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Wait, wait, what? The Heartbeat Straightjacket is a standard canvass straight jacket, with some fancy electronics, to create a sweet costume with remote wireless capabilities. To get the full effect, a person holds a special stethoscope to their heart, and they see their heartbeat displayed on the straight jacket that I am wearing. This tutorial is about my Halloween costume from 2007. I'm just 18 months behind!
Ok, why? I wanted to create a project around EL (electroluminescent) wire - sometimes called cold neon. It's this neat, very bendable, very bright wire that glows nicely in low-light situations (like Halloween!). With a little work, you can control EL and get some interesting displays. On top of that, I needed a warm costume as the weather in Boulder can get pretty ugly at the end of October (it usually sleets). A straight jacket was the perfect platform for the electronics, warm, and just weird enough for Halloween (and Burning Man).
Stethoscope: I wanted to create a neat way to interact with the jacket. I could have used a heart-rate strap or some other way of picking up the user's heartbeat, but I wanted to use a stethoscope because I thought it would be more friendly that asking someone to strap on a heart band. I figured I could use a microphone to 'hear' the heartbeat and then transmit the beat to the jacket.
What did I learn? I learned that this project was more of a sociological experiment! A lot of people are rather timid about putting a stethoscope against their bare skin (no, they don't work through clothing). Maybe it's the fear of sterile, cold metal against their skin - the doctor's office coming back to haunt them. Maybe they're afraid of contracting a deadly viral skin chewing horse disease (silly). I'm not sure. The other funny thing I learned was that people can rarely find their own heart (oh no! I'm dead!).
I found that you could run my inverter without a load, without problems. Results may vary. So you can see that strand of EL wire uses around 25mA(DC) depending on length. On par with LED power consumption.
The next step was to get the wireless working between the straight jacket and the stethoscope. I choose the nRF2401A because I heart Nordic. These low power, low cost ICs are great for a simple 'Hey! Beat the heart' type signal. The signal that will be broadcast by the stethoscope is a low data rate (I need something like 4 bytes) and low bandwidth (4 bytes * ~70 beats per minute). In retrospect, I should have used the nRF24L01. It is easier to use and has even better specs than the nRF2401A, but either will work.
Now using all this learned knowledge from the breadboard of the triac and the radios, I created two PCBs, the EL-controller and the Stethoscope transmitter.
Here you can see that we send high voltage (HV) AC through the connector where the string will be attached. When the EL_E pin goes high (3.3V), this will turn on the triac, allowing AC to flow in and out of EL wire causing it to light up. Be sure to include a current limiting resistor into the triac! 1k works well.
Now some testing! The first step was to get the EL sequencer to control one string, within the jacket:
Here is the sequencer hooked up to LiPo, inverter power, AC output, and one string there on the end.
The next step was to create the series of 5 hearts. This involved a bit of work. If you've never worked with EL wire, here' s the step-by-step process to soldering to this evil phospherous beast.
Before you go diving into an EL wire project, think about how the EL strands are going to connect to your controller. I highly recommend JST 2-pin friction connectors:
Ok, now repeat. 4 more times. Ugh. This was the most painful step of the project. Pretty slow. But once complete, I had hearts!
I programmed the jacket so that if it wasn't receiving a heartbeat, to go into demo mode where the heart beats on and off at a varying speed.
The next phase of the project was to wire up the stethoscope. I wanted the scope to look as 'off-the-shelf' as possible.
I bought a cheap stethoscope online and then began to hack it.
I found the smallest electret microphone on Digikey I could find. Unfortunately I've lost the part number. Here's one that looks promising.
The next step was to solder wires to this little guy. Not trivial, but not too hard. Be sure to get two mics in case you lift a pad during soldering. Notice the tape to hold the mic still while I was working with it. I used 30AWG wire wrap wire so that I could easily feed the wires through the flexible tubes of the stethoscope.
The head of the stethoscope comes off the tubes if you pull hard enough. And the diaphragm covers come unscrewed on this cheap version. I used small heat shrink to keep the wires together.
I created a small hook so that I could fish the wire through the head. 30AWG wire is a little hard to work with so I first fished a 'pull' wire through the head, attached the pull wire to the actual microphone wires, and then pulled everything back through the head.
Next, I cut a small slit in the rubber tube roughly where the electronics would be attached to the tube. I used thicker 22AWG wire, again so that I could fish down the tube, attached the mic wires to this thicker wire, and pulled it all back through.
Re-assembling the head to the tubes, I wired the microphone wires to the RF Stetho board.