Switching EL Wire
My motorcycle has an been inspiration for many project ideas. The most recent of these to come to life is electroluminescent wire under-lighting tied into my turn signals.
EL wire needs (relatively) high voltage AC to operate – typically around 100V at 2kHz. This provided me with a few design challenges: 1. Convert 12VDC to 100VAC 2. Switch voltage to multiple EL strands 3. Avoid leaking AC into the bike’s DC systems 4. Do the above using the least space and weight necessary
Challenge 1 was fairly easy. Originally, I had intended to design and build an inverter. After size and effort considerations, I settled, instead, on purchasing a purpose-built inverter from adafruit.
The remaining challenges proved to be more interesting. I have been switching DC for years, mostly with jellybean transistors and microcontrollers. I’ve done some AC switching with small mechanical and solid-state relays, but don’t want the noise or space typically involved when using relays. Enter the opto-isolator and triac.
Two quick electronics lessons: >A triac, or triode for alternating current, is essentially a transistor that is capable of passing current in both directions.
An opto-isolator uses an LED to turn on a phototransistor, allowing two circuits to be connected, but electrically separate. This allows a low-voltage/current device, like a microcontroller, to switch higher voltages without exposing it to potentially hazardous levels. I used an opto-isolated triac, as the larger triac requires an AC signal to switch on.
Using KiCAD, I designed a simple board consisting of an opto-isolator and triac for each of two channels.
I used PCBNew, which is packaged with KiCAD, to convert the schematic into a board layout, which I could then use to make a usable circuit board.
Lesson learned: Verify pin identifiers during and after layout, before printing/etching. KiCAD had the triac pins numbered incorrectly. As I had turned off the silkscreen layer during layout, this went unnoticed, leading to frustrating hours of hardware debugging.
Once I had the board(s) laid out, it was time to actually make the thing. At HeatSync Labs, we have a few methods available for creating PCBs. My favorite so far is using the laser cutter. Unfortunately, the laser is unable to cut or etch copper, so we use black spray paint as a mask.
First, the copper is covered with a layer of black spray paint. Once dry, it’s placed on the laser cutter’s bed, and the design file is imported to the control software. Since the software isn’t very well-written, importing requires several file conversions. I have found the best path to be: 1. Export from KiCAD as SVG 2. Import SVG into CorelDraw, export as bitmap (b/w, 600dpi, ensure scaling is 1:1) 3. Import BMP into LaserCut 4. Invert image
This creates a negative mask, i.e., white portions will still have copper when finished
This is critical for back-side/bottom-layer copper!
- Verify all dimensions are what you expect
- Follow laser procedures to “engrave” board
Etching a masked PCB has been covered many other places, so I won’t go into that here.
Once etched and cleaned, I drilled holes for the components. I did this with a Dremel drill press and a tiny high-speed drill bit. Be careful when you’re doing this; Most of the drill press accessories I’ve seen are very wobbly, and that doesn’t work well with these bits. They’ll shatter and fly everywhere at the slightest lateral pressure. I tightened all screws/bolts, then added extra tension on the drill mount by tying it to the frame with some Velcro strips.
After drilling, components were soldered into place.
The boards were tested using a typical bench power supply. Third try was the charm!
As excited as I am to have this set up on my bike, I haven’t installed it yet for two reasons: End-of-semester crunch, and I don’t like the color or intensity of the wire. I purchased high-intensity yellow wire, but it turned out to be more of a sorta-bright greenish. I’m looking for a better source now, but might install this anyway in the meantime.
When it is installed, the board will be housed in a weather-resistant enclosure mounted to the front of the bike’s frame, since that’s close to where I need to pull power and signal status from. The enclosure I have should be large enough to also house my future headlight modulator.
I am currently searching for EL wire that is better suited to this purpose and matches my bike better. At some point in the near future, I’ll be expanding the system to accommodate a third channel for a brake indicator, and adding an extension to light my jacket. Stay tuned.