I've built a home made ECG to these designs:

Unfortunately, while I can see the QRS complex, the P & T waves get lost in the noise. It looks like everybody has atrial fibrillation - not good!

Here's the physical device:

As you can see, I have surface mounted it inside a tin can, intending to reduce the interference.

I've also used shielded audio cable (single core, braided outer) for the electrode leads. The braiding is earthed to the tin can via the outer rim of the RCA plug.

This is a view of the 'inside' of the can. As you can see, the three blue wires connect the 'leads' to the RCA plugs. I don't think they're the source of interference, even though they're not shielded, as there is no interference effect when the RCA cables (the black ones in the previous picture) are plugged in.

Finally, here's a trace from the ecg, as recorded by my computer's sound card. The QRS is clearly visible, but the S & T are pretty much obscured by that lovely 50hz (i think it's 50hz) hum. Our power in australia is 240v/50hz, so this would make sense.

So, i've put the electrics in a tin box. I've shielded the wire (although this may not be the correct procedure). I've earthed all the circuit earths to the main earth. I've even earthed the tin box to my computer's case. The noise persists.

I'm totally out of ideas - and would love any input on how to reduce this interference! ECG's are really prone to noise because of their design - amplifying a tiny signal many times.

If you can't help - I hope you enjoyed the brief project outline anyway