Polar heart rate monitors work very well; a lot of them also have "5Khz" output which cuts out the whole bluetooth/ANT+ stack - basically they send out a short

Polar heart rate monitors work very well; a lot of them also have "5Khz" output which cuts out the whole bluetooth/ANT+ stack - basically they send out a short range inductive pulse at 5KHz on each heartbeat. For a simple demo, plug an old-school telephone pickup (or any old coil) into a soundcard and listen - you'll hear a short click or chirp on each heartbeat (this how most gym kit that is compatible with polar HRM's work).