broadcast the data of the HRM sensor

Hi,

The easy way to do it is simply to have this code on the Bangle:

Bangle.setHRMPower(1);
Bangle.on('HRM',function(e) {
  NRF.setAdvertising({},{manufacturer: 0x0590, manufacturerData:[e.bpm]});   
});

That'll advertise the current BPM as one byte in a custom UUID (the one reserved for Espruino's use).

More info at http://www.espruino.com/BLE+Advertising and http://www.espruino.com/Reference#l_Bangle_HRM for the HRM event

However there is actually a standard HRM bluetooth service that can be implemented - Adafruit have an example at: https://learn.adafruit.com/bluefruit-nrf52-feather-learning-guide/custom-hrm

So potentially that could be implemented (using NRF.setServices), and it might work out of the box with some apps. If you need more help doing that I could post something up