Sense HAT node library

The Sense HAT is an add-on board for the Raspberry Pi family. It features an LED matrix, a joystick, a gyroscope, an accelerometer, a magnetometer, and temperature, barometric pressure, and humidity sensors.

This library pulls in hardware specific modules for the Sense HAT and exports them in a single module.

LEDs

The LED library is the sense-hat-led module. Example

It supports setting individual pixels, flipping the display horizontally or vertically, displaying characters, and batch pixel jobs (clear, fill). We recommend you check the documentation for the library above for the entire API documentation.

Setting an LED

// Using the standalone hw-specific library
const matrix = require('sense-hat-led');
// Using this library
const matrix = require('node-sense-hat').Leds;

const x = 3;
const y = 3;
const red = [255, 0, 0];

// Set a single pixel
matrix.setPixel(x, y, red);

Switching off an LED

const matrix = require('node-sense-hat').Leds;

const x = 3;
const y = 3;
const off = [0, 0, 0];

matrix.setPixel(x, y, off);

Filling the matrix

const matrix = require('node-sense-hat').Leds;

const O = [0, 0, 0];
const X = [255, 0, 0];

const cross = [
	X, O, O, O, O, O, O, X,
	O, X, O, O, O, O, X, O,
	O, O, X, O, O, X, O, O,
	O, O, O, X, X, O, O, O,
	O, O, O, X, X, O, O, O,
	O, O, X, O, O, X, O, O,
	O, X, O, O, O, O, X, O,
	X, O, O, O, O, O, O, X,
];

matrix.setPixels(cross);

// To fill with a single color instead
matrix.clear([127, 0, 0]);

Clearing the matrix

const matrix = require('node-sense-hat').Leds

matrix.clear()

Joystick

The joystick library used is the sense-joystick module. Example

The joystick library exposes an EventEmitter interface, so that you can register a function to be called when a joystick event occurs.

Possible joystick events are 'press', 'release' and 'hold', and the possible direction values for these events are 'up', 'down', 'left', 'right', and 'click'.

Using the joystick

When using the joystick, the first thing that needs to be done is retrieving a handle to the device. Behind the scenes this involves finding the correct Linux /dev file and reading the descriptions. As this can be quite a long process, the getJoystick function returns a promise.

const JoystickLib = require("node-sense-hat").Joystick;

JoystickLib.getJoystick().then(joystick => {
  // We now have a handle the joystick hardware
});

Registering for events

const JoystickLib = require("node-sense-hat").Joystick;

JoystickLib.getJoystick().then(joystick => {
  joystick.on("press", direction => {
    console.log("Joystick pressed in " + direction + " direction");
  });
  joystick.on("release", direction => {
    console.log("Joystick released in " + direction + " direction");
  });
  joystick.on("hold", direction => {
    console.log("The joystick is being held in the " + direction + " direction");
  });
});

IMU

The Inertial Measurement Unit library used is the nodeimu module. Example

The use of the IMU is slightly more involved—we recommend you check the example above. We've provided some snippets below to detail the simple cases.

Data collection

const imu = require("node-sense-hat").Imu;

const IMU = new imu.IMU();

IMU.getValue((err, data) => {
  if (err !== null) {
    console.error("Could not read sensor data: ", err);
    return;
  }

  console.log("Accelleration is: ", JSON.stringify(data.accel, null, "  "));
  console.log("Gyroscope is: ", JSON.stringify(data.gyro, null, "  "));
  console.log("Compass is: ", JSON.stringify(data.compass, null, "  "));
  console.log("Fusion data is: ", JSON.stringify(data.fusionPose, null, "  "));

  console.log("Temp is: ", data.temperature);
  console.log("Pressure is: ", data.pressure);
  console.log("Humidity is: ", data.humidity);
});

Tilt heading correction

Getting the tilt correction is more involved again:

const imu = require("node-sense-hat").Imu;

const IMU = new imu.IMU();

const headingCorrection = (heading, offset = 0) => {
  // Once you have your heading, you must then add your 'Declination Angle', which is the 'Error' of the magnetic field in your location.
  // Find yours here: http://www.magnetic-declination.com/
  const declinationAngle = 0.03106686;

  heading += declinationAngle + offset;

  // Correct for when signs are reversed.
  if (heading < 0) {
    heading += 2 * Math.PI;
  }

  // Check for wrap due to addition of declination.
  if (heading > 2 * Math.PI) {
    heading -= 2 * Math.PI;
  }

  return heading;
};

const headingToDegree = heading => {
  // Convert radians to degrees for readability.
  return heading * 180 / Math.PI;
};

IMU.getValue((err, data) => {
  if (err !== null) {
    console.error("Could not read data: ", err);
  }

  console.log("Tilt heading is: ", headingToDegree(headingCorrection(data.tiltHeading, Math.PI / 2)));
});

Interface

These modules are exposed by the following entries:

Joystick: joystick library
Leds: LED library
Imu: nodeimu library