This is a wireless, battery-powered physiological sensor that sends electrocardiography (ECG), galvanic skin response (GSR) (AKA electrodermal activity) and respiration (plethysmography) signals to a remote computer in real-time. The circuit uses instrumentation amplifiers, some basic analog filtering, digital acquisition with an Ardweeny (Atmega328P), wireless transmission with a NRF24L01+ chip. A GUI runs on Scilab to display the data and to compute heart & respiration rate.
Yes I know about the BITalino thing but I found it a bit expensive and it's more fun to design all the sensors from scratch. Plus it doesn't have a respiration sensor and I find that it's a valuable addition to a physiological analysis. Open source documentation for the hardware on such projects is almost inexistant so here is my contribution!