Two years ago my son decided he wanted to be Luke Skywalker for Halloween, so I set out to make him a light saber with cool lights and sounds. What started out as a kid's costume quickly turned into an obsession, and I've spent many hours thinking about how to build the ultimate DIY light saber. My focus has been on how to make a really, really bright blade, and this instructable describes what I've built.
Step 1: Other Designs
One high-power LED
Early on I decided that I would not be happy with the DIY light saber design you often see on the web, with a single, high-power LED mounted in the handle that shines into a plastic tube lined with diffusion material. For one, it is very hard to make the light uniform (it always looks brighter near the handle). In addition, you can't get the distinctive "scrolling" animation when you turn it on and off.
Chains of through-hole LEDs
My first few sabers were built around a blade made from a long string of ordinary through-hole LEDs. You can find instructions for building these blades on this site and others. The basic idea is to bend the legs of the LEDs so that they can be packed together, top-to-bottom, with all the cathodes soldered together on one side and all the anodes soldered together on the other side. It is tedious work, but you can create a fairly nice blade with about 70 to 80 LEDs. You can get the animation effect by making segments of 10-12 LEDs and wiring them separately to different PWM pins on an Arduino. Be careful with power, though -- most micro-controllers cannot supply more than 50-100mA of current from their pins. Instead, you need to use the pins to control a transistor (use a MOSFET for efficiency). I'll discuss power in more detail later. The disadvantage of conventional LEDs is that they are designed to project most of their light straight out the front, but in this case we want light to project from the sides of the LEDs. Filing or sanding the tops of the LEDs improves diffusion somewhat, but a lot of light is still wasted.
Overall, this is a nice design (see photos above), but I was looking for something more powerful.
I briefly considered using flexible strips of surface-mount LEDs -- you can find these things for sale on many different sites. Some are fixed colors, while others are individually addressable RGB pixels, which opens up a lot possibilities for cool effects. There are two issues with these strips, however. First, it is hard to diffuse the light enough so that you cannot see the individual pixels. Second, many of them consume power even when off, which can be a problem for battery-powered devices. But an interesting alternative worth considering.
Step 2: Chip-on-board LEDs
What really piqued my interest was discovering chip-on-board (COB) LEDs. Originally developed for commercial lighting applications, these LEDs come in modules of various sizes and shapes, but they all share a similar design: they have a large number of tiny LEDs connected directly to an aluminum plate and covered with a clear protective plastic. It is not unusual to find COB modules with 80 LEDs on a 14cm strip -- far denser than any of the flexible LED strips. Here is a nice description of how COBs are built:
You can buy COB LED modules from many sites on the web. The price has come down significantly, and one of the cheapest ways to buy them is to get modules designed for use as aftermarket fog lights for cars and trucks. You can get a pile of them on eBay for not much money. Here are some factors to think about when choosing:
Color: COBs come in various shades of white, red, blue, and sometimes green, but not nearly the array of colors you'll find for discrete LEDs. I chose a very cold white color called "ice blue", which is a great match for Luke Skywalker's light saber.
Voltage: COBs come in a few standard voltages. Each individual LED is typically 3V, and they are wired in parallel to create a 3V COB. Some modules are 12V, which consist of four 3V parallel arrays wired in series. Occasionally, you can find 6V or 9V modules. As you'll see later, you can easily wire two or three 3V COBs together yourself to make a 6V or 9V module.
Size: I'll discuss the options in more detail later, but you need COBs that will fit inside your tube.
Step 3: Build COB Triangles
The first real step here is to solder together multiple COBs to form a single module that will shine out in all directions. I found that using two COBs back-to-back does not create a uniform light -- you can tell when you're looking at the edge, which is darker. Instead, I took three 3V COBs and wired them in series, then folded them into a triangle. You can see the test module in the picture. Technically, I should drive this module with 9V, but I'm using an 11.1V (3C) LiPo battery because when a bunch of them are wired together, there is a big voltage drop down the chain. I'm using a 10 Ohm resister (the big fat 10 watt resister) to make sure I don't blow out the LEDs during testing.
Each triangle (3 COBs) will form a single segment of the blade.
Think carefully about the size of each module. Mine are about 5cm by 1cm, so to make a 70cm blade I'll need 14 triangles, for a total of 42 COBs. I wanted small COBs so that I could create a smooth animation up the blade. If you don't care much about the animation (or you can't easily control 14 segments -- see discussion later), get longer COBs and you'll save yourself a lot of work.
Step 4: Wire the Segments Together
In my design, the individual COB segments are controlled by N-channel MOSFETs, which allow my Arduino to switch a lot of current without loss of power. It's particularly important in this case because the COB triangles are now 9V, while the Arduino I'm using is 3.3V. N-channel MOSFETs are "open drains", meaning that they switch the circuit on the ground side, not the power side. So, all the anodes of the COB modules (the positive sides) are connected to single (big fat) power wire. Each cathode has a separate wire, which will be soldered to the drain on a MOSFET.
In order to give the whole blade some structure I wrap the COB triangles around an aluminum tube. You can find tubes like this one at hardware stores and hobby shops (and online also, I'm sure). I drilled a hole for each cathode wire, so that I can thread them through the tube. This way, they won't interfere with the light.
You can try to thread the anode wire through the tube as well, but I found it difficult, so instead I ran it along the outside of the tube (the red wire in the pictures).
Step 5: Wire to the Controller
You can make any kind of controller you like. The simplest is a single MOSFET that controls all of the COB segments. Connect the ground wires (from the COB segment cathodes) to the drain pin on the MOSFET. Connect the sink pin on the MOSFET to ground (e.g., the negative terminal on your battery). Connect the gate pin on the MOSFET to a pin on your Arduino. You can use a PWM pin to ramp up the power on the blade, but with a single transistor it will control the whole blade as a single light.
For an animated blade you need one MOSFET (and Arduino pin) for each segment that you want to light separately. I ended up making a custom PCB with a 16-channel PWM controller (PCA 9685), but it works well even with 5 or 6 PWM pins directly from the microcontroller.
Step 6: Light It Up!
This set up requires two batteries: one for the microcontroller (3.7V LiPo) and one for the blade (11.1V LiPo). The blade pulls a lot of current quickly, so I use a high-discharge battery that is designed for remote-control vehicles. They aren't cheap, but they are worth it.
My blade is made of 42 COB modules that draw 3 watts and generate 70 lumens -- in theory this blade can put out almost 3000 lumens and consume 126 watts!
I am planning to put up another instructable with information about the circuitry (including Eagle files and code), so stay tuned!