A while back I posted about my first attempts to make a stethoscope amplifier as an input to PyroCardium.  This has worked well, though it has required some tweaking since then. First, it turned out that the resistor network used to power the electret mike and to bias the output signal to within the range of the single supply op-amp I’m using was too low resistance, and was substantially loading the relatively high impedance output of the electret. Secondly, getting a good seal between the microphone and the stethoscope tubing is critical for getting a decent signal out. Tweaking those things gave us a setup that worked pretty well for Alchemy and Maker Faire.

That said, there are some definite problems with our current stethoscope/microphone setup. First, it turns out it’s surprisingly hard to position the stethoscope on your chest to really pick up your heartbeat, especially as our current setup doesn’t have audio feedback so you can hear whether you’ve found it or not. Secondly, the stethoscope does a pretty good job of picking up ambient noise.  It works great it a quiet room but less well at a noisy, crowded event.  In fact I was able to get it to trigger on the beats of the music playing at Alchemy with just a little tweaking. This is great for some other things we have in mind for Priceless but less good for picking up your heartbeat.

So, we’d like to find a better system for Burning Man.  Ideally we’d like something where there’s no ambiguity about where to place the sensor and a sensor that robustly picks up your heartbeat, regardless of environmental noise, the fact that you’re running in place, or anything else.  So far we’ve had two ideas - use some kind of simple EKG system, or use a pulse oximeter.  This weekend I tried building this simple EKG circuit but met with no success whatsoever - all I could see at the output was 60 Hz line noise.  I’m not really sure why (the high common-mode rejection ratio of the instrumentation amp is supposed to take care of that) but I’m not sure I have the skill set to make it work.  I’m also not sure that this EKG system will be robust enough or easy enough to use on the playa, and there are also safety issues with hooking it up to a line powered system.

So, my current plan is to buy a cheap pulse oximeter and see if we can rip it apart and get it to interface to a PC.  If anyone out there reading this knows about pulse oximeters and how to do this let me know.  Also, if you’re good at analog design and want to make the ekg circuit work, that would be good too :).

This entry was posted on Monday, May 12th, 2008 at 7:16 pm and is filed under PyroCardium, Workshop.