Can ya hear my hearbeat?

Ok, I decided to setup the basic single channel circuit and test it out to see if i could atleast register my own heartbeat. Easier said than done. A little bit of theory…

INA118 gain is determined by a single resistor Rg. And the Gain formula is : Gain = 1 + (50kilo-ohm/Rg)

Now, I thought i could manage to record my heartbeat with a nominal gain of 100 or so, but I actually had to crank up the gain to get a decent recording by using a 100 ohm resistor for Rg. Anyways, got that all setup and now, for the electrodes. I haven’t got down to creating the active electrodes yet, so I decided to simply use a pair of home-made ones. Each passive electrode simply consisted of a plastic cap with an office clip (yes! the all-useful office clip) and a sponge soaked in saline water. Here’s what it looks like:

Passive electrodes

A word of advice: Watch out for all that chest hair when you tape the electrodes to your chest. It can get excruciatingly painful when you try to pull the electrodes off! Don’t ask me how i know! 😀

Electrode positioning: Main electrode (V+) on the left chest close to the heart, other electrode (V-) on the right chest and the Right leg electrode on the right ankle (duh! obviously!!). The electrode cables were simple single cores with shielding.

OK, time for Testing! As I suspected, the circuit worked better with a dual power supply rather than a single 5v supply. (just hook up a couple of 9V batteries to 7805 & 7905 with supporting components and you’ve got your dual supply). Also, the right leg driver didn’t really improve the quality of the readings. In fact, I got better readings by simply connecting the Right Leg electrode to Vref which, in the case of a dual power supply, would be the Ground. The reading I got was pretty noisy but not bad, considering my electrodes were home-made. Recorded my heartbeats through the soundcard using Adobe Audition. Here’s a screenshot for y’all:

testing INA118 - heartbeats

It’s definitely not the cleanest of recordings… which brings to me to the next stage… Filtering!

See y’all in a while!

~ by teknomage on November 11, 2007.

One Response to “Can ya hear my hearbeat?”

  1. Fascinating process. Did you know that a simplified EEG now has toy applications? You might have a future as a game developer…


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