The human heartbeat can be made audible by using this cardiophone circuit. It is basically an audio circuit coupled to a probe made specially for the purpose of picking up electric signal from the human heart.
To get the best signal, place the probe’s electrodes to a point close to the heart. The preferred point is just below the left breast with negative electrode pointing the the left of the sternum.
After constructing the circuit, the output A must be calibrated to null through the potentiometer P1. This is important for the circuit to function properly. The signal coming from output A can then be connected to either a low-frequency amplifier or an oscilloscope.
The signal coming from output B is a square wave in sync with the heart rhythm. This signal can be used to trigger a final stage amplifier or other circuits. The heartbeat can be heard from the final amplifier’s speaker.
The special signal probe is shown in bellow figure and the simplest way to make this probe is to use a 1 cm x 10cm blank pcb board. Following the design, the non-shaded parts of the pcb board must be etched away. The un-etched copper plate must be covered with solder to protect it from corrosion and to facilitate good contact with the skin. Take note that two of the probe’s electrodes are marked negative and positive respectively. It is of utmost importance to use a shielded twisted pair wire for the cable connecting the probe with the cardiophone circuit.
Cardiophone PCB layout