During my wanderings of the Internet I came across this project: http://tinkerish.com/blog/?p=130 It's a heart rate logger based around a Polar heart rate monitor strap reciever module from Spark Fun: http://www.sparkfun.com/commerce/product_info.php?products_id=8660 Reading through the datasheeet I came across the table on page 5 which lists "Reception Frequency: 5.5kHz". This produced a bit WTF in my mind: who uses a frequency *that* low in a device this small? Anyway, it turns out that this was simple to check as 5.5kHz can easily be recieved by a computer sound card. I strapped on the heart rate monitor I've got (PulseTronic brand) and connected the closest antenna at hand (a ~50cm long aligator lead) to the end of a 3.5mm plug lead and took a recording. Sure enough there's a very obvious pulse train near 5kHz: Click to view full size! I plan on building a radio reciever for these pulses, a wire loop + a bunch of op-amp filters should do the trick well enough. This will then be fed into a microcontroller which will log the data to some kind of memory: eeprom or SD, most likely.