Custom Remote WiFi Mains Socket Switch

Here’s again one of these small hacks that I’ve been wanting to do for a while but never found the time…

There’s actually not that much to it, the most interesting part is probably just being amazed at how easy it is nowadays to pull off this kind of hack…

I bought (for probably only around 5£) this Sonoff S20 wireless remote control socket, and while pretty pleased with the build quality, I immediately disliked the software stack.

One is supposed to install the eWeLink Android app, which not only requires an account set up, but also sends the data externally to some of their servers.

More importantly it requires this switch to connect to an existing WiFi network, but I want to use it to turn on/off my very WiFi / internet router/modem.

What’s great however about this device, it’s being based on an ESP8266, which can be easily programmed even with the Arduino IDE. To the extend that they’ve even provided the 0.1″ spaced 4 holes to solder in the standard header for serial programming:

Internals

It took some time to put together the exact pinout of that header as well as what pins of the ESP8266 the various inputs/outputs are connected to, so here they are, all these details in one place:

ESP8266 pin usage

ESP8266 Pin	Function	GPIO (Use in Arduino Schetch)	Connected to
10 – MTDI	GPIO	GPIO 12	Relay + BLUE LED (HIGH on)
12 – MTCK	GPIO	GPIO 13	GREEN LED (LOW to turn on)
15 – GPIO0	Flash	GPIO 0	Switch (LOW when pressed)
25 – RDX	UART RXD	GPIO 3	J1 Pin 2
26 – TXD	UART TXD	GPIO 1	J1 Pin 3

Pinout

VCC	Where the white arrow/triangle is
RX
TX
GND

And finally the small Arduino script that:

• creates its own Access Point (essential for me, given the above)
• listens to presses on the physical switch to control the device (and does some basic debouncing)
• starts a very basic web server on port 80, that turns on/off the mains switch depending on which page is accessed

#include <ESP8266WiFi.h>
#include <ESP8266WebServer.h>

IPAddress localIP(192,168,4,1);
IPAddress gateway(192,168,4,1);
IPAddress subnet(255,255,255,0);

#define BUTTON_PIN 0            // LOW when pressed
#define GREEN_LED_PIN 13        // LOW ON
#define RELAY_BLUE_LED_PIN 12   // HIGH ON
#define DEBOUNCE_DELAY 200      // in millisecs

ESP8266WebServer server(80);
volatile bool isOn = false;

void handleRoot() {
  server.send(200, "text/html", "
<h1>Use either /on or / off</h1>
");
}

void handleOn() {
  if(isOn) {
    server.send(200, "text/html", "
<h1>Already ON, nothing to do.</h1>
");
  } else {
    isOn = true;
    digitalWrite(RELAY_BLUE_LED_PIN, isOn);
    server.send(200, "text/html", "
<h1>Turned ON.</h1>
");
  }
}

void handleOff() {
  if(!isOn) {
    server.send(200, "text/html", "
<h1>Already OFF, nothing to do.</h1>
");
  } else {
    isOn = false;
    digitalWrite(RELAY_BLUE_LED_PIN, isOn);
    server.send(200, "text/html", "
<h1>Turned OFF.</h1>
");
  }
}

void setup() {
  pinMode(BUTTON_PIN, INPUT);
  pinMode(GREEN_LED_PIN, OUTPUT);
  pinMode(RELAY_BLUE_LED_PIN, OUTPUT);

  digitalWrite(GREEN_LED_PIN, HIGH);   // Turn Green LED OFF
  digitalWrite(RELAY_BLUE_LED_PIN, LOW);  // Turn relay OFF

  // Attach an interrupt to the pin, assign the onChange function as a handler and trigger on changes (LOW or HIGH).
  attachInterrupt(BUTTON_PIN, onSwitchChange, CHANGE);

  // create access point
  // void softAP(const char* ssid, const char* passphrase, int channel = 1, int ssid_hidden = 0);
  WiFi.softAPConfig(localIP, gateway, subnet);
  WiFi.softAP("sonoff", "******", 1, 0); // hide the SSID

  // start web server
  server.on("/", handleRoot);
  server.on("/on", handleOn);
  server.on("/off", handleOff);
  server.begin();
}

void loop() {
  server.handleClient();
}

volatile long lastEventTime = 0;
volatile bool lastState = HIGH;

void onSwitchChange() {
  long currentTime = millis();
  bool currentState = digitalRead(BUTTON_PIN);

  // debouce by making sure this event doesn't happen too often
  // debouncing is done on ALL changes
  if(currentTime - lastEventTime > DEBOUNCE_DELAY) {
    // toggle the relay and store its current state
    // toggling is done only on FALLING events (when switch is pressed -> LOW)
    // we need to check that it's actually Falling as opposed to just LOW to avoid dupe events
    if(currentState == LOW && lastState == HIGH) {
      isOn = !isOn;
      digitalWrite(RELAY_BLUE_LED_PIN, isOn);
    }
  }

  lastEventTime = currentTime;
  lastState = currentState;
}