Why I built it
It was the design project for a BME lab class: build a circuit that turns the raw heart signal from the skin into a readable ECG.
The hard part
The hardware was the hardest part. Every joint had to be soldered cleanly, and two of the three boards we received had manufacturing defects. Whenever the signal looked wrong, we had to determine whether the cause was our soldering, a defect in the board, or signal acquisition, and we kept iterating until one board produced the clean trace in Fig. 2.