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Conscious of the Pump

October 3, 2010

I spent a good part of yesterday listening to my heart. Most of the time it beats regularly like a metronome in the background, keeping time. Sometimes it races, and I need to consciously work at slowing it down. Rarely am I aware of it beating too slowly. I can use breathing and activity to modulate it while I am awake, but how does it behave when I am asleep?

I probably should just go buy the Polar Heart Rate Monitor, but I stubbornly thought it would be interesting to try first to build a working circuit myself.  That is partially why I’m here after all.  I knew from DanO’s comments that I’d need an Op-Amp to help distinguish the heart from the rest of the electrical noise my body is giving off.  (By the way — regarding my post last week about Galvanic Skin Response (GSR) — in discussing the waveform I got with Eric Rosenthal, he said it sounded like all I was getting was the Common Ground Signal, and that I needed an Op-Amp to amplify that signal as well.)

Justin Downs’ tutorial at the ITP sensor wiki ( is extremely helpful, but I wanted to begin with an EKG, not an IR monitor.

I began with Forrest Mim’s Differential Amplifier:

And then added a 0.1μF capacitor between the two inputs.  I read that this helps to stabilize the signal and reduce high frequency noise (  Here’s the board:

Using DanO’s SerialFiler sketch (, I got much more convincing data than last week.  Here it is at hook-up.  I’m touching it 1/3 into the screen, and have attached it either side of my heart at about 2/3:

Here I’m practicing a pretty rough form of Pranayama breathing to race my heart:

Here I’m slowing it down:

And here I’m trying to remain as still as possible:

So I think there’s something there, but there’s still a lot of noise.  I’m going to try to work out some filters, and if that doesn’t work by midweek, I’m moving onto the Polar hardware and sensor from Sparkfun (  I’m all for stubbornly cracking this problem, but also for keeping my eye on the larger project, which is to try to get a sense of how my body behaves in the unconscious state of sleep, where I spend at least one quarter of my life.

So I haven’t answered the question yet of how my heart behaves while I’m asleep, but I have begun to be able to see it while I’m awake, and that is an important awareness of as a point of comparison.

I also don’t think I’ve begun to answer the question in a visually appealing way that tells a story.  In a graphical interface, the EKG graph can tell you the most at a glance, but it could still have more information density.  Scrolling left and right, it could change color on a ROYGBV scale depending on how far away my heart rate is from a base “normal”.  It could also be scalable on the time axis so that I could see more detail or more of the bigger picture.  It could poll me randomly via SMS or equivalent so that I could report my mood back with a simple keystroke, and that could be tagged to the graph at that point.  There could be a pie chart that breaks down my heart rate variation into categories and compares it to an optimal distribution.

We spend too much time in front of our screens, though, and a physical counterpart to this might be more helpful.  Just as wearing a watch makes one more aware of time, wearing a device that subtly hints at such a relationship, say, between heart rate and mood might make one more aware of that as well.  Riffing on the idea of relating current heart rate to a base “normal” state through an ROYGBV color range, perhaps this could be translated into a cufflink, with a base state of green, red racing, and blue calm:

Of course, this means cuffs every day.  A mood ring might be more convenient.


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