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A Power Efficient Refrigerator Monitor

An Introduction To Squirrel Applications Example 8

In this example we will create a refrigerator monitoring application. The application monitors the temperature and humidity of the refrigerator and sends alerts if the temperature or humidity is higher than a set threshold for too long. The application also monitors the refrigerator door using the accelerometer to wake on motion and the internal light sensor to determine the door status. The application saves power by sleeping between readings when the door is closed, and only connecting periodically to upload readings or when an alert is triggered. This code can be easily configured for use with an imp006 Breakout Kit, impExplorer Kit, impAccelerator Battery Powered Sensor Node or impC001 Breakout Board.

Skill level


This example will focus on writing Squirrel code. Please visit the Getting Started Guide on the Electric Imp Dev Center to learn how to configure your device with BlinkUp™ and how to use the Electric Imp IDE, impCentral™.

What You Learn

  • How to use Electric Imp libraries.
  • How to send data to a cloud service such as Initial State.
  • How to use a Hardware Abstraction Layer (HAL).
  • How to write a class in Squirrel.
  • How to program your device to run offline.
  • How to configure sensors to take asynchronous readings.
  • How to send data between device and agent using the Message Manager library.
  • How to configure and use nv storage.
  • How to put the imp into deep sleep.
  • How to change the default connection policy.
  • How to wake on an interrupt.
  • How to configure the accelerometer’s click interrupt.

What You Need


// ---------------------------------------------------
// HAL's are tables that map human readable names to
// the hardware objects used in the application.

// Copy and Paste Your HAL here
ExplorerKit_001 <- {
    "LED_SPI" : hardware.spi257,
    "SENSOR_AND_GROVE_I2C" : hardware.i2c89,
    "ACCEL_I2C_ADDR" : 0x32,
    "PRESSURE_I2C_ADDR" : 0xB8,
    "POWER_GATE_AND_WAKE_PIN" : hardware.pin1,
    "AD_GROVE1_DATA1" : hardware.pin2,
    "AD_GROVE2_DATA1" : hardware.pin5
  • Assign your hardware class variables. In the Application class before the constructor you will find a number of class variables. You will need to re-assign the hardware variables so they look something like the example below. Do not copy and paste from this example, as these values may differ from the ones in your HAL.
// --------------------------------------------------------
// Application code, take readings from our sensors
// and send the data to the agent

class Application {

    // Time in seconds to wait between readings
    static READING_INTERVAL_SEC = 30;
    // Time in seconds to wait between connections
    static REPORTING_INTERVAL_SEC = 300;
    // Max number of stored readings
    static MAX_NUM_STORED_READINGS = 20;
    // Time to wait after boot before disconnecting
    static BOOT_TIMER_SEC = 60;
    // Time to wait between checks if door is open
    static DOOR_OPEN_INTERVAL_SEC = 1;

    // Accelerometer data rate in Hz
    static ACCEL_DATARATE = 100;
    // Set sensitivity for interrupt to wake on a door event
    static ACCEL_THRESHOLD = 0.1;
    static ACCEL_DURATION = 1;

    // The lx level at which we know the door is open
    static LX_THRESHOLD = 3000;
    // Alert thresholds
    static TEMP_THRESHOLD = 11;
    static HUMID_THRESHOLD = 70;

    // Time in seconds that door is open for before door alert triggered
    static DOOR_ALERT_TIMEOUT = 30;
    // Number of seconds the conditon must be over threshold before triggering env event
    static TEMP_ALERT_CONDITION = 900;
    static HUMID_ALERT_CONDITION = 900;
    // Time in seconds after door close event before env events will be checked
    // Prevents temperature or humidity alerts right after is opened
    static DOOR_CONDITION_TIMEOUT = 180;

    // Hardware variables
    i2c             = ExplorerKit_001.SENSOR_AND_GROVE_I2C; // Replace with your sensori2c
    tempHumidAddr   = ExplorerKit_001.TEMP_HUMID_I2C_ADDR; // Replace with your tempHumid i2c addr
    accelAddr       = ExplorerKit_001.ACCEL_I2C_ADDR; // Replace with your accel i2c addr
    wakePin         = ExplorerKit_001.POWER_GATE_AND_WAKE_PIN; // Replace with your wake pin

    // Sensor variables
    tempHumid = null;
    accel = null;

    // Message Manager variable
    mm = null;

    // Flag to track first disconnection
    _boot = false;

    // Variable to track next action timer
    _nextActTimer = null;

    constructor() {...}
  • Copy and paste the Agent Code into the Agent Code pane in the impCentral code editor.
  • Sign into Initial State.
  • Find your Streaming Access Key on the My Account page.
  • Navigate back to impCentral.
  • In the Agent code enter your Streaming Access Key into the Application class static variable STREAMING_ACCESS_KEY on line 24.
  • Hit the Build and Force Restart button to start the code.
  • Note the agent ID in the logs.
  • Navigate back to Initial State, find the Bucket that matches your agent ID.
  • Watch your data update in the Source, Lines, Waves and Tile views on the Initial State website.


Device Code

Agent Code