I made some modifications to the original circuit I talked about in my previous post and made it on a bread board. Here is the resulting circuit

 

05 - Heart beat sensor circuit.jpg

05 - Heart beat sensor circuit 2.jpg

 

I placed my finger on the sensor, and I got a quite clean signal

 

05 - Heart beat sensor signal.jpg

The signal is 200 mV peak-to-peak. Since the ADC has a 10 bits resolution over a 5V range, the signal will be read as 80 ADC steps, which should be enough to make heart beat detection easy