The emonTH V2 is an open-source, wireless, battery powered temperature and humidity monitoring node. It is an easy to deploy tool for monitoring building / room temperature and humidity. Data from the emonTH is transmitted via wireless RF (433MHz) to an emonPi / emonBase web-connected base-station for logging to Emoncms for data logging, processing and graphing. Room temperature and humidity data can be used to inform a heating control system, feed into a building performance model or simply for general interest!

Features

Temperature and humidity sensing - using the onboard Si7021 temperature and humidity sensor and (optionally) an external DS18B20 temperature sensor.

• Easy to set-up - the unit comes pre-assembled and pre-loaded with Arduino compatible firmware. If desired the code can easily be modified using Arduino IDE or PlatformIO and uploaded using an USB to UART cable to the on-board ATmega328 microprocessor.

• Long Battery Life - The emonTH is powered by two AA batteries through a high efficiency DC-DC boost converter circuit. Taking a temperature and humidity reading once every 60 seconds the emonTH batteries can last for several years (real world tested: 3 years+). See emonTH V2 battery estimation calculations & AA battery considerations (blog post).

• Expansion Options - An optical pulse sensor can be attached to the screw terminal connector for interfacing with a pulse-output utility meter. See hardware wiki for optical pulse sensor connection info.

• RF node ID DIP switch Selection - Up to four RF node IDs (23,24,25 & 26) can be set via physical onboard DIP switches. See hardware wiki.

Specification

• Microcontroller: Arduino compatible ATmega328p
• Internal Sensor: Si7021 (internal temperature & Humidity)
• External Sensor (OPTIONAL): DS18B20 external temperature sensor. See hardware wiki for connections
• Power: 2 x AA from on-board holder, LTC3525 3.3V DC-DC boost converter to extend battery life
• RF Radio: RFM69CW 433Mhz
• Acrylic enclosure
• 3 year+ battery life 
• External dimensions: 71 x 71 x 27 mm

Si7021 Temperature & Humidity Sensor

• Silicon Labs SI7021-A20-GM1R
• Power supply 1.9V - 3.6V
• Operating range humidity 0-80%RH; temperature -40~125 deg C
• Accuracy humidity +-3%RH
• Accuracy Temperature: +-0.4 degC
• Active current: 150uA
• Sleep current 0.06uA
• Measurement time: 2.6ms = 10ms

Si7021 (emonTH V2) Vs. DHT22 (emonTH V1) comparison 

Documentation

• emonTH V2 Setup Guide
• emonTH V2 GitHub (firmware & hardware CAD design)
• Technical hardware Wiki Documentation (emonTH V1.5)