When your heart beats, it generates an electrical current, or signal.  The range of that signal can vary from .05 Hz to 150 Hz.  Anything outside of that range isn't coming from your body, so to the first step in building our EKG sensor is to filter out all signals that outside of that range.  We're doing that using two second order Butterworth filters (one high-pass and one low-).  I used this hand dandy online filter calculator to figure out what components I would need.


SallenKeyLP.gifThe low pass filter diagram.


After building the filters, we'll amplify the signal and pass it into a dedicated arduino for some processing before it gets sent to the Master Controller that's sending all the data off of the sensor array.


But I'll have to build those sensors first.  I'm currently on my third try.  I made the same mistake on the first two: not reading closely enough the spec sheet on the op amps I was using.  The first one didn't work at all, but the second one cutoff at the correct frequency.  It also clipped all of the negative input.  This time I've read the spec sheet thoroughly, and I'm feeling good about getting it working.  I'll post an update after my build this weekend.