To the left is a small circuit that generates a 0 V to +5 mV square wave at 1 Hz, to somewhat simulate a heart rate.

On the breadboard to the left is a voltage divider to prive the virtual ground at half of the battery voltage.

Then is the AD620 inamp for the initial amplification.

Then I used a TL082 for both an additional amplification as well as summing, using a potentiometer to shift the level up or down.

Then the signal is read by the Arduino which simply reads the analog signal and translates it to a 0-1023 integer value (where 1023 would mean the signal is at 5 V) and sends it to the laptop over serial.

Possible improvements:

• Make the input signal even closer to that of a real heart rate, by decreasing the duty cycle and make it a sine wave.
• Modify the voltage divider to put the virtual ground higher (the battery is 8 V so the maximum right now is +4 V).