Update: The DIY ECG project has had several iterations. The latest one can be viewed here: https://www.swharden.com/wp/2019-03-15-sound-card-ecg-with-ad8232/
Warning: This post is several years old and the author has marked it as poor quality (compared to more recent posts). It has been left intact for historical reasons, but but its content (and code) may be inaccurate or poorly written.
I kept working on my DIY ECG machine (I had to change the values of some of the resisters) and it looks like I’m getting some valid signals! By recording the potential using my sound card (the microphone port seems to be a nice analog to digital converter that every PC has) I was able record my ECG with sound recording software, smooth it, and this is what it looks like.
This was based on a circuit I made using a single op-amp (A LM324 from RadioShack $1.49). The op-amp amplifies micro-potential generated by my heart and outputs it in a way that I can connect it to a microphone jack. The signal is very noisy though. I’m thinking about making the more advanced circuit (with 6 op-amps) to aim for a better signal-to-noise ratio, but first I’ll try coding my way out of the noise…