I happened to buy a whole bunch of cheap LEDs a few days ago and figured I'd use a couple of them as lighting on my scratchbuilt plane. After doing that I've decided to post a very, very basic little how-to for anyone who wants to try their hand at adding LEDs to their plane, but isn't quite sure of how to wire them. To those of you with some electronics knowledge, perhaps you can help me out on the bits that I'm not too sure on
I'm going to assume pretty much zero electronics knowledge for this, so the over simplification isn't just me being patronizing. I'm not going to explain what series and parallel are though, you can use the internet for that.
First off, I've only got a basic understanding of this myself, so if you follow these instructions and your plane fails to work/catches on fire/looks silly/becomes sentient and steals your car, it's not my fault. You have been warned.
Anyway, here's how I went about doing it:
1: First off, you'll have to get some LEDs. I bought a bunch of 5mm LEDs from Dealextreme, though there are many, many places to buy them cheaply. I'd recommend against getting them from an actual shop, as they tend to be way more expensive. For what you'll be using them for, they don't need to be top quality brand name LEDs. Cheap ones from the miraculous internet will suffice. For this little project I went with 2 bright blue LEDs, for a couple of reasons. Firstly, I didn't have any resistors on hand so the more LEDs I used, the harder it would be to find suitable resistors in the clock radio I'd torn apart for this very purpose. Secondly, my green LEDs hadn't arrived yet, so to do one on each wingtip, green and red in the traditional way wouldn't work. Hence why I went with blue.
2: The second step for me was to lightly sand my LEDs to make them a bit less directional. As they were, there was very little light emmited from the side and a helluvalot emmitted from out the front. Sanding them down a bit makes them look better from more angles.
3: Now you've gotta decide how your going to power them. I decided to make mine plug into a spare channel on my receiver (the batt channel in this case). It seems that my particular reciever is wired with all the neutral pins wired together, and all the positive pins wired to eachother. I'd assume that all receivers are like this but I can't be sure, so it's best to check how your particular set is wired before you do this. Because of this, it means that by plugging my LEDs into the spare channel, I'm basically plugging my LEDs straight into my ESC's built in BEC.
If you decide to power your LEDs in this way, be mindful of the current draw. Each LED only draws about 20-40 ma, and this will all come out of the availabe current the BEC is supplying which also has to run your RX and servos. In my case, my ESC had a 1.5 amp BEC, so seeing as I'm only running two servos from it, my little twin LED system isn't going to be a problem at all.
You do have a couple of other options on where you get the power from. The first is to wire an adapter so that you are using the voltage straight out of your flight battery. The upside of this is the higher voltage you'll get, which means it's easier to wire lots of LEDs and you'll need less resistors to do it. The downside is that the battery voltage will fluctuate, so you risk either running too much current through your LEDs, or having them get very dim as the battery wears down. Not really an ideal solution.
The other option is to use another power regulator, either a standalone BEC or some other circuit to run your LEDs off. It'll add weight, but it means that if you stuff up your LED wiring and they short in midair, you are less likely to come plummeting out of the sky, which is what will happen if your LEDs short out and you're using the same power supply that your receiver is running off.
4: Next up, it's time to start drawing your circuit. You can choose to wire them in either parallel or series, or a combination of both. The basic rules you need to know are these:
Every series string of LEDs needs at least one resistor to limit the current flow. This will prevent the LEDs from entering 'thermal runaway' and burning up. Basically the resistor will limit the current and keep your LEDs safe.
Every parallel circuit needs its own resistor as well. You can't just wire two leds in parallel with one resistor. You would need one resistor for each LED. Think of each LED as it's own little series circuit: You can have as many LEDs in these little series circuits as you want, but you must have a resistor in there too. Essentially, every single LED will need at least one resistor in series with it
. So 4 LEDs in series will need only one resistor (as each LED will still be in series with a resistor), but if you broke that circuit up into two smaller ones in parallel, with two LEDs in each, you would need two resistors (so that every LED will still be in series with one resistor) Confusing? That's what happens when I don't proofread and edit for clarity. Moving on...
As you add LEDs in series, the voltage required adds up. When you read 'foward voltage' in the specs of an LED, it just means the voltage that LED runs on. The 'foward' is because current only flows through an LED in one direction. You can't have a string of LEDs with a total voltage requirement which is higher than your supply voltage, as the LEDs won't be able to get the voltage they need to run correctly. It is OK to put LEDs with different voltage requirements in both series and parallel, it doesn't affect anything really.
In any series circuit, the current will be the same through the whole thing. This means that if you've got LEDs with different current requirements, DON'T put them in series, as you won't be able to give each LED the right current. They will all have the same current flowing through them. If you want to use an LED with a different current requirement in your circuit, put it in parallel and give it it's own resistor in series (as we must).
Apparently (I'm not too sure on this myself), you are supposed to make sure your series LEDs require at least 2 volts or so less than what your supply puts out, to make up for the voltage lost across the resistor. I'm not sure if this is accurate though, because it seems to me that the formula used to figure out the resistors we need ought to compensate for this by selecting a low resistence. I dunno. I'll wait until somone who knows what they're talking about posts and then edit this. Until then, try and leave a few volts between the required voltage and your supply voltage.
5: Now you've got to draw a diagram of your circuit, and so do I. As I couldn't be bothered scanning my terrible diagram from earlier, I'll draw up a quick one to show how I wired my circuit. As previously mentioned, I'm running my LEDs from the 5V present at the receiver, and I'm running the two blue LEDs in parallel, with their own resistors.
Aah, there we go. Paint skillz to the rescue:
Ok, that's the diagram of my little circuit. To those of you with utterly no electronics experience, the little bit down the bottom reperesents my power supply, the triangle type things are the LEDs (I've probably drawn them backwards), and the little sqiggly bits arn't where I got the wires tangled: Those are the resistors. We'll find the resistance they need to be in a moment. So lay out your diagram, making sure the series bits don't require more voltage than you've got and ensure you've got a resistor in series with every LED. Now it's time for...
6: Math: There's a simple little formula you can use to find out what resistors you're going to need, though there are also online calculators that will do it. We'll use the formula though.
Essentially, to work out what resistors to use, you do the formula for every series circuit you've got. I've got two series circuits (in parallel with eachother). I'd have to do the formula twice, but it turns out both of my resistors will be the same, as both of my series circuits are identical.
The forumula is this: R= (Vs - Vl)/I
Because I didn't use the proper subscripts there, I'll explain what I've written to avoid confusion. Vs and Vl arn't
VxS and Vxl. 'Vs' is the voltage of the supply (hence the s), and 'Vl' is the combined voltage required by the LEDs (hence the l (I know, it looks like an I)) For multiple series LEDs, you add their total required voltages to find the value you need to use for Vl. The I stands for the current, in amps
, you want to flow through the LEDs.
So in my case, resistance (R), equals 5V (Vs, voltage of supply) minus 3.4 (The voltage required by my single LED, Vl), all divided by 0.03 (the current in amps I want my LEDs running at). Remember, you'd usually have to do this formula once for each resistor, but seeing as mine will be the same I don't have to.
You'll typically want about 20-30ma (milliamps) for this type of LED, with the voltage pretty easy to guesstimate by the colour. Google it and you'll find charts showing typical voltages in relation to LED colour. Remember, you must put the current into amps for the formula (so divide ma by 1000)
So the result of my formula, R=(5-3.4)/0.03 is 53. So ideally I need a 53 ohm resistor. Unfortunately I don't think 53 ohms is a standard value. So what I should
do is use the next highest value (as this will put the actual current just a little below what you wanted). What I actually
did was use a resistor of about 50 ohms, for two reasons. One, my LEDs ought to be able to handle a bit more than 30 ma of current. Two, I didn't have any spare resistors on hand and 50 ohms was the closest I managed to find by prodding around in an old clock radio with a multimeter
7: So now it's time to solder it all up. I used some very thin, enamal coated wire to save weight and increase the risk of short circuiting the whole thing when I inevitably nick the wire with somthing abrasive. I soldered my 50 ohm resistors to one leg of each diode (it doesn't matter which), then soldered those two circuits in parallel. Make sure you get the polarity of your LEDs right:
They typically have one leg a bit longer: This is the positive lead. Some others have a little flat spot on the base of the LED's plastic: This side is negative. If you get them backwards, they won't light.
Once I'd soldered those together, I grabbed an old servo extension lead (you can use one of these, or steal the lead from a bad servo). I cut it, and then removed the yellow wire which takes a signal from the RX. We only need the red and black (in my case, red and brown) wires which are the positive and negative leads. I soldered the positive lead to the positive side of my circuit, and likewise with the negative side and put heatshrink all over the exposed bits. The resistors arn't visible in the final product as they are hiding under the heatshrink near the leds. Here's some pictures:
After all was said and done, I grabbed my heaviest, most overweight and unaerodynamic plane, and proceeded to cut holes in it and add some more superflous wiring, and voila! LIGHTS!
Now, I haven't had a chance to fly it but my LED's arn't getting hot (I measured the total current at 65ma), the plane's still working and I've seen no magic smoke, so it would seem that my methods are correct.
There are a few downsides to this though: Firstly, these lights will run a battery down with no regard to the fact that lipos shouldn't get run down too far. Crash in a high tree with these lights and they'll happily ruin your lipo while you ruin the airframe by throwing sticks at it trying to make it fall. You also can't control these lights via the reciever. When the battery is on, so are they. I also didn't wire a switch in so that I could turn them on or off, because as the cable is so short on my setup I'm just going to unplug them when I don't need them.
So yeah, a long winded, confusing explanation of somthing that you all probably knew anyway, but hopefully it'll help somone at some point, maybe. This should also work for making lights for helicopters such as the blade CX, though I'm not sure how many amps their internal BEC is.