While browsing SparkFun‘s catalogue I noticed the RMCM01 Polar heart module. This is an OEM reader for Polar’s heart rate monitors bands. After looking at the data-sheet, it looked like a very simple project. I ordered one from SparkFun and borrowed a Polar T31 heart rate band from a friend. Now time for some hacking fun.
How do heart rate monitors work?
Heart rate watches work like medical EKG. Whenever the heart is about to beats, the heart’s natural pacemaker emits an electric charge that causes all the cells in an atria to contract at the same time. The Electrodes in the band detect this electric emission and a transmitter in the module sends a signal to the receiver module.
Each transmission is about 1msec long. In order to prevent cross-talk between several transmitter-receiver pairs, each transmitter gets automatically coded to a receiver.
Wiring the module is straight forward. The circuit uses a 32khz crystal and a pullup resistor for the reset pin. The module pinout is as follows:
Download the sources (.pde file) here.
The idea behind the code is to accumulate several readings and calculate an average. At any time we keep 8 samples. The samples are kept in a cyclic buffer, so the oldest is dropped.
Heart rate is printed to the serial every second. The code is fairly straight forward and documented.
Adding a GPS and a logger for SD can make a pretty simple project. Make sure to check out the nice heart-rate logger project at tinkerish. Sources are available upon request.
You may also want to look at a heart monitor breakout board with plethora of interfaces with the RMCM01.