Pulse Necklace 17Nov2009
Hack Notes, Pulse Choker, Nov 17th, 2009
Typing them up a second time, cause Eric forgot to save
3M Red Dots vs TENS pads vs conductive fabric chest strap
We hooked up an o-scope (the Tektronix 2235 works, the 1241 didn't) to our open-ecg-project output signal and tried it with various electrodes.
3M Red Dots: works well, little noise. Signal is about 1/2 size for Chung-Hay compared to Eric. If the sticky stuff is secure, signal does not vary with movement. If it's not, signal will show noise when you move.
TENS Pads: same signal quality as Red Dots, but because the stickiness is less, movement is more apt to make the signal have noise. We found that with the heavy wires, we really have to hold the TENS pads on to get them to stay noise free.
Fabric Chest Strap: can be made to work, but really susceptible to noise because it's just not that secure. On Eric, shoulder shrugs and leg movements cause it to have huge movements. On Chung-Hay, it's almost impossible to see the ECG signal for the ambient noise, and any movement at all will really swamp it. Concentration is required to make it work at all :-(. If this is the best it can do, the chest strap probably won't work for us.
Experimenting with high-impedance op-amp
We have a LMC662CN high-impedance op-amp. We wired it up, and tried it with 3M Red Dots and fabric chest strap. In both cases, what we got was a rail-to-rail 500 Hz pattern. We talked about it for quite a while and then it occurred to us to look up the op-amp in the open-ecg-circuit, and it turns out it is very high input impedance already, 100GOhm. So we probably don't need this at all. Much more important is shielding and keeping other sources of noise out. So we think the real way to proceed on the electronics now is to focus on miniaturizing the electronics and getting some shielding in. Then we can use that miniaturized circuit with either TENS pads or fabric chest strap, whatever works.