I'm happy to say now is possible add WIFI to your DIY controller with less than $5.
For it is needed a NodeMCU and a logic level shifter because it works with 3.3V.
I'm using a NodeMCU V3 because it has an ESP-12 and USB interface built-in and it's very cheap.
1 x NodeMCU V3 ;
1 x Logic level shifter 5 to 3.3 V
See the video to learn how to set up the MQTT client.
Warning: Don't forget to turn off the UART debug log in ESP-link interface.
Watch on youtube.com
PS. Any model of ESP8266 supported by ESP-link can be used but please do not ask me how to make the wiring to change the firmware.