headlights for my revo..

[jokerjustin]

im trying to set up some leds to run in my revo... at my local bashing spot there is decent lighting but in some landing zones it gets a bit dark and i lose track of my truck.. i was trying to find somthing that would work off my 3rd channel in the radio so its easily turned off without the need for a switch... i would like them to be as bright as possible without dimishing my runtime...
plus i have a 20$ gift card from the local radio shack so i would like to use it.

i was looking at these since they are the highest intensity ive found..
http://www.radioshack.com/product/in...ductId=3125355

but my prob is that the max voltage of them is 4v.. and im not positive but i think the mmm puts out 4.8? whats a good way to set this up?

i want it to be as small of a setup so its easily hidden

[BrianG]

First of all, those types of LEDs aren't going to be bright enough or have a wide enough dispersion pattern to be useful IMO. LEDs are pretty directional. There are 1W and 3W LEDs that will work better for what you want. You can get the 3W LEDs from various places, but it might be cheaper just to get a couple/few of these and take out the parts you need.

And no matter what voltage the MMM BEC puts out, you are going to want to use an LED to limit the current or it'll blow. Since you are going to use 2 or 3 LEDs, put them in series (along with the proper resistor) and run off the main battery pack. More efficient this way and won't load the BEC for nothing.

Then, you are going to need a switching device to convert the pulses on CH3 to an on/off signal. DimensionEgineering has such a device: http://www.dimensionengineering.com/PicoSwitch.htm

[jokerjustin]

dam.. ill look for some flashlights i can take apart at radio shack... i really want to use the gift card money... otherwise ill prob never use it......

and would each individual led need its own resistor and how would i know which is the correct one?
edit found a led calculator and figured it out...

im using a 4s lipo.. should i use 16.4 as the source voltage or use 14.8?

[BrianG]

If you use regular white LEDs, they typically have a Vf value of 3.5v and a nominal current of 20mA. From experience, they are still quite bright at 15mA. So here is where the math gets a bit sticky.

I would design it such that the LEDs draw 20mA at the max 16.8v. That way, when the battery voltage gets down to around 14.8v, the current will still be high enough to still be bright. For example; if you use three LEDs in series (10.5v total), and want to draw 20mA @ 16.8v battery, you need a 315ohm resistor. 330ohms is the closest standard value, which will result in 19mA. When the battery voltage drops down to 14.8v, the LEDs will be getting 13mA. At that point, you may start noticing some dimming. The problem is the fluctuating supply voltage on a static resistor and LED.

A better way, albeit more complex, is to use an LM317T regulator configured as a CC source so that the LEDs get the same 20mA no matter what the battery voltage is as long as the battery voltage is at least 2v higher than the total LED voltage drop (10.5v for 3 LEDs). Here is such a circuit (sorry about the crude drawing - it was quick):



A similar circuit can be made using transistors, but a little more complex. RadioShack has the LM317T regulator which is why I suggest it. And configuring it as a CC source is very simple.

[jokerjustin]

awesome and great drawing.. so i would need to purchase 3 leds a 68 ohm resistor and the 317 t regulator wired just like your drawing.....

i was planning on putting 3 leds per side.. i think it should be enough?

[BrianG]

Yup, that'll do it. The LM317T, being technically a 1.25v voltage regulator, will provide 1.25v higher than the adjustment pin, which will generate 18.38mA using a 68ohm resistor. This configuration is what makes it constant current. If you want a little more current, a 56 ohm will generate 22.3mA. The three LEDs will drop around 10.5v, add the "overhead" voltage of 1.25v for the 317T, and a little extra for stable running, will be a good match for a 4s pack with a 3.2v/cell cutoff. And the good news is that no changes are needed if you decide to run 5s, 6s, or even more.

Either way, the resistor needs to be at least 1/8w or more, which should be easy to find. BTW: those 68 and 56 values are common values, but RS doesn't seem to have a 56ohm on their site. You can either use a 68 ohm or use 150ohm+100ohm resistors in parallel to give you 60ohm total.

If you want a total of 6 LEDs, you could probably get away making a "3s2p" LED pack () and use a single 317T IC, but due to LED characteristic differences, I would use a 317T, resistor, and 3 LEDs for each side.

So, parts list for two circuits:

2 x LM317T
2 x 68ohm resistor (or 2x100ohm and 2x150ohm) 1/8w
6 x LEDs

Just be sure you hook up the 317T pins correctly or it won't work. There should be a device diagram on the package, but you can find the pinout at all kinds of places on the web.