HOWTO make a cheap ECG with an old PC sound-card

Here's a HOWTO for building your own electrocardiograph -- many such plans exist, but this one lowers the cost and part-count by ingeniously repurposing the sound-card from a PC.

Digitization: Once amplified, the ECG signal along with a bunch of noise is in analog form. You could display the output with an oscilloscope, but to load it into your PC you need an analog-to-digital converter. Don't worry! If you've got a sound card with a microphone input, you've already got one! It's just that easy. We'll simply wire the output of our ECG circuit to the input of our sound card, record the output of the op-amp using standard sound recording software, remove the noise from the ECG digitally, and output gorgeous ECG traces ready for visualization and analysis!

DIY ECG Machine On The Cheap (via Make)