I have 5 of these plastic moldings that each have 2 RGB LEDs inside along with a microcontroller that rotates through the color spectrum for aesthetics. I want to bypass the microcontroller so that I can control the LEDs with my own supply.

I plan to use a 5v source to power them. I plan to wire them like below, except there would be 10 LEDs for each color instead of 2. To simplify my question, let's assume that I just wanted to power all LEDs at full brightness (white).

LED diagram

I've taken some measurements of these LEDs while they're operating (the color was white so they're all at full brightness at the time).

Here's what I measured from their current setup from the microcontroller:

  • Red: 2.0v drop, 16mA at full brightness. I calculated I would need a 19 ohm resistor (R1) to bring the voltage down to 2.0v.
  • Green: 2.85v drop, 11mA at full brightness. Calculated 20 ohm resistor (R2) needed.
  • Blue: 3.0v drop, 10mA at full brightness. Calculated 23 ohm resistor (R3) needed.

Are these three resistors all I need to account for with this set up? Do the LEDs need to be current-limited, or is that already done with the resistors?

  • 1
    \$\begingroup\$ You have everything you need. The resistors is what limits the current. Multiply total current draw with 5 V to get the power consumed. \$\endgroup\$
    – winny
    Apr 15, 2018 at 10:11

1 Answer 1


You've got it covered pretty well. Just a couple of very minor points.

It appears by the LED's not having dimmer capability you want white light. The low power LEDs do not mix very well. You should use LEDs like the Lumiled C line or CZ line which are designed to have good color mixing. White comes in many varieties of CCT and CRI. CCT determines the Kelvin Temperature (warm, netural, cool) and CRI is how close they match natural sunlight. White LEDs are less expensive and are specified in CCT and CRI.

When selecting LEDs you may want to consider the Photopic Luminous Efficacy of the Relative Sensitivity Curve for the C.I.E. Standard Observer.
According to this curve when compared to a 555nm green with a radiometric intensity of 100, a 450nm blue LED would be perceived as 3.8 and a 660nm red would be 6.1. If you are using a 470nm blue they are usually specified in Luminous units (cd or lm) rather than the 450nm deep blue which is specified in radiometric watts. Same applies to 660nm red (deep red) vs. 625nm red-orange (commonly called red)

There is usually a big difference in the specified luminous and radiometric intensities between R, G, and B LEDs. If the LED's intensities are specified in cd or lm you may want to adjust the resistors to match the actual luminous intensities. Matching radiometric LEDs also requires the calculating Photopic Luminous Efficacy.

Links: Photopic Luminous Efficacy
Irradiance Conversion Factors

A 3.3V supply would be better than 30% more efficient.

It is best to use the actual measured Vf to calculate the resistor values.

I get slightly different values for the resistors.
Red 18.75Ω (actual 18.7 or 19.1)
Green 19.55Ω (actual 19.1 or 19.6)
Blue 20Ω

BTW: You said "19 ohm resistor (R1) to bring the voltage down to 2.0v." the resistors limit the current. The LED's forward voltage (Vf) characteristics determines the voltage.


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