I have an old holiday light display – a tree of lights – that I am refurbishing. It features ornaments that are made out of lights. In its first incarnation, these ornaments were made from incandescent light strings hot glued to armatures made out of wire. They took a long time to build, the lights were always coming loose from the armatures, and if the lights burnt out, it was a huge pain to try to replace them.
This time, I wanted to do something different, and decided to build my own ornaments out of high-power LEDs, mounted in plastic sheets.
Step 1: Create a pattern
The first step is to find a pattern for your ornament. You will need a simple outline for it to work well. You can find numerous examples on the internet; I ended up using cookie cutter outlines as they were simple and easy to deal with.
Once you have the pattern, you will need to enlarge it to the desired size. I did this by importing the picture into PowerPoint and then expanding it.
If you are doing an ornament that is symmetrical (such as a star), you may want to import one segment of it and then mirror it so that all segments are the same.
I used Visio for all of this because it made it easy to put a scanned pattern in the background and overlay it with the locations of the LEDs, and then hid the scanned pattern when I printed it out. Any drawing tool with layers should work well for this.
Step 2: Determine the placement of the LEDs
To create the outline, the LEDs need to be evenly spaced along the shape outline. You could use a flexible ruler for this:
I used a three-sided ruler, which allowed me to use different scales to get the number of LEDs that I wanted.
If you use a straight ruler such as this, you will need to bend it around the curve rather than just measuring from point-to-point.
You will also need to figure out how many LEDs you want. This is a tradeoff between cost and the quality of the outline. I ended up with 41 red LEDs for my candy cane. The number is important; go read section 8 and make sure you understand how it impacts the number of LEDs.
After I stepped along the outline, I scanned the pattern back into the computer, and did a cleaner drawing.
Step 3: Transfer the pattern to the plastic
I picked up some pieces of 1/8” plastic from my local TAP plastics. The pattern is taped to the plastic, but then I need a way to transfer the pattern. I used an automatic center punch, a really cool tool that you press against the material, and a spring will load up and then release a hammer and drive the tip in. This gave me nicely visible marks on the plastic. You could probably use an awl or a nail and a hammer and get similar results:
After marking each point, we remove the pattern, and we are left with a marked piece of plastic:
Step 4: Drill the holes
This step is a bit problematic. The LEDs that I am using are 5mm in size, and it is hard to find metric-sized drill bits. The closest fractional size is 13/64 = 5.16mm, which will be a little bigger than I would like. It’s also only available as standard twist bits, which can crack plastic when you drill it.
I ended up buying a 5mm end-mill off of Ebay; this got the size exact but was a bit problematic because the end mill would skid around a bit at times. It was manageable in my drill press but would not have been good with a hand drill. If you do go the end-mill route, make sure to get a split-tip version.
As backing, I used a piece of Azek PVC trim. After 43 holes, we have the following:
Getting the plastic prepared was a considerable effort. If I had it to do again, I would probably create the pattern in software and have it laser-cut instead of drilling it myself.
Step 5: Select the LEDs
LEDs have a number of different properties that we need to consider:
The “standard” colors that are easily (and cheaply) available are red, green, blue, yellow, and white. You can find some other colors, but they may not be as bright. The yellow that I used was labeled as “amber”.
Brightness is usually measured in Microcandela (MCD). Anything over 1000 mcd is pretty bright.
LEDs are brightest when you look directly at them, and get less bright as you move off to the side. Different LEDs do this differently, and this difference is roughly quantified by the viewing angle of the LED. If you want to know the details of how the light drops off, the data sheet will show the details. For ornaments, I’d recommend angles of at least 30 degrees.
Forward Voltage Drop
Forward voltage drop is determined by the construction and chemistry of the LED, and generally varies from color to color. This will be listed as a single value on the data sheet, but if you dig deeper there will be a graph that shows how the voltage drop varies as the amount of current changes.
Acceptable Forward Current
Acceptable forward current is the amount of current you can push through the LED without compromising its longevity. The data sheet will also talk about this value. Note that this is not the maximum current.
After looking at a bunch of different choices, I ended up settling on the following LEDs, all from Digikey:
The first five are all made by Cree, and I’m pretty happy with them. The orange is a bit disappointing; it is advertised as a 20 degree LED, but if you look at the graphs, it’s really only about 14 degrees, and I really doubt the 4200 mcd brightness that Kingbright claims.
The prices listed are for singled LEDs; there are discounts for larger quantities.
Superbrightleds.com carries some additional colors (violet and pink), but they are pricey.
For the candy cane, I ordered up 50 red LEDs, so that I would have a few extra.
Step 6: Wire up the LEDs
The LEDs will be wired up in series. Depending on the type of LEDs you are using, their color, and the number, you may be chaining all of them together, or you might be creating separate circuits. If you are using multiple colors, multiple circuits are a good idea, as the brightness at a given current may vary drastically between colors. See Step #8, Choosing the Dropping Resistor, to figure out how many circuits you use.
Each of the LEDs has a long lead and a short lead. To chain them together, adjacent LEDs must be connected from short lead to long lead. Here is approach that will help you keep them straight:
- Bend the long lead at a right angle.
- Snip off both leads, leaving them long enough to go from one LED to the next. This distance will depend upon how far apart your LEDs are in the pattern.
I did this for almost all of the red LEDs at once.
Now we can start chaining them together. Take one LED, bend the long lead to the side, but only trim the short led. Put that in a hole with one of the prepared LEDs next to it:
Once they are in, bend the vertical LED from the first led towards the second one, and solder them together.
Put another LED in the next hole. Align it towards the last LED, and then bend the vertical LED of the last led over towards the new one. Solder them together.
We continue the process all the way around the outline, until we get back to the beginning.
Step 7: Building the power supply
There isn’t a traditional power supply in this project; we are going with a transformerless power supply, one that drives the LEDs directly from the 120 VAC wall supply, just the way commercial strands of LED holiday lights do.
|This project involves line-level voltages that could injure or kill you if you do not take proper precautions. Be careful and thoughtful.
Now that that is out of the way, the power supply is very simple. The incoming AC power goes through a full-wave rectifier (you can use a half-wave rectifier, but the LEDs will flicker slightly and will not be as bright. If you see commercial light strings flicker, it’s because they don’t have a full-wave design), which gives us pulsating DC. If we applied that directly to the LED string, we would put a ton of current through them and blow them up, so we need a resistor to limit the current. We’ll cover that in the next section.
I highly recommend using a fuse on the input of the power supply; it can protect you against a lot of problems. I re-used the plug from old holiday light strings, as they already have an integral fuse, and the price is right.
Step 8: Choosing the dropping resistor
The size of the dropping resister depends upon the voltage that we need to drop and the current that we want to use. Time for a bit of math.
Voltage (resistor) = Voltage (line) – Voltage(LEDs)
Voltage(LEDs) = # of LEDs * LED voltage drop
If we look at the data sheet for the LED (follow the link I listed earlier), we will find that the red LEDs have a typical forward drop of 2.1V, and I have 43 of them, so:
Voltage(LEDs) = 43 * 2.1 = 90.3 V
Voltage (resistor) = 120 – 90.3 = 29.7 V
The dropping voltage should be at least 10V to keep the current stable.
Now that we we have the voltage for the resistor, we need to choose what current to use. The red LEDs that I use can take up to 30 mA of current and last a long time (that value came from the datasheet), but they are BRIGHT at that current. I recommend starting at 3mA and then adjusting from there; I ended up having to adjust most of them down in brightness. The resistance value comes using ohms law:
Voltage = Current * Resistance
Resistance = Voltage / Current
In this case:
Resistance = 29.7 / 3mA = 29.7 / 0.003 = 9900 ohms.
10K ohms is the closest standard value, so we’ll try that (in actuality, 3mA was too bright, so I dropped the red LEDs down to about 2 mA).
Are we done? Well, not quite. We need to figure out how much power the resister will dissipate, which is figured by the following:
Power = Voltage * Current = 29.7 * 0.003 = 0.089 Watts
The resisters that I am using are 1/4 watt versions, so this is fine – a single 10K resister will work.
Let’s take another example – say we are using 40 blue LEDs.
Voltage(LED) = 40 * 3.2 = 128 Volts
That is too much, so we will need to break it into two 20 LED strings. That gives us:
Voltage (LED) = 20 * 3.2 = 64 Volts
Voltage (Resistor) = 120 – 64 = 56 Volts
If we want to drive these at 20 mA, we get:
Resistance = 56 / 0.02 = 2800 ohms.
2700 ohms is the closest standard value.
Power = 56 * 0.02 = 1.12 Watts
That is more than 1/4 watt, and, in fact, that’s the amount of power on each of the two LED strings. We have a few choices.
- We can increase the number of LEDs to reduce the resistor voltage.
- We can get higher-wattage resistors.
- We can use multiple resistors. In this case we would need five 1/4 watt resistors, each 560 ohms.
Step 9: Wiring in the power supply and resistor
Once you have the resister chosen, you can wire it into the chain of LEDs:
Now we can wire the bridge rectifier and resistor into the chain of LEDs. Before you do this, hook it up both ways; it will not cause an issue if you hook it up backwards. Be very careful that you do not touch the incoming AC terminals yourself or touch them to the LED chain.
|I’m really not happy with the bridge rectifier. It’s pretty big, and it’s not easy to deal with. If I had to do this again, I think I would build the bridge rectifier out of individual diodes on a separate small board (perfboard or maybe a PC board). It would be flatter and easier to deal with.
At this point, you should be able to carefully plug it in (make sure none of the wires cross), and check that it works.
Step 10: Encapsulation
We need to fix the LEDs in the plastic, insulate the exposed wires, and, if it’s going to be outside, waterproof it as well. I did it with clear silicone from a caulking tube:
The silicone goes on from the side; I’m trying to get it to ooze under the wires so that the wires are totally encased. We go all the way around from the outside, then from the inside, and then over the power supply. Here’s the result:
Step 11: Enjoy