Simply connecting a pair of electrodes across the chest and measuring the difference is not an ideal way for measuring this potential difference. While the signal can be detected, it is commonly superimposed on 60Hz noise thanks to the electrical grid distribution so this needs to be reduced as much as possible. One method for doing this is to use three electrodes instead of two with the third electrode being connected to the left leg and then being connected to the ground plane on our circuit. The second electrode, CHEST1, is then connected to the right side of the chest, and the last electrode, CHEST2, is connected to the left side of the chest. By doing this, we ground most of the body and make the 60Hz signal common to everywhere.

So, with a lot of the noise removed, it's time to detect our heartbeat! The first stage in our circuit is a series resistor and two reversed biased diodes. These parts are there for safety with the resistor acting as a current limiter while the diodes conduct during high voltage spikes and mitigate electric shock against the user. The next stage is the instrumentation amplifier (U1) which takes these two tiny voltages and amplifies them. The amount of amplification depends on the value of RV3 but is it advised that RV3 is configured as to provide a resistance of 2.2KΩ across pins 1 and 8. The amplified signal is then added to an offset voltage of 100mV (potential divider formed by R1 and R5), to provide a small DC offset which improves the LM358 signal detection. This signal is then amplified by U2A which is an adjustable gain amplifier whose gain is controlled by RV1.

This signal will still contain some 60Hz noise and this is further removed by R3 and C1 which form a low pass filter. This filter will pass the heart beat signals very well but not the hum from the mains and so this filter produces a very clean signal. This heartbeat signal is then passed into a comparator (U4A) and a buffer (U4B). The comparator is set to trigger upon a heartbeat which blinks the LED while U4B is used to view the signal on a computer or other circuit (such as an oscilloscope).

Construction

This circuit can be built using most (if not all)

construction techniques such as PCBs

, stripboard, matrix board, and even solderless breadboards. The project here was mounted on a wooden base with some connectors to demonstrate the final circuit board. However, if this was to be used on a regular basis then this circuit would be better mounted in a proper enclosure with exterior connectors. All files needed to produce this project are included as a project zip and include CNC G-Code with auto leveling.