sht21.js

// Module to talk to the SHT21 temperature and humidity sensor over I2C

var i2c = require('i2c');

// Register addresses
const TRIGGER_T_MEASUREMENT_NO_HOLD  = 0xF3;
const TRIGGER_RH_MEASUREMENT_NO_HOLD = 0xF5;
const WRITE_USER_REGISTER            = 0xE6;
const READ_USER_REGISTER             = 0xE7;
const SOFT_RESET                     = 0xFE;

const STATUS_BITS_MASK = 0x0003;

function SHT21(i2c_address, i2c_options) {
  i2c_address = i2c_address || 0x40;
  i2c_options = i2c_options || {};
  this.wire = new i2c(i2c_address, i2c_options);
}

function parseTemperature(data) {
  var st = (data[0] << 8) + data[1];
  return 175.72 * st/(1<<16) - 46.85;
}

SHT21.prototype.readTemperature = function(cb) {
  var wire = this.wire;

  wire.writeByte(TRIGGER_T_MEASUREMENT_NO_HOLD, function(err) {
    if (err) {
      cb(err, null);
      return;
    }

    setTimeout(function() {
      wire.read(3, function(err, data) {
        if (err) {
          cb(err, null);
          return;
        }
        cb(null, parseTemperature(data));
      });
    }, 100);
  });
};

function parseHumidity(data) {
  var srh = (data[0] << 8) + data[1];
  return 125 * srh/(1<<16) - 6;
}

SHT21.prototype.readHumidity = function(cb) {
  var wire = this.wire;

  wire.writeByte(TRIGGER_RH_MEASUREMENT_NO_HOLD, function(err) {
    if (err) {
      cb(err, null);
      return;
    }

    setTimeout(function() {
      wire.read(3, function(err, data) {
        if (err) {
          cb(err, null);
          return;
        }
        cb(null, parseHumidity(data));
      });
    }, 50);
  });
};

module.exports = SHT21;