This instructable will show you how to decorate a pair of headphones with electroluminescent wires (EL wire) that react to the music. This project uses over the ear headphones, EL wire, an inverter taken from a T-shirt EL panel graphical equalizer display (T-qualizer knock-off), and an mp3 player. I call this project "Level Headed" because it dynamically displays the levels of the audio you're listening to on your headphones.
Materials and Tools
over the ear headphones
5 strands of EL wire
7x1 ribbon cable
7x1 crimp terminal housing
21 female crimp terminals (order extra in case you mess up)
220 ohm resistor
3.5 mm stereo cable with a male connector (cut a cheap loopback cable in half)
3.5 mm stereo splitter
clear heatshrink tubing
crimper or pliers with knurled jaws
knife (for removing phosphor)
xacto knife (for cutting PCB traces)
small phillips screwdriver (for disassembling the inverter)
Step 1: ATTACHING THE EL WIRE
To connect the EL wire to the inverter I simply copied the same type of connector used by the EL panel. This connector uses ribbon cable with female crimp terminals in a 7x1 housing. The downside to this approach is that all of the EL wires have to be together where they meet the ribbon cable so that the corona wires can share the common connection. If each EL wire had an individual common connection then the ribbon cable could be split and the EL wires separated. This would allow two EL wires to be used on opposite ends of the project without wasting EL wire to cover the distance from the shared common to the desired location. Since the EL wires will be grouped together in my headphone decorations the shared common is preferable. If you'd like to use the individual common technique then you'll need to rewire the inverter with a ribbon cable that has ten lines, with common connected to every other line.
My EL wire came pre-stripped, but I did have to scrap the phosphor off the core wire. The picture above shows a razor blade, but I think a regular pocket knife works better for removing the phosphor. Here's a link to a tutorial on EL wire which includes instructions on how to properly strip it. Please read it if you aren't familiar with how to work with EL wire. The main takeaway is to be careful not to cut the corona wires when removing the insulation and to run the inverter only when the EL wires are connected. I used electrical tape to isolate the core wires once they were soldered to the ribbon cable. You will probably have an easier time with heat shrink tubing. Then I hot-glued the soldering to insulate the corona wires/common and core wires. This made a nice, rigid connection. You may want to use a large piece of heat shrink to insulate the connection while preserving more flexibility than you get from hot glue.
To construct the connector, first examine a single female crimp terminal. The set of tabs at the edge are meant to wrap around the wire's insulation. The other set of tabs near the middle are meant to contact bare wire. Insert one strand of ribbon cable in to the connector to determine how much insulation you should remove. Remove that much insulation. Reinsert the wire and line it up so that the tabs will grab the proper part of the wire when they are crimped. Normally a crimping tool is used to press down the tabs but you can use pliers with knurled jaws to do the same thing. Here's a link to a very detailed article explaining how to attach female crimp-on terminals. Once all the terminals have been attached you can insert them in the housing. The terminals have a metal catch that springs out when it reaches the hole on the side of the housing. Therefore you should make sure all the terminals are facing the same way, that you insert them into the housing the proper way, and that you insert them far enough. Wrap some electrical tape around the housing to insulate any exposed metal since EL wire is driven by high voltage that can shock you.
Step 2: MODIFYING THE INVERTER
The T-qualizer inverter uses an electret microphone to sense sound and drives five lines connected to an EL panel made to look like a graphic equalizer. The number of lines it drives depends on the amplitude of the sound sensed. The inverter drives the lines additively which means if line three is being driven then lines two and one must also be driven. The first and last wires of the ribbon cable are both connected to common and the connector is not keyed, so it is possible to reverse the order in which the panel's colors light up.
A modded inverter has a couple of advantages. First, it allows the inverter to be triggered by a remote source via a headphone extension cable. Note that the audio source doesn't have to be outputting sound through a speaker for the inverter to work. Second, a custom waveform that steps its amplitude up or down can be used to precisely control which levels the inverter lights up. So you can use a specially-made audio file instead of music to animate the EL wire.
I've only worked with one of these inverters, so I can't say how likely you are to have one just like mine. I'm hoping that these modifications are generic enough that you will be able to apply them to a different inverter. First I removed the screws holding the inverter's case together and inspected the circuit board. The sensitivity pot feeds power to the electret microphone's internal amp. The output of electret mic is then capacitively coupled to an inverting single transistor (M28S) amplifier. The output of the transistor is capacitively coupled to the EL driver IC. To make room for the audio cable I removed the barrel power socket. Mouseover the image notes to see the places on the board I'm describing.
Next I desoldered the electret microphone and cut the trace connecting the potentiometer (thumbwheel) to the IC's power pin. Doing this let me directly input an audio signal into the circuit. I had some success capacitively coupling an audio signal onto the microphone's output trace before making any modifications, but it didn't work quite right.
I found that the dynamics of the panel covered its full range nicely when the audio signal input was at a listening level that is slightly below normal. This means that if the volume was increased to a normal or loud listening level that the panel would be pinned to the red most of the time. Therefore I decided to reuse the potentiometer to attenuate the audio signal before it is input to the inverting transistor amplifier. To do this I cut the trace connecting the upper and middle legs of the pot and connected the left audio channel's wire to the upper leg and audio ground to the circuit board's ground. Now the thumbwheel allows control of the driver's dynamic range without having to change the listening volume of the music. My 2nd generation ipod shuffle has a noisy output, so it causes the lower level EL wires to light up even when music isn't playing. Using the thumbwheel to increase the input resistance fixes this problem.
Unfortunately, these modifications make the inverter susceptible to noise picked up by the audio cable. To fix this I used a 220 ohm resistor. I soldered the first leg to the audio line where it joins the thumbwheel and the second leg to where the negative battery terminal meets the PCB. Normally the barrel connector socket bridges the PCB's negative battery terminal pad to ground but since I removed it I used the remainder of the resistor's leg to connect them.
Step 3: DECORATING THE HEADPHONES
Step 4: DECORATING THE HEADPHONE CABLE
To do this I used several pieces of clear heatshrink tubing to attach the EL wire to the headphone cable. Since EL wire and the clear tubing are stiff you want to use several pieces of tubing instead of one long piece to allow the assembly to still have some flexibility when you're finished. The hardest part is keeping the wires untangled. First, cut several pieces of clear tubing and slip them over the headphone cable. Then thread the EL wire through the tubing. Now starting from the EL wire plug, untangle the EL wire and heatshrink the first piece of tubing. It's best to put the first piece of tubing a good distance away from the headphone plug, otherwise the mp3 player and inverter would be too close together and any time you wanted to push a button on the mp3 player you'd have to juggle the inverter too. Work your way up the headphone cable by untangling the EL wire and shrinking new pieces of tubing. Make sure the headphone cable doesn't twist. When you get to where the headphone cable splits you can either trim the EL wire or split the lines such that some wires follow the right ear wire and the others go to the left. To attach the EL wire to the headphone wires after the split I used clear tape. You could probably use clear tape for the entire length of the cable, but it won't be as sturdy as the clear heatshrink tubing.