The heart signal itself is a very small signal (in the µV range) that is easily lost in the noise. To make matters worse, your body has a varying DC voltage that can be quite substantial. The basic idea is to connect two probes to your body and feed them into an instrumentation amplifier with a high CMRR (common mode rejection ratio). This instrumentation amplifier will reject the noise because it is common to both inputs and it will amplify the difference. I chose to use an IC solution since it has high precision balanced resistance needed for such a high CMRR. Additionally, the datasheet for the IC that I chose shows a "right leg drive" connection based off of op amps to eliminate the DC voltage that floats around your body. As you can see from the screenshots I got pretty good results right off of the bat!


the schematic

I got the circuit straight from the datasheet for the INA114 instrumentation amplifier that I used. It is towards the end in the applications section. It is easy to spot because it shows a diagram of a body with the connections hooked up. I took great care to make sure that I used a sheilded cable for the probes. I used an old audio cable and soldered alligator clips onto the end. I also purchased some EKG electrodes that you simply peel the backing off of and stick onto your chest. They can be purchased for around $13 for a pack of 50 on the net. For power I used 4 AA batteries. I did this because YOU SHOULD NEVER CONNECT ANYTHING PLUGGED INTO THE WALL TO YOUR CHEST! Even with a battery, small currents could kill you. Also I needed to be able to tap off of the middle to use as the GND.

Here are some pictures. Please excuse the bad quality. I wrote this entire page and posted all the pictures in about 20 min. It was an either do it now or I'd never get around to it type of thing ...