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Did you make a robotic coffee pot which implements HTCPCP and decafs unauthorized users? Show it off here!
By Kamiquasi
#171596
SparkFun carry the Polar Heart Rate Monitor Interface which is exactly for that Polar model. It's not exactly cheap, though.

Perhaps you can make Pulse Sensor work, though. Doesn't seem like anybody tried it on the chest, and you'd have to provide your own strap of some sort.

Others will hopefully chime in with other solutions (off-the-shelf or otherwise).