I'm building a 3 ft scale model of a Peterbilt truck with high detail to include multiple 3mm colored LEDs. What type of regulator for differing 2 to 3.2 volts can I use?

Yellow LEDs 2 - 2.2 volt Red LEDs 2 - 2.2 volt Green and white 3 - 3.2 volt


2 Answers 2


I'll assume you're using a suitable power supply like 5V or 12V.

The simple way is to use a current-limiting resistor. You can determine the resistor value in three steps:

  • determine your required current from the LED datasheet

Typical continuous current values for this size of LED (T1-3/4 / 3mm) are 10-20mA. Let's assume 20mA for example. You will probably want to adjust this (more below.)

  • determine your required resistor IR drop

You already know your LED forward drop from the datasheet. You then want the resistor to drop the rest of the voltage.

For a 5V power supply and 3.2V LED, required IR drop is 5V - 3.2V = 1.8V.

  • determine your resistor value at target current

With those two values in hand, we calculate the resistor using Ohm's Law of R = E/I. We have 1.8V / 20mA = 90 Ohms.

For a 2V LED, the calculation is similar: (5V-2V) / 20mA = 150 Ohms

That all said, you will likely want to adjust your values a bit to achieve better realism. Your running and tail lights will probably want less current than the headlights for example.

If you're using light pipes or fiber optics (Have you considered this? It would help with realism) you will want to adjust for those too.

There are two other methods to control LED brightness:

  • Electronic current limiter

Simple current limiters can be made inexpensively from just two transistors. A limiter delivers the same current regardless of supply voltage or LED forward drop. More here: How to drive an LED using a DIY steam generator?

If you have a higher supply available you can combine groups of LEDs in series so they can share the same limiter circuit. This has the benefits of cutting down your wiring and matching the LED currents.

More about that here: Why is my LED circuit drawing 50% more mA than I calculated?

  • Pulse-width modulation (PWM) control

PWM allows you to use a lower value dropping resistor and ‘chop’ the drive to meet an average value and achieve a target brightness. The benefits are greater efficiency and easier control.

The drawbacks? A strobing effect if there is motion involved. It gets especially bad for photography: PWM lamp strobing is a common artifact that shows up on car shows a lot (Audi running lights for example.) Annoying as hell, I hate it; you probably would too on your model Pete.

PWM drive also requires a more sophisticated driver, typically a microcontroller or a timer like a 555 chip.

  • \$\begingroup\$ That would be a painstaking wiring job. (PITA) for hundreds of LEDs \$\endgroup\$ May 25 at 17:08

The truck brand is PeterBILT.

A better solution is to choose StripLEDs that run from 12 to 14.5V with built-in current limiting resistors.

You can get 8 mm tape mounted LEDs cheap and paint over the strip to match the truck and reveal only the LEDs with a remote control on colours.

You choose brightness and/or 1 to 4 primary colours with intensity based on Watts per meter or Amps per meter.

Single colour strips are often 12V.
Multicolour Strips are usually 5V

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There are too many unknowns in your question.

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Truck std. width is 8.5' with a front 10 LEDs shown or approx 0.5" or 12.5mm spacing per LED for 20:1 from 100"

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    \$\begingroup\$ You do understand that OP is building a model, right? 3’ long, which would be about 1:20 scale. \$\endgroup\$ May 19 at 22:12
  • \$\begingroup\$ Yes I understood @hacktastical (-1) pls don't insult my intelligence. thats 10 LEDs on front bumper with 12.5 mm spacing for 100" / 20 = 5" or every 0.5" which is possible on some stripleds. This was not a shopping answer but here's one example of higher density stripleds aliexpress.com/item/1005002246574329.html \$\endgroup\$ May 20 at 0:00
  • \$\begingroup\$ FWIW I didn't downvote you (I've grown sour on the whole thing.) That said, if you did understand that then maybe tune up your answer? \$\endgroup\$ May 20 at 1:14
  • \$\begingroup\$ Thinking about OP's desire for a realistic model, I don't see how strip LEDs would work so well. They will want probably to replicate the shapes and relative intensities of the various lights, which isn't possible with a strip. That's why I suggested fiber optics or light pipes. \$\endgroup\$ May 20 at 1:14
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    \$\begingroup\$ I think the strip lighting would save work if one could be found to be a good match, and there are so many lighting products out there, I think there's a good chance, so good idea, Tony. \$\endgroup\$ May 25 at 16:23

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