This is kinda a dangerous project – for one the circuit used to sense cardio activity will include a path that crosses the human heart – thus if a shock would occur (now you know what will happen) you might get hurt. Thus as a warning don’t use high voltage sources (including step down power supplies) and you may use the information in this project at your own risk!
The concept behind this project is fairly simple – when your heart beats it produces a small potential difference (Voltage) across different points in the human body. This potential difference is sensed using three electrodes each electrode will go to certain parts of the body on on each thumb and the third one could be on one of your feet. The electrodes is connected to inputs of three amplifiers the two from the thumbs is connected to an in amp while the third one is connected to two other amplifiers (one configured as an integrator while the other configured as a x100 inverter) to be used as a common mode voltage – the third one is for common mode rejection. the output of the amplifier stage is then fed into an ADC input of the propeller which samples the signals.
To plot the data into a PC – through serial interface, an opto isolator is used to protect both human and the computer from electrical surges.