Heartbeat Straight Jacket

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Contributors: Nate, Toni_K
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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.

Stethoscope

I found the smallest electret microphone on Digikey I could find. Unfortunately I’ve lost the part number. Here’s one that looks promising.

Mic

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.

Wired Mic

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 next step was to insert the mic into the head of the stethoscope.

Mic Compared to Head

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.

Hooks to pull through Mic

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.

Successfully Inserted

Success!

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.

Running wire through tubes

Re-assembling the head to the tubes, I wired the microphone wires to the RF Stetho board.

Completed Stethoscope

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.

RF Stetho Board

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’.