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Build with Ardunio IDE and Board Extension ESP8266WiFi //

Using the following pins:

Function Pin Description
Force WLAN Config D4/GPIO2 Causes acess point to open for WLAN config during normal operation
Transmitter D6/GPIO12  
USB/Flashing D9/GPIO3 fixed pin
  D10/GPIO1 fixed pin
Required for reset D8/GPIO15 must be low for reset
(bad with I2C pullups)
  D4/GPIO2 sometimes used for buildin LED
(must be high for reset)
  D3/GPIO0 must be low for reset

Libraries required:    

Adafruit_MQTT.h Required Adafruit MQTT Lib Version >=0.16.1

Sorcecode is available at

For the hardware use any ESP8266.
For beginners use a module with "buildin" USB (Like NodeMCU or WEMOS D1 mini or D1 minipro).

The transmitter MUST be operated with 3.3V (therefor the most "Arduino transmitters" don't work) and you MUST attach an antenna (at least a cable with exactly 17,3cm).
If you coil up the antena you reduce range (or use one of the listed below).

For Gemany:
ACHTUNG! Stellen Sie vor Inbetriebnahme sicher, diese nur mit der zulässigen Sendeleistung zu betreiben. Die aktuellen Bestimmungen können unter überprüft werden.;jsessionid=26A12072F86B6B3DECFD82321C669DA9?__blob=publicationFile&v=5

For other countries:
Check your allowed frequency (mostly 433MHz or 315MHz), max transmitting power (for example in Germany 10 mW) and what other laws you have to follow.

Easiest way: Use only official and licensed modules for your country , no self build stuff.

Hardware examples (only examples, I used other parts, but don't have the links anymore):

433MHz transmitter