In my earlier post today about the Blinky EKG, I wrote
About the only thing I can think of is that there is too large a DC offset between the EKG electrodes, as the Blinky EKG uses a large gain on the first stage and a relatively small gain on the second stage. The DC-blocking high-pass filter is after the first stage. The EKG built on the protoboard used a smaller first-stage gain and larger second stage gain, so wouldn’t saturate the first stage as easily. (I’d learned more about EKG electrodes by the time I’d designed that circuit.) I could fix the gains by changing a few resistors on the EKG blinky board, which may be worth the pain of unsoldering and resoldering resistors. That may be worth trying today.
I finally got a chance to test that this afternoon. I replaced the 470Ω Rgain resistor on the instrumentation amp with a 12kΩ resistor, reducing the first-stage gain from 175.2 to 11.667. With this change, I could see my heart beat with no problem, if I turned the second-stage gain all the way up to 111, as high as the trimpot would let me go. This gain was not enough to light the LED, though, so I replaced the 1kΩ resistor below the trimpot with a 100Ω resistor, which allows the second-stage gain to be adjusted from 10.9 to 1101. The combined gain is thus adjustable from 127.2 to 12845.
UPDATE: 2013 July 5. I realized this afternoon that I would have been better off leaving the 1kΩ resistor alone, and changing the 100kΩ resistor above the trimpot from 100kΩ to 510kΩ, giving me a second-stage gain varying from 47.36 to 521, and a total gain of 552.6 to 6078.
At high gain, the LED is always on, and at low gain the LED is always off, but when the gain is around 2300, the LED blinks nicely and the waveform recorded with the Arduino data logger is good.
I believe that the problems I’ve been having with the Blinky EKG board have been the result of DC bias in the EKG electrodes saturating the first stage, so reducing the gain of the first stage and increasing the gain of the second stage was the right fix. I’ve been thinking of redesigning the blinky EKG board to be more decorative (so that it could be worn as a pin or a pendant)—if I do that, I’ll certainly change the gain.