LED Light Bars are a super-easy way to add some extra-bright and colorful illumination to your project. Each Light Bar is essentially a set of three super-bright 5050-size LEDs. They’re offered in a variety of colors including white, red, blue, and green.
While these bars are very simple devices, they do have a few quirks when it comes to using them. Like the fact that their nominal operating voltage is 12V. In this tutorial we’ll go over some of the important specifications of these LED Light Bars. Then we’ll dive into some example circuits that can help you get the most of these nifty little LED assemblies.
A glance at the LED Bars will reveal that there’s not a whole lot required to interface with them. There are two pairs of wire pigtails coming off the sides, labeled ‘+’ and ‘-’. The darker-gray wire connects to the ‘+’ pin, and the white wire connects to ‘-’ on both sides.
These wires supply power directly to the LEDs. A nominal voltage of 12V should be applied to these wires. A lower voltage will work (to a point) but result in dimmer LEDs. Either of the wire pairs can be used to supply power to the LED, and the unused pair can either be trimmed or connected toanother LED bar.
The bars themselves measure about 3 inches across and half an inch wide. Each has three super bright SMD LEDs, which are spaced by about 1.07″.
For mounting purposes there are drill holes on either side of the board, and a peel-away sticky foam on the backside.
The “nominal” voltage for these LED Bars is 12V. “Nominal” as in that’s what’s recommended by the manufacturer. They will work at lower voltages, although that’ll mean sacrificing some brightness.
The table below shows some of the characteristics for each of the LED bar colors. These are values we found while testing the bars out. The minimum voltage was the lowest voltage where the LEDs were at recognizably lit up, although very dim. We recommend that you at least give the LEDs around 7V. The higher the voltage, the brighter your LED will be.
|Color||Minimum voltage||Current @ 7V||Current @ 9V||Current @ 12V|
|White||4.84 V||8.35 mA||25.8 mA||55.2 mA|
|Red||2.8 V||13 mA||29.8 mA||54.0 mA|
As far as current pull goes, both LED colors consume about the same when powered from between 9 and 12V, up to about 55mA when powered at the nominal voltage.
Reverse Engineering the Light Bar Circuit
Looking at the visible components on the bars, it’s apparent that there’s not a lot to them. Three six-pin, SMD LEDs, and an equal number of resistors. We can easily reverse-engineer this circuit to find out exactly how these things work.
Each SMD LED is actually a collection of three equal LEDs. The LED bar’s PCB is set up to string those LEDs in series, with the resistor in-line to limit current. The values of the resistors depend on the color of the Bar. The red bar, for example uses 330Ω and the white bar uses 150Ω resistors.
In most cases, Light Bar assembly begins with stripping some wire. The wire pigtails on the bars are 20 AWG, and should be easy enough to strip with any, old wire stripper.
The wire lengths can be extended, if need be, with a little splice. Don’t forget to cover your splice with heatshrink!
Alternatively the stripped pigtails can be tinned, crimped, or plugged directly into a mating connector.
The ‘+’ and ‘-’ wires of one bar can be connected to another to string them together. More and more bars can be stringed until you start to approach the current limit of the 20 AWG wires – about 1.5A. With some back of the napkin calculations – 55mA per bar, 1.5A max – that’d be 25-ish bars.
There are a variety of ways these LED bars can be controlled and illuminated. Let’s look at a few example circuits:
If you don’t care about dimming the LEDs, the easiest way to power them up is to connect them directly to a 12V power supply. Stick them in your enclosure or project, plug the supply in, and forget about them. If you’re looking for a supply that can source 12V, either a wall wart or a moregeneral power supply should be able to do the job.
Using a 555 Timer
If you don’t have an Arduino around, or are looking for a more analog/elegant/cheap solution, you could use a 555 timer and a handful of common components to generate the PWM signal. Here’s an example circuit:
Most 555 timers can work at up to 16V, so you can run it directly off the 12V supply. Then twist the potentiometer to adjust the brightness. Woo 555 timers!