Holiday project - building an internet connected door sensor for £10

• board

I’m on holiday this week, so I’ve had a chance to do some fun stuff at home. My 8 year old son saw the Google Nest products on YouTube and wanted to create his own version with a Raspberry Pi with a camera module.

Rather than spend £30 on a Raspberry Pi, £12 for a camera module etc., I suggested we could build a minimal viable product first using a much cheaper ESP 8266 (£3) hooked up to a reed switch on the door, plus I had one lying around. I ordered a reed switch from Amazon for £2.

First step was to get the ESP 8266 to react to switch events on a digital input. We got it working based on this [0] but instead of lighting an LED, sending the switch status using Serial.write and using the Ardiuino IDE Serial Monitor to check it worked.

• https://www.arduino.cc/en/tutorial/switch [0]

• alarm_floor

Once that was working, we wanted a Web server to notify. The cheapest and easiest way to do this is to use the Serverless Framework to set up an API inside the AWS infrastructure. I chose JavaScript, because my son knows a bit of that, but I should have stuck to Go, because I had to try and explain callbacks and promises to an 8 year old.

Once it was printing to the console, I set the API up to email him when the API receives a POST.

At this point, we had an API and the ESP 8266 handling switch notifications, the next part was to get that ESP 8266 onto the home wifi, and send a POST notification to the API.

Here’s all the code:

#include <WiFiServerSecure.h>
#include <WiFiClientSecure.h>
#include <ESP8266WiFi.h>
#include <ESP8266WiFiMulti.h>
#include <WiFiUdp.h>
#include <ESP8266WiFiType.h>
#include <ESP8266WiFiAP.h>
#include <WiFiClient.h>
#include <WiFiServer.h>
#include <ESP8266WiFiScan.h>
#include <ESP8266WiFiGeneric.h>
#include <ESP8266WiFiSTA.h>

#include "RestClient.h"

#include "Debounce.h"

// Uses:
//  * https://github.com/wkoch/Debounce
//  * https://github.com/esp8266/Arduino/tree/master/doc/esp8266wifi
//  * https://github.com/csquared/arduino-restclient
// To install libraries:
//  * cd ~/Documents/Arduino/libraries
//  * git clone <repo>
//  *   (For https://github.com/csquared/arduino-restclient, git clone https://github.com/csquared/arduino-restclient RestClient)

const char *ssid = "";
const char *password = "";

RestClient client = RestClient("xxxxx.execute-api.eu-west-1.amazonaws.com", 443, 1);
const char *eventDoorOpened = "{ \"event\": \"door_opened\" }";
const char *eventDoorClosed = "{ \"event\": \"door_closed\" }";

const int inPin = D1;

void setup()
{
  pinMode(inPin, INPUT);
  Serial.begin(115200);

  // Using 
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED)
  {
    delay(1000);
    Serial.print("Connecting..");
  }
  Serial.println("Connected...");
}

Debounce doorSwitch(inPin); // the number of the input pin, D1 is set for the ESP8266.
int previousState = -1;

void loop()
{
  int newState = doorSwitch.read();
  if (previousState == -1) {
    // On the first iteration, just set it to the latest value and don't trigger an event.
    // This makes the device succeptible to a power outage making it not work.
    previousState = newState;
  }
  if (newState != previousState) {
    if (newState == HIGH) {
      Serial.println("Door was closed");
      postSecure(eventDoorClosed);
    } else {
      Serial.println("Door was opened");
      postSecure(eventDoorOpened);
    }
  }
  previousState = newState;
}

void postSecure(const char *event)
{
  String response = "";
  Serial.println("Posting...");
  int statusCode = client.post("/dev/sensor/detect", event, &response);
  Serial.println("Results (status / response)...");
  Serial.println(statusCode);
  Serial.println(response);
}

I simplified the micro-controller code by using a few libraries. A debounce library to handle switch noise, and a HTTPS RestClient library, since the default doesn’t support HTTPS. Since there’s hardly any RAM and no disk, there’s no space to store information about valid root certificates etc. For a toy project, there’s no need to protect against invalid or expired certificates.

Once we had it working on the table it was time to upscale it. I bought 25m of doorbell cable from the local ScrewFix for £3, and soldered the ends of it onto a bit of stripboard I had lying around, along with the required resistor. I did this more for the fun of introducing my son to soldering than anything else, because we could have just used a connector block for a single resistor and a couple of cable connections.

Then, we put it in an ice cream tub to make it more professional.

• alarm_in_box

So, we’re onto the final step - installation and decoration of the box!

Of course, I already had a few pre-requisites (laptop for coding, multimeter for testing, screwdrivers, optional soldering iron) but the parts themselves cost less than £10:

• ESP 8266 - £3
• Breadboard - £2
• Cable - £3
• Switch - £2
• Ice cream tub - free