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Heartbeat Straight Jacket

Heartbeat Straight Jacket

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!).


Things I had to find:

  • EL Wire - There's quite a few companies selling this on the internet. I ended up using 0.125" (3.2mm) diameter blue. 15' was enough for this project. ~$1.30 per ft.
  • EL Driver - Basicly a DC to AC inverter. I found one that inputs 3V (for battery operation) and outputs the needed AC to driver EL. This one is $7.50.
  • Straight jacket - Start by looking on EBay. Monkey Dungeon is pretty epic. ~$60
  • Stethoscope - Shop around on Froogle. I spent around ~$25. "Sprague Rappaport", whatever that means.
  • Buttoneer - The creator of this device probably never expected it to be used to attach EL wire to clothing, but it works magnificently!
  •  Fancy electronics - That's what we discuss here! Most of this can be bought (get ready for shameless plug) on the SparkFun website. We offer the cool board that controls EL wire - it's called the EL Sequencer.


Figuring out how to use EL wire:


This first thing I had to do was to get my ATmega168 microcontroller to be able to control a single channel. In other words, turn on and off one string of EL wire. EL wire is a bit intense when it comes to control. EL wire requires something like 125V AC at 425Hz. This is a pretty high voltage, and a weird frequency, but with very little current. I might be able to design a circuit to generate this, or I could just buy and inverter.

Note: The 125V/425Hz coming out of the inverter was enough to tingle the fingers pretty good. Not enough shock to really worry about, but enough that you should watch out for exposed AC connections.
Connecting the AC inverter to the EL wire, sure enough, it lit up. Now I wanted control. How do I turn the AC power going to a strand of EL wire on and off? With a Triac! These are nifty 'switches' that allow you to turn on/off an AC source. Perfect. Pull out the Digikey bible and try to find a good one. I settled on part: MAC97A4GOS-ND. Cheap, TO92 through hole easy to solder, and can handle up to 0.6A at 200V. Wow. Let's hope we don't do anything nearly that big.

Here's the DC current readings I found going into the inverter:
  • 86mA no load
  • 110mA load under control

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.


Initially, I used two radios in the same breadboard so that I could get firmware sending and receiving. Eventually, I boiled all this down to a few nRF libraries that work pretty well. When in doubt, RTFM for the Nordic ICs.

Control Boards:

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.

(Note: We now carry the EL Sequencer as a product. It can be used in this and other projects.)


Board shown without RF populated, and without Triacs populated. The EL sequencer receives RF triggers and controls the EL strings, up to 6 independant channels.

Files: In this case, each channel is an EL wire in the shape of coincentric hearts. Admitedly, this PCB layout is really quite bad, and we've learned a lot since 10-2-07 when I laid it out. The main goal is to show you how you can use a microcontroller to control EL using triacs. To do that, checkout the schematic:

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.


The Stetho RF board that goes into the stethoscope had to be made as small as possible. It is powered by a small lithium polymer battery and does nothing but read the analog level on the microphone in an attempt to pick up and heart beat, and broadcast a 'Hey! Beat the heart' signal to the straight jacket.


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.



Working with EL Wire:

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:

This type of JST connection will allow you to quickly remove a strand or remove a controller from the piece of clothing.
Step 1: Cut and strip the outer sheath

You can't even see the threads in this picture, but the way that EL wire works is there is a phospherous center core (the thick white wire in the picture above) and hair like wires that spiral around the core. If you strip the wire too harshly, you can easily cut through (or tear) the hair like wires. It takes practice. I recommend picking up an EL stripper. These stippers help, but it still takes practice.

Step 2: Scrape off some phosphor

Using an x-acto knife, scrape the phosphor from the core. In the picture above, you can see the two hair-like spiral wires.

Step 3: Solder the core


For my own sanity, I wired all the cores with red wire, and all the hair like wires with black wire. Remember to put the heat shrink on the red and black wires before you solder! I used tape to anchor everything while I was soldering. Worked well.

Step 4: Solder the hairs


Now carefully solder the hairlike wires. You can see I wrapped the hairs around the black, solid core wire. I used the x-acto knife blade to help coerce the hairs into place.


After soldering, shortly before heat shrinking.

Step 5: Heat shrink 'er


Notice the individual wires are heat shrinked. This helps give the hairs some support, and it also isolates the core connection from the hair connection. To shrink heat-shrink, I like to use a cheap heat gun. In a pinch, I have used matches or a lighter.

Step 6: More heat shrink

Adding a final overall sheath of heat-shrink will significantly add to the resiliance of the connection. These connections may seem very fragile at first, but I've beat up on these connections over the past two years. One connection has failed - it was the first one that I did and was pretty poor. A little practice and you'll be creating great, solid EL wire connections.

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.


Here is the inside of the jacket with LiPo battery, AC inverter, EL Sequencer, and the various lines going to the EL wire.

To form and attach the EL wire to the straight jacket, I used the handy Buttoneer. This device uses this neat plastic 'staples' to re-attach buttons to clothing. It also works surprisingly well for attaching EL wire to clothing!

Hacking a Stethoscope:

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.

I used a hacked microphone board to amplify the audio signal. Hot glue secured the fragile 30AWG wires in place. The amp'd signal was then fed into the ADC on the ATmega168. I had to leave the electronics partially exposed so that I could re-charge the battery and turn the board on and off.

In practice, the microphone input is barely able to pickup the audible sound of a heartbeat. It works, but it's not clean.  When it did work, I find the effect really astounding! However in public, most people found a blinking straight jacket far more intriguing than the stethoscope 'feature'.

Twas a fun Halloween costume!

Nathan Seidle

Comments 10 comments

  • Wow. That’s a really cool project. It just gave me an idea: an Arduino EL shield.

    • It’s already on the way. They should hit the storefront soon!

      • so - can we expect the Boarduino breakout version of the sequencer any time before mid july or even before the burn at all?
        I had an epiphany today when i realized i could drive el-wire with triacs. Needless to say, i was about ½ way thru designing a breakout for the boarduino, and moseyed over here to read some of your posts on having PCBs made…
        either way - i need something i can mate with a uC dev(arduino xxx) that has more pins, and some leftover for interactive input… im thinkin mega.. but thats a bit overkill.
        Im still buying 2 of these tho…

  • If you had put the microphone at the end of the ear piece, you might have got a better sound/signal. The bell (the big end you stick on someone’s chest) is essentially the amplifier. By putting the microphone IN the bell you’re kinda defeating its purpose.

    Just a thought.

    Excellent pick on the Buttoneer! That would be perfect to secure EL wire to various fabrics, without sewing! (Not that I mind sewing…)

  • this is unbelievably unbelievable!
    all i can say is… wow.

  • I’m always a little heartbroken (ha) when I find that a project I’m trying to do is really similar to someone else’s. I decided to try super-bright LED’s with side-emitting fiber optics rather than EL-wire. I think it’ll be more reliable and last longer, but it’s certainly not as efficient. The biofeedback part came from a junked treadmill: it clips on your earlobe and reads heartbeats with infrared transmission through it. We’ll see how well it turns out!
    I’ll keep an eye out for the straight jacket on the Playa this year. I seem to remember seeing it, but heck: EL-wire animated hearts? Not too uncommon a sight.

  • This looks really cool. I actually thought of a wearable electronics project similar to this, and I’m wondering if anyone is working on something like it:
    Heart rate monitor display on shirt/vest.
    Numeric display of heart rate and LED/EL all over shirt that corresponds with heart rate. Pulsing LEDs would be cool (ala macbook) The Polar HRMI sold here would be an obvious candidate.

  • Just checking in to see if there’s an ETA for the Arduino shield for the EL sequencer? Excitedly waiting!

  • Thanks for sharing this! I was thinking the other day about repurposing a stethoscope head for geekier ends (if you can imagine) – this’ll definitely help.