Our own Hack-A-Day emeritus took some time out from his showgirl pursuits to pass this along. The ECG was built to pad a resume, but it doesn’t make it less useful. Be warned, too much power could stop your heart. If you build one, triple check everything before wiring yourself up. Basically, an electrical differential is generated by the contraction of muscles in the heart. This slight signal is amplified by an op-amp and sampled with an A/D converter.