SenseAir S8 CO2 module on an ESP8266 ESP-12E. The client publishes over MQTT and updates a channel on ThingSpeak.

Moz-TW uses ThingSpeak instead of MQTT. The hardware is a LoLin NodeMcu V3. The wiring follows,

NodeMCU    S8
      G -- GND
     D7 -- TX
     D8 -- RX
     VU -- Vcc ( 5V)

Real-time CO2 value in MozTW is published on ThingSpeak.

ESP8266 Core is licensed under LGPL. My implementation of S8 ModBus follows MIT license.