Using LED's on R/C models has become very popular. The can dress up a scale model and the latest high-lumen LED's are so bright that they are perfect for night time flying. The only trouble is that most folks are a bit perplexed by the current limiting resistor. Every LED needs one, but determining its resistance and wattage can be confusing. Not any longer -- this online calculator will turn that chore into an easy matter.

This web page is based on work done by Rob Arnold, the creator of a general purpose LED calculator that I found on the web. Although I have added many new features, at the core of the javascript code is basically his talented work.

If you have experimented with LED's then I suggest that you move on down to the calculator and get on with using it. If LED's are still a bit of a mystery to you, then please continue reading.

Basic LED Information

LED's are not at all like a common "light bulb."  They do not have a filament, nor due they use any of the other traditional methods to produce light. Instead, they are a cousin to common diodes (that is why they are called Light Emitting Diodes) and are truly solid state. That means that if they are not abused by excessive current, they will probably last longer than any of us mortals.

LED's are actually current operated devices rather than voltage devices. That means that you can safely use any voltage that is higher than the LED's forward voltage (more on that in a minute) as long as you manage the current. That is the purpose of the resistor. So, to use an LED you will need to install a current limiting resistor and then apply a suitable DC voltage from a battery. In our R/C application, the expected battery voltages range from about 3.6VDC to about 24VDC. 

LED's are polarity sensitive, so you will need to observe how you connect the battery. The LED's anode is always positive (+) and is usually identified by a longer lead (there are exceptions). On LED's with a flat spot around the lens base, the flat mark ALWAYS indicates the cathode lead, which connects to the battery negative (--) terminal. By the way, the resistor can go on either lead.

At one time nearly all LED's would work on voltages as low as about 2VDC. However, with the introduction of new colors, the minimum operating voltages, called the forward voltage (Vf), are now all over the map (1.7 to 4.5 VDC). Unlike a light bulb, low voltages do not cause dim operation. Instead, if the battery voltage is below Vf, no light will be produced. Nada, Zip, Zero. It is important that you note the Vf rating of the LED since this value will be needed when you attempt to use the calculator. Guessing is not an ideal option.

In most cases, you will be using an LED with a Vf of about 4 VDC for blue and white and 2V for all other colors. However, the actual value will come from the data sheet of the part you bought. You can ignore the Reverse Voltage (Vr) specification since it is only important if you use the LED on AC power sources.

Generally speaking, the higher the current, the brighter the LED will be. The operating current you choose will nearly always be about 20 to 30mA. Higher currents are used on some LED's, sometimes as much as 50mA. But, please be aware that if the chosen current is excessive then the part will sadly go to diode heaven. Also, high currents will cause the LED to get too hot, which causes them to dim. My point is, do not push the LED current unless you are sure you know what you are doing.

How Do I Power My LED?

All R/C models have onboard batteries that are great sources of power for running your LED's. Usually there is a 4.8V or 6.0V rechargeable battery that powers the R/C receiver and you can just share this voltage with your LED's. And if you don't go crazy, the extra load on the pack is minimal. 

Rather than connecting directly to the battery, a preferred method is to have the LED's use a spare servo jack on the receiver. If you do not have a spare output, then just use a "Y" cable adapter and have the LED share an output with one of the servos. Tapping into the servo plug is easy. There are three wires; the center one is positive and the outside brown or black wire is negative. The third wire is not used by the LED.

Using the servo output on the R/C receiver is especially useful with electric powered models. They often use 7.2V and higher battery voltages (which is fine for LED use). But, if you connect to the receiver's servo output, then your LED brightness will be consistent as the battery pack is used in flight. This trick capitalizes on the Battery Eliminator Circuit (BEC) that is in the motor's Electronic Speed Control (ESC).

The BEC output provides a regulated voltage of about 5V which your receiver can share with your LED's. However, do not overload the BEC with too many lamps or you may loose control of your model during flight. The number of LED's you can connect will depend on the BEC's current rating, cell count, number of servos, and LED current draw. Please do not ask me for advice on how many LED's your ESC can handle -- I will not know.

The drawing on the right shows how a pair of wingtip LED's are wired into a standard R/C servo connector. Because it only draws 40mA when used with a 5VDC source, this simple circuit can be plugged into a spare receiver channel or Y-connector'd  with an existing servo.

The LED's are polarity sensitive, per the "A" (anode) and "K" (cathode) notations. The resistors go in series with each LED. You will soon see how to use the LED calculator to determine the resistor values.

LED Roundup

LED's come in hundreds of choices, so finding those that are good performers for lighting up a model aircraft is a daunting task. Nighttime R/C pilots are interested in good lamp brightness (high mcd) and a wide viewing angle.

I ended up buying dozens of LED's from Mouser, Digi-Key, Super Bright LEDs, and Electronic Goldmine. I checked them for brightness and illumination spot size. I was surprised to find that many of the LED's did not perform as well as expected, at least in the cases where the data sheets hinted that I should have been more impressed.

My simple tests consist of a projector screen and variable LED current source. On a one-by-one basis, I observed each LED in a dark room. I measured the spot size at a fixed distance and judged the brightness. Using currents in the 20mA to 50mA range, I determined the most efficient value (good light output at a reasonable current). At the end of my tests I assigned a score to each LED using a 0 - 9 scale value (0=poor, 9=excellent).

The table below shows the results from several LED's that I tested. Those not shown were such poor choices that I won't bother to clutter the table with their data. My overall Score is shown in the comments section in the table below.

Legend: Green =Best choice,  Orange = Fair choice, Violet = Poor choice, Red = Don't use.

Part No.	Source	Color	Typ mcd	View Angle	Size	Typical mA	Vf	Score/Comment
604-L7104VGC/H	Mouser	Green	11000	34º	3mm	25mA	3.7V	9 / Wide spot, very high brightness. Recommended.
RL5-W6030	Super Bright LEDs	White	6000	30º	5mm	25mA	3.2V	8 / Wide spot, high brightness. Perfect for Landing lights.
RL5-A7032	Super Bright LEDs	Aqua	7000	32º	5mm	20mA	3.6V	8 / Medium spot, high brightness.
RL5-R8030	Super Bright LEDs	Red	8000	30º	5mm	20mA	2.2V	8 / Medium spot, high brightness.
604-L7104QBC/D	Mouser	Blue	1500	25º	3mm	25mA	3.5V	7 / Wide spot, high brightness. Good for wing tips.
160-1512	Digi-Key	Amber	1800	60º	7.6mm Sq	45mA	2.2V	7 / Huge Spot, medium brightness.
G12702	Electronic Goldmine	Blue	3000	25º	5mm	25mA	3.2V	7 / Wide spot, high brightness. Recommended.
RL5-W10015	Super Bright LEDs	White	10000	15º	5mm	20mA	3.4V	7 / Narrow spot, very high brightness. Good for strobe use.
604-L7114QWC/D	Mouser	White	3200	20º	5mm	25mA	3.5V	7 / Medium spot, high brightness. Good for landing lights or strobe.
RL5-G8045	Super Bright LEDs	Green	8000	45º	5mm	20mA	3.5V	6 / Very Wide spot, medium brightness.
604-L7104QWC/D	Mouser	Blue	2200	34º	3mm	20mA	3.5V	6 / Wide spot, medium brightness.
G12703	Electronic Goldmine	White	2500	15º	5mm	25mA	3.5V	6 / Narrow spot, medium brightness. Good strobe light.
G12993	Electronic Goldmine	Yellow	3000	15º	5mm	20mA	2.2V	5 / Medium spot, low-med brightness.
MV8305	Digi-Key	Yellow	2000	20º	5mm	20mA	2.0V	5 / Narrow spot, med-high brightness.
604-L53SRCE	Mouser	Red	3500	30º	5mm	20mA	1.9V	5 / Narrow spot, medium brightness.
604-L7104SRC/J	Mouser	Red	2300	34º	3mm	20mA	1.9V	4 / Medium spot, low-med brightness.
CMD333UWC	Digi-Key	White	2000	20º	5mm	20mA	3.8V	4 / Medium spot, medium brightness.
604-L7113SYC	Mouser	Yellow	1200	20º	5mm	20mA	2.0V	4 / Narrow spot, medium brightness.
G12769	Electronic Goldmine	Green	3000	15º	5mm	20mA	3.5V	4 / Narrow spot, medium brightness.
G12766	Electronic Goldmine	Org-Red	4000	25º	5mm	20mA	2.1V	4 / Medium spot, low-med brightness.
604-L934SRCF	Mouser	Red	1200	50º	3mm	20mA	1.9V	3 / Medium spot, low brightness. Skip this one.
604-L813SRCE	Mouser	Red	3000	40º	10mm	20mA	1.9V	3 / Huge lamp. Smaller than expected Spot, medium brightness. Skip this one.
404-1114	Digi-Key	Yellow	425	70º	3mm	20mA	2.2V	2 / Wide spot, low brightness. Skip this one.
G12922	Electronic Goldmine	Light Green	?	?	5mm	20mA	2.2V	1 / Very Dim. Skip this one.
604-L934SGC	Mouser	Green	300	50º	3mm	20mA	2.2V	1 / Narrow spot, low brightness. Skip this one.

Using the Calculator

To use the calculator all you need to do is enter three simple parameters:

1. Enter the Battery Voltage. If you are using your ESC's BEC output, then enter 5VDC.
2. Enter the LED's forward voltage (Vf) specification. Get this from the LED's data sheet. If you are using two LED's in a series connected string, then add all the Vf's together and enter it as one value. Total Vf's must be less than your source voltage.
3. Enter the desired LED current you wish to use (20mA works well for most R/C applications). Do not exceed the maximum current rating shown on the data sheet (reduce the value shown by at least 20%).

1. It tells you what the calculated current limiting resistor value is.
2. It finds the standard resistor value from the common 5% tolerance offerings.
3. It suggests the standard minimum resistor wattage you should use.
4. It shows you the resistor's color code.
5. It determines dissipated power of the LED and resistor. You will be warned if it appears unsafe.
6. It looks up the Mouser and Digi-Key part numbers for you. Of course these parts are nothing special, so get 'em from Radio Shack if you wish.

Geez, does it get any better than this?

Step 1: Enter your requirements