 1-GPIO port and be safe please inform me..
    0_1518300277580_37e1e015-d602-4b2b-bfb0-209f1c0d6c58-image.png

    {Spec information of my Blue LED}
    Part Number: RL5-B5515 - Super-Blue LED (GaN)
    absolute maximum ratings: (TA=25°C)
    PARAMETER SYMBOL RATING UNIT
    Power Dissipation PD 120 mW
    Continuous Forward Current IF 20 mA
    Peak Forward Current (1/10th duty cycle, 0.1ms pulse width) IFM 50 mA
    Reverse Voltage VR 5 V
    Operating Temperature TA -40~+85 °C
    Storage Temperature TSTG -40~+85 °C
    Reverse Current (VR=5V) IR 10 "a
    Lead Soldering Temperature (3mm from body) 260C (for 3 seconds)
    optoelectric characteristics:
    PARAMETER SYMBOL MAX TYP UNIT TEST
    View Angle of Half Power 2ø1/2 30 Degree
    Forward Voltage VF 4.0 3.5 V IF=20mA
    Dominant Wavelength ÿ D 467 nm IF=20mA
    Full Width at half Max FWHM 25.46 nm IF=20mA
    Luminous Intensity IV 5500 mcd IF=20mA



  • I suppose I am going to my local Radio Shack and seeing if they have the proper calculated resistor that I think need.

    with the 5v pin as my +
    the LED Forward Voltage at 3.5v with a Max 4.0v
    and the forward current rating of 20mA

    I calculated that I need an 86ohm resistor at 1/8W to provide a stable power draw...
    Perhaps a 52ohm at 1/8w to be safe and account for the possible Max 4.0v forward voltage??? Perhaps even the 68ohm at 18w and ride the middle road?

    Can anybody confirm my assumptions are correct?
    Am I completely over thinking this?

    I simply like to know I am running stable and safe when it comes to any of my equipment; and mathematics is my kryptonite.

    Thanks to any helpers again...



  • @psychosious This is not as hard as you may think. You will be using ohms law for this and a chart of preferred resistor values. If we assume then that the forward voltage of your LED is 3.5V and it is going to need 20mA for it to light up nicely, we calculate based upon a 5V supply. R = V / I so R = (5-3.5)/I and in this instance, I = 20mA so the full calculation is R = (5-3.5)/0.02. It is important to remember that 20mA = 0.02A and it is this unit that must be used. The answer is 75 ohms. Now if we go to the chart of preferred values, the E24 series will provide you with that value exactly. If you have resistors around, I would be using anything just above or below that so 82 or 68 ohms would be fine because we just calculated based on a 20mA current draw. The led will be fine up to 25mA. These days, I buy LEDs with built in resistors to save board space and improve simplicity of design.



  • @psychosious You can also try to connect to 3.3 rail directly!
    This might work for a status LED (on/off/error). If you want to use it for illumination effect the 3.3V can be a bit low.
    Usually the forward current for blue LEDs are 2.9-3.1V
    Keep in mind, the 3.3V rail delievers only 100mA!
    That's enough for a few microcontrollers and a bunch of LEDs - not enough for your XMas tree!


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