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 UHF radio (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 which will last over 4 years. Use alkaline batteries for longest battery life or rechargable eneloops for best environmental benefit.

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
4 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