Heart Rate Monitor Interface

(photo: Sparkfun)

New! : You can download a new firmware package that returns the period (in milliseconds) between pulses here.

The Heart Rate Monitor Interface (HRMI) is an intelligent peripheral device that converts the ECG signal from
Polar Electro Heart Rate Monitor transmitters into easy-to-use heart rate data. It implements a sophisticated algorithm for computing an average heart rate even with noisy or intermittent data from the transmitter.

The HRMI was born from the
Heart Rate Telemetry system I designed. I wanted to do something with the algorithm I had designed for that product and pitched this product to Nathan Seidle, Sparkfun CEO. He agreed with a handshake and went off to layout the board for Sparkfun’s manufacturing process. They build the board and I provided the firmware, documentation and demo code. You can buy one here.


  • Multiple interfaces: USB, Logic-level serial and I2C
  • Dual heart rate processing algorithms: averaged and raw
  • Uses the RMCM01 Polar OEM receiver
  • Compatible with coded and non-coded Polar transmitters including T31, T31C, T61C and Wearlink®
  • 32-entry heart rate data history buffer
  • Four 8-bit ADC inputs
  • Up to a 5 channel digital input/output utility port
  • Simple command/response interface
  • Programmable power-on default operation
  • Custom exercise equipment
  • Portable heart rate monitoring devices
  • Bio-feedback devices
  • Heart beat aware body-worn electronics