| I am always seeing questions about how to build
LED spotlights. In this how-to I will try to describe one way to build
very simple, no frills LED spotlights, from start to finish. There are
several excellent sites out there that describe how to build LED Spotlights
for people that have experience with electronic projects, but none that I
know of that get into the details of selecting LED's, calculating the
circuit, and then building the circuit. So for those of you that want
to try and build something like this, let's get started.
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| What to look for in an LED: For LED's in a spotlight, I have found the most important characteristics are the intensity (measured in mcd), the size (measured in mm) and the viewing angle. The higher the Intensity, the brighter. And the smaller the viewing angle, the more directional the light coming from the end of the LED will be. I have had the best luck with 5mm LED's, 10mm did not produce a clean beam of light, for example see the the green lit tombstone here, you can see the bright circles in the center of the beam. I have never tried the 3mm LED's. A good intensity rating depends on what you want to use the spotlight for, I suggest you find a picture with the effect you want and try to find out what they were using. For example this is in a poorly lit room and is a capture from a video. The spotlight has 7 5mm 13,000mcd LED's. This spotlight was also used to light the scarecrow/skeleton in this picture. I have had good luck with 4-6 LED's with around 10,000 mcd intensity, but I encourage you to experiment. There are also LED's available at 20,000 mcd or more. In this how to we test the spot before soldering, so you can try out the combination you select while it is still easy to take it apart and add or subtract LEDs. If you are interested in UV, (blacklight) LED spotlights, I have a small page showing some test results here. In terms of viewing angle, anything 20-25 degrees or under should be fine. Click the thumbnails below for some examples of the specs you will see online, with the values the size, intensity and viewing angles underlined in red:
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| Where to find
ultra-bright LEDs: My favorite way to get ultra-bright LED's is through ebay auctions, just go to ebay and search on "LED ultra bright". 20,000mcd plus LEDs can be found by searching on "LED xtra bright". If you are patient you can sometimes find specials with free shipping. I have always used BestHongKong or led-hk, shipping can be high and you buy in bulk (50 LED's minimum) but it works out to be inexpensive for what you get. For example as I type this I found a set of 100 5mm 10,000mcd blue LED's, 20 degree viewing angle on ebay with a buy-it now price of $2.95, with shipping of $10.95. That works out to 14 cents an LED! There are other manufacturers on ebay (see the links under the spec diagrams above for example), so you are not limited to BestHongKong or led-hk. Before you buy, you might want to determine what resistors you need, and purchase those and the 9 volt battery clips described later at the same time to save shipping.
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| How to figure
the resistors: There are a lot of LED calculators out there, I use http://led.linear1.org/led.wiz. The tricky part is figuring out what to plug in.
Some examples of specs you will find online, with the Diode Forward Voltage and Diode Forward Current underlined in red:
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| Sometimes they will list a range,
if that is the case just pick a value in the middle. For example, if
the Forward Voltage is 3.2-3.8, use 3.5. Don't worry if it is not the
exact right value, you just need to be close. Once you have the
values, plug them into the calculator and it will design your circuit for
you.
I will give an example of calculating and wiring a 5 LED spotlight using some UV LED's I have sitting around. I don't have the specs any more for these LED's, so a search on ebay for 'LED ultra bright UV' came back with a UV LED with the specs shown in (figure 1) below from Asia Engineer. Forward voltage of 3.6 (half way between 3.4 and 3.8), a forward current of 20, and a Source Voltage of 9. This calculator has two features I like; One is the ability to select the output (I like the schematic, but a wiring diagram might be easier to understand), and second is you can check the box for help with resistor values and it will tell you the color stripes to look for. See (figure 2) below for how the calculator works with the values plugged in, and what the results look like. Another common problem I hear about is not being able to find the specs for the LED's you have. I have found that the Diode Forward Voltage and Current basically depend on the color, so if you have a 5mm red ultra bright, then find some specs for a blue ultra bright 5mm LED, average them out and use that. For example I looked up 5mm blue ultra bright and found the following specs:
So I would try 3.4v and 25ma. You may not end up with the most efficient circuit, but it should work.
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| Figure 1. |
Figure 2. |
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Now it's time to look at the circuit diagram and arrange it so you can compare it to the spot you are about to build. The circuit as returned by the calculator is shown in (figure 3). The Arrows with the lines at the point are LED's. The symbol that looks like teeth is a resister. The circle with the '+9V next to it is the positive terminal of the 9 volt battery (the smaller post on top of the battery) and the 3 lines stacked on top of each other is the negative terminal on the 9 volt battery. So this circuit has 5 LED's, 3 resistors and a 9 volt battery. To match the orientation of the circuit as we will build it , first rotate the circuit, then exchange the center branch of the circuit that had 2 LED's with the branch with a single LED. We can do this because it is a parallel circuit; it does not matter which order the 3 branches are in, as long as you have those 3 branches. Finally, with the circuit oriented as in (figure 4) you can look at (figure 5) and see what the bottle cap with the LED's on it will look like, and hopefully you can see the similarity between the LED's in figures 4 and 5
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Figure 3. |
Figure 4. | Figure 5. | |||||||||||
