Mobile phone bike computer sensor from Puck.js - Bluetooth CSC service

Hello,

Here is today's installment of "What Colin did in lockdown today".

I was teaching someone to program yesterday, and we made a bike computer using a Pixl.js to display speed and distance on the Pixl display using an sensor from an old broken wired Cateye bike computer we had. This sensor is a reed switch and a magnet.

I thought "I have a Puck.js with Bluetooth and magnetometer" so had a go at making a sensor for my phone using the Puck.

When running this code the Puck appears as a bluetooth cycling sensor ("CSC profile") to use the jargon and can be read by apps on a mobile phone. I tried "Jepster" on Android and the nRF toolbox app.

The theory is that the Puck could be mounted on the front fork of a bicycle, with a magnet on a spoke. When you bring the magnet nearby the LED on the Puck lights, and a reading is sent to the phone. When the magnet rotates away the LED goes off.

There is a commented out bit of code that uses setWatch so the same code could run with a bike computer sensor - this probably gives longer battery life than using the magnetometer.

I did discover if you alter the time between pulses the app seems quite happy to believe I can cycle at 80,000 km/h - take that, competitive Strava users :-)

I haven't actually tested this on a bike, but I think it works. Posting here in case it is of interest to anyone!

/*
Espruino code for Bluetooth Low Energy CSC (Cycling and Cadence Sensor) profile
https://www.bluetooth.com/wp-content/upl­oads/Sitecore-Media-Library/Gatt/Xml/Cha­racteristics/org.bluetooth.characteristi­c.csc_measurement.xml

Data is returned via the Measurement characteristic:

flags = 0x01 = Wheel data; 0x02 = Crank data; OR together for both
uint32 = count of wheel revolutions;
uint16 = last wheel event time in units of 1/1024 seconds
uint16 = count of crank revs
uint16 = crank event time

Usage:

csc=create_csc_service()

At each wheel revolution

csc(time_in_seconds)

...each call to this will notify the phone etc

*/

function create_csc_service() {

  // Dataview and buffer to hold the measurement data
  var buffer = new ArrayBuffer(11);
  var dv = new DataView(buffer, 0);

  var revs=0;
  
  // Given # wheel revs, time in seconds, format the buffer 
  // for the CSC measurement characteristic.
  function format_csc(revs,wtime) {
    var time_csc_units = wtime*1024; // Convert to units of 1024th of a second
    dv.setInt8(0, 0x01); // Only wheel data is available => 01
    dv.setUint32(1, revs, little_endian=true);
    dv.setUint16(5, time_csc_units, little_endian=true);
    dv.setUint16(7, 0); // Crank revs not used
    dv.setUint16(9, 0); // Crank time not used

    return buffer;
  }
 
  // Update the characteristic. Call once per revolution passing
  // time in seconds. This is our "public" interface
  function revolution(time_secs) {
    revs+=1;
    format_csc(revs,time_secs);
    // Now notify...
    NRF.updateServices({
        0x1816 : { // CSC UUID
        0x2A5B: { // Measurement UUID
          value : buffer,
          readable : true,
          notify : true,
        }
      }
    });
  }

  // Start - advertise that we're a CSC
  NRF.setServices({
    0x180F : { // Battery service UUID
        0x2A19: { // Battery value UUID
          value: [90], // Battery level percentage
          readable : true,
        },
    },
    0x1816 : { // CSC UUID
        0x2A5B: { // Measurement UUID
          value : format_csc(0,0),
          readable : true,
          notify : true,
        },
        0x2A5C: { // Feature UUID - need to fully understand this
          value: [ 0x01,0x01],
          readable: true,
        }
    }
  }, { advertise: [ '180F', '1816' ], uart: false});

  return revolution;
}

// Version with ex bike computer magnet+reed sensor on GPIO pin D13
//pinMode(D13,"input_pullup");
//setWatch(function(e) {
//  csc(e.time);
//},D13,{edge: 'falling', repeat:true, debounce:20});

// Version for Puck.js using its in-built magnetometer

// Two values for hysteresis. Determine mag reading for magnet near and far.
// Might need to change this depending on magnet or bicycle positioning
MAGNET_NEAR_VALUE=150000;
MAGNET_FAR_VALUE=130000;

function on_mag(reading) {
  reading.x -= mag_zero.x;
  reading.y -= mag_zero.y;
  reading.z -= mag_zero.z;

  // Pythagoras theorem, but no need to
  // sqrt as we're only interested in
  // change
  var magnitude = reading.x*reading.x
      + reading.y*reading.y
      + reading.z*reading.z;

  // Magnet detected near?
  if (magnitude>MAGNET_NEAR_VALUE && !magnet_near) {
    magnet_near=true;
    LED.write(1);
    csc(getTime()); // Update the bluetooth service
  }

  // Magnet moved away?
  if (magnitude<MAGNET_FAR_VALUE && magnet_near) {
    magnet_near=false;
    LED.write(0);
  }
}

function onInit() {
  csc = create_csc_service();

  mag_zero = Puck.mag(); // Get base case for magnetometer reading
  magnet_near = false;
  Puck.on('mag', on_mag);
  Puck.magOn(80); // 80Hz sample rate
}