Building a LED Flashlight
by Jeff Polston (October 13, 1997, updated: February 5, 2004)
Astronomers crave those pitch black skies. They'll even drive, hike, or climb the tallest mountain to obtain them. They rave about that super dark rural site, where you can't even see your hand in front of your face. The first thing they do upon arrival is grab a flashlight so they can see!!!
Let's face it, no matter how great a dark site is, you'll still need light. You've got to see how to operate your telescope. You've got to see your eyepieces so you'll know which one to pick up. And, you've got to be able to see your star chart. A normal white light flashlight is out of the question because it will destroy your night vision. What you need is some sort of red light to help preserve your precious night vision.
Most of us started out with a flashlight with some sort of red lens over the front of it. I still carry around my Mini-Mag flashlight with a red plastic lens cover. I have a headband to mount it in (for that Borg look) and I have an accessory attached to the end of it so that I can hold it in my teeth. But, a red light-emitting diode (LED) is much better to use for astronomy. It has a deeper, more pure red color, which helps to better preserve your night vision.
There are now quite a few of these flashlights on the market. You can get them in the standard flashlight look. These seem to be your basic flashlight but they have an LED in place of the incandescent bulb. You can buy weird looking devices with adjustable "goose necks". For a couple of bucks you can buy a tiny LED light for your key chain. My favorite light is one called Starlite, which is made by Rigel Systems. This is a small, box shaped flashlight that has two, super bright LEDs and a thumb wheel for adjusting the brightness.
If you don't favor purchasing a commercial unit, whether it be to save a few bucks or religious reasons, you can also fashion your own little LED flashlight out of parts found at your local electronics store (like Radio Shack). They are very simple devices to make and you can customize it to your heart's delight.
About the only thing different with a LED is that it can't generally be hooked up directly to a battery. It needs to be wired in series with a resistor. The resistance needed can be easily calculated with the following formula:
R = (Vbattery - Vled) / (Iled),
where R is the resistance in ohms, Vbattery is the voltage of your battery or battery pack, Vled is the LED's "forward voltage", and Iled is the "forward current" of the LED. The forward voltage and the forward current can be found on the LED package. The longer of the two wires from the LED should be connected to the positive lead from the battery. If you wire it the other way around, the LED will not work.
Here's a simple drawing showing the circuit. The battery (light blue) is at the bottom, the switch (blue) is on the right, the LED (red) is at the top, and the resistor (yellow) is on the left. Notice that the long lead of the LED is wired through the resistor to the positive terminal of the battery.
A calculation example is the Archer Jumbo Super-Bright LED from Radio Shack. According to the package, it has a forward voltage of 1.85 volts. The forward current is 20 milliamps (0.02 amps). If you were using a 9 volt battery, you would need a resistance of:
R = (9 - 1.85) / (0.02) = 357.5 ohms
The resistance doesn't have to be exact. Just get something in this range. I used ¼ watt resistors in my project. If you want variable brightness, just wire a 1000 or 5000 ohm potentiometer in series with the LED and resistor. If you want a wide, diffuse glow from the LED then roughen up the lens a little. You can use sandpaper. You can enclose these components in a Radio Shack project box or build something yourself.
Here's an image of the inside of my home built LED light. I used the a project box sold by Radio Shack. I also bought the brightest red LED and the brightest orange LED from Radio Shack. You don't really need two, but I was just having fun with the project. I also got two LED holders to try and make the flashlight look professional. Unfortunately, as you can see by the gobs of hot glue, I drilled the holes too big for the LED holders. Always have the proper size drill bits!!! :)
Anyway, I put two LEDs in my flashlight. I figured the brighter orange LED would be for finding my way around, and the slightly dimmer red LED would be for reading charts. I was trying to make this light kind of simple (despite the two LED approach) so I didn't use a potentiometer. Each LED (with its calculated resistor) is attached to a momentary switch on the side of the box. Most would probably prefer to have on/off switches, but I also let my baby daughter play with the light (hence the need for it not to stay on all the time).
My LED light works pretty well. I have used it on several observing sessions already. To make it an all purpose light, I have been thinking about adding a white light to it. I would like to use a white LED but the only company that I could find them at wanted too much money. I might add an incandescent bulb at a later date when I figure out a good way to do it. For now, both my daughter and I enjoy this neat little flashlight.
Multiple LEDs and Batteries
This article has generated enough email to cause me to add this updated section. It seems that everyone loves the idea of building their own LED flashlight, but they want to add more LEDs and batteries and want advice on hooking them up and figuring out the correct values for the formulas. Since I'm not an electronics expert, I always have to think and brainstorm a little to figure it out. So hopefully this update will help those of you who want to go beyond my simple LED flashlight design.
First of all, for a brighter LED light (whether it be red for night vision or white for just common use), you add more LEDs. Of course the more LEDs you add, the more current it will need, so the quicker your battery will run down. For a brighter, longer lasting flashlight, you might consider more LEDs with more batteries.
Batteries in normal flashlights are typically arranged in a series circuit. In other words, they are end to end. Just think about a 3 or 4 cell flashlight. All the batteries are lined up, with the positive terminal of one battery connected to the negative terminal of the next battery. When they are hooked up this way, you add the voltage of each battery to get the total voltage of the circuit. If you have three AA batteries (which are 1.5 volts each), your total voltage would be 4.5 volts (i.e. Total voltage = 1.5 + 1.5 + 1.5 = 4.5).
If you hook up the batteries in parallel, with the positive terminal of one connected to the positive terminal of another (and the negative terminals connected to each other), your total voltage stays the same. If you connected two 9v batteries together in parallel, the total voltage would still be 9v. But now you would have a system that can product more current, or in other words power the LEDs longer.
The choice is up to you. If you're connecting everything yourself, you can do it about anyway possible. But since I favor pre-made battery holders and connectors such as those from Radio Shack, I'd probably hook up multiple AA or AAA batteries in series with a battery holder, and I'd probably hook up multiple 9v batteries in parallel with a battery connector and wire. Sometimes you use a certain battery type and configuration because of space limitations in your device. I personally like dealing with 9v batteries because they are compact and you only need a small snap-on connector. Plus it seems pretty trivial to add a second 9v battery in parallel to get a longer lasting light. Use whatever battery you want for whatever reason, be it size, convenience, price, etc.
As I've already mentioned, if you want a brighter light, you put more LEDs in it. Just pick your color and if you want it bright, buy the brightest ones they offer. Oh by the way, you can now buy white LEDs pretty cheap at places like Radio Shack. When I wrote my original article above, you could only order them from a few select companies and they were very costly. As for hooking them up, I suggest you hook them up in parallel, with a resistor for each LED (as opposed to one resistor for the entire circuit). Although this is more work, the reasoning behind this is that if one LED happens to go bad, it won't affect the other LEDs as much. If you only had one resistor for the entire circuit and one LED went bad, that would mean that the other LEDs are now running on more current and if they can't handle it, they will fail prematurely. And if that bad LED actually fused into a short, a single resistor circuit wouldn't even power the other LEDs now because all the power is going through that fused LED. Of course you may deem the chances of something going bad remote enough not to deal with the hassle of extra resistors. That's logical thinking and to be honest, I'm not sure I wouldn't follow that path myself. I just want to point out that the more secure design would probably be a parallel circuit, especially if you are putting a lot of work into it and plan on really depending on it a lot.
Okay, you know the voltage of your batteries. If they are in series, add them up. If they are in parallel, the voltage is the value of one of the batteries (assuming the batteries all have the same voltage).
Case 1: array of parallel LEDS, with one resistor for entire circuit
If you just want to use one resistor before the array of parallel LEDs, use the same formula given above (i.e. R = (Vbattery - Vled) / (Iled) ) and just add up the total current for the LEDs. In my example, I was using a single LED with a current of .02 amps. If I hooked up 5 of these in parallel, the total current through the entire array of LEDs would be .1 amp. In my example of a 9v battery and a single LED having a forward voltage of 1.85v, the resistor value needed was 357.5 ohms. If I hooked up 5 of these LEDs in parallel, then the single resistor needed would be 71.5 ohms. (i.e. (9 - 1.85 ) / .1 ). Notice that I'm using the forward voltage value from just one LED. Of course with more LEDs, the more current you are drawing, so you'll need a resistor that can handle it (i.e. higher wattage). But with small LEDs for flashlights and stuff, it ain't that big of a deal.
Case 2: array of parallel LEDS, with a resistor for each LED (recommended)
In this case you just use the formula as if the LED was a stand alone circuit. You use voltage and current values for the LED in question. So if you hooked up a parallel array of any number of LEDs, just use the value of each LED to figure out what resistor to use with it. With my example of a LED with a current of .02 amps and a forward voltage of 1.85v, the corresponding resistor to use with a 9v battery would be 357.5 ohms (i.e. R = (Vbattery - Vled) / (Iled), so R = (9 - 1.85) / .02 = 357.5 ohms). Another reason I recommend this is if you decide to use different value LEDs for some reason, the resistor is specific for the LED it is paired with. In other words, it makes it easier to actually have different types of LEDs if you so desire.
Here's a simple drawing showing three LEDs in parallel, with two batteries in parallel.
Case 3: LEDs in series
In the case of the LEDs being in series, just add everything up. The total voltage and current of the LEDs would be their cumulative sum. So in my example, if you have three LEDs with a current of .02 amps and a forward voltage of 1.85v, the corresponding single resistor to use with a 9v battery would be 57.5 ohms (i.e. R = (Vbattery - Vled) / (Iled), so R = (9 - (1.85 + 1.85 + 1.85) / (.02 + .02 + .02) = 57.5 ohms ).
Hope all of this makes sense. I've tried to incorporate solutions for all the questions I have received on my original article. Someone once asked me about making the LEDs flash. Well, you can buy flashing LEDs from Radio Shack too. I've also recently converted the lights in my camper to LED lights, but I bought commercial units instead of building them myself. I converted to LEDs to help conserve the battery when I'm camping away from power hookups. The reason I bought commercial units, which consisted of 19 LEDs in a bulb housing that would fit my camper sockets, is because the price has finally come down far enough for me to be willing to buy them rather than try to make them. The reason I'm saying this is because if you are strictly building a multiple LED light yourself to save money, check the commercial alternatives again because they might be low enough in cost now.
Of course, building your own LED light can be fun and rewarding. You can custom build it to suit your needs, and still probably save money over any commercial unit you could by. And it's neat to show someone your own handiwork. Good luck with all your projects!
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