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emonTx TH

emonTx TH Setup with internal and external]


The emonTH is an open-source, battery powered, Temperature and Humidity monitoring wireless node.

It's been designed to be an easy to deploy tool for monitoring building / room temperature and humidity.

The data from the emonTH is transmitted via wireless (433/868Mhz) to an emonBase web-connected base-station (we recommend a Raspberry Pi with an RFM12Pi) which then posts the data onto an emoncms server (e.g. emoncms.org) for logging, processing and graphing. The 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!


  • Temperature and Humidity sensing options - Using DHT22 temperature and humidity sensor, or if humidity is not required just a DS18B20 temperature sensor. Both DHT22 and DS18B20 can be used together as shown above for internal and external readings.

  • Easy to set-up - the unit comes pre-assembled and pre-loaded with Arduino compatible firmware. If desired the code can easily be changed by using an USB to UART cable and the Arduino IDE to upload a sketch of your choice onto 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 reading once every 60s the emonTH batteries should last for 1-3 years. We recommend rechargeable alkaline batteries for best performance and environmental impact (see blog post).

  • Expansion Options - If desired the emonTH function can easily be expanded: remote DS18B20 temperature sensors can be screwed into the terminal block for outdoor temperature monitoring, multiple DS18B20 temperature sensors can be connected at once on a digital one-wire bus.
    • An optical sensor can be added for interfacing with a pulse-output utility meter or a relay board could be connected for controlling an appliance.
    • Update: the emonTH now supports multiple DS18B20's, see blog post

  • Flexible choice of RF radio - Using the RFu328 with ATmega328 microprocessor as a common platform emonTH can work with either an RFM12B or Ciseco SRF RF radio


  • Microcontroller: ATmega328 on RFu328 board
  • Sensors: DHT22 (temperature & Humidity) / DS18B20 (temperature) sensor options
  • Power: 2 x AA from on-board holder, LTC3525 3.3V DC-DC boost converter to extend battery life
  • RF Radio: Using the RFu328 with ATmega328 as a common platform emonTH can work with either RFM12B or Ciseco SRF radio
  • Battery life: 1-3 years expected, see blog post
  • On-board LTC3525-3.3 DC-DC boost converter see emonTH hardware blog post

Port Map

EmonTH V1.4 PortMap.png

emonTx TH Setup with internal and external


See getting started with the emonth: Getting started with the emonTH


DHT22 Temperature and Humidity Sensor

  • Power supply 3.3-6V DC
  • Output signal digital signal via single-bus
  • Sensing element Polymer capacitor
  • Operating range humidity 0-100%RH; temperature -40~80Celsius
  • Accuracy humidity +-2%RH(Max +-5%RH); temperature <+-0.5Celsius
  • Resolution or sensitivity humidity 0.1%RH; temperature 0.1Celsius
  • Repeatability humidity +-1%RH; temperature +-0.2Celsius
  • Humidity hysteresis +-0.3%RH
  • Long-term Stability +-0.5%RH/year
  • Sensing period Average: 2s
  • Independent sensor test report

DS18B20 Temperature Sensor

  • Power supply range: 3.0V to 5.5V
  • Accuracy over the range of -10°C to +85°C: ±0.5°C.
  • Storage temperature range:-55°C to +125°C (-67°F to +257°F)

RF Connectivity


The Rfu328 sold through the OpenEnergyMonitor shop is ready setup for use with the RFM12B, just make sure you solder the RFM12B with the correct orientation if soldering it yourself: the crystal on the RFM12B should be on the same side as the tiny 16Mhz ATmega328 crystal on the RFu328. To use the RFM12B on the RFu328 a modified version of the JeeLib RF12 library has been created. This is called the RFu_JeeLib see GitHub readme for the modifications which have been undertaken. Once using the RFu_JeeLib the RFM12B on the RFu348 can be used exactly the same as before, all code written for the emonTx V2, JeeNode etc should work just fine using using the RFu_JeeLib library.


A Ciseco SRF module can be used on the emonTx V3. The easiest thing to do is to purchase a complete RFu328 SRF setup from Ciseco.However a SRF can be used on the RFu328 purchased from the OpenEnergyMonitor shop with one small modification. The RFu328 sold through the OpenEnergyMonitor shop comes read setup for use the RFM12B, to use it with the SRF you will need to remove one SMT resistor and rotate another by 90 degress. Follow these instructions from Ciseco IN REVERSE!

Electrical Characteristics

Power Consumption

See blog posts:



Enclosure Case

The emonTH by default is shipped without the SMT USB mini-B connector since the standard case does not allow for use of the USB port to power the emonTH. However there has been a community contributed 3D printable case design contributed which does allow for use of the USB connector. The 3D CAD design for this case is up on Thingiverse:


[File: http://thingiverse-production.s3.amazonaws.com/renders/3f/4b/1a/1c/93/photo_2_preview_featured.jpg]


Arduino compatible, a UART to USB cable is required to upload code

EmonTH Github repository

Open-Hardware Design Files

All the Design files for the emonTH are hosted on Git based SolderPad, see: http://solderpad.com/git/openenergymon/emonth

emonTx TH Setup with internal and external


Proudly open source

The hardware designs (schematics and CAD files) are licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

The firmware is released under the GNU GPL V3 license

The documentation is subject to GNU Free Documentation License

The hardware designs follow the terms of the OSHW (Open-source hardware) Statement of Principles 1.0.

Environmental & Life Cycle

We are passionate about sustainability and are aware of the embodied energy and use of resources involved in electronic manufacture. We try our best to reduce environmental impact wherever possible:

  • We have recently been inspired by a few projects taking a lead in promoting and making steps towards Ethical and Sustainable Electronics, see our blog post we wrote on the topic .
  • The PCB's are manufactured in the UK by a manufacturer who uses lead free techniques, complies to the highest environmental industry standard and is actively investing in techniques and equipment to reduce wastage and environmental impact (e.g water treatment and recycling). Hot-air leveling was chosen instead of immersion gold finish to reduce environmental impact.
  • The assembly is also done in the UK with all components being RoHS compliant and free of conflict materials.
  • Surface freight is used in preference to air shipping when ordering parts in bulk. This consumes 33 times less energy.
  • We have strived to optimise electrical consumption in our hardware to be as low was possible and recommend the use of green rechargable batteries, see blog post



The OpenEnergyMonitor system is sold as a development kit to empower members of the openenergymonitor community to to get involved with the OpenEnergyMonitor open-source energy monitoring development project.

Your safety is your own responsibility, including proper use of equipment and safety gear, and determining whether you have adequate skill and experience. OpenEnergyMonitor and Megni registered partnership disclaims all responsibility for any resulting damage, injury, or expense. It is your responsibility to make sure that your activities comply with applicable laws, including copyright. Always check the webpage associated with each unit before you get started. There may be important updates or corrections! All use of the instructions, kits, projects and suggestions given both by megni.co.uk, openenergymonitor.org and shop.openenergymonitor.org are to be used at your own risk. The technology (hardware , firmware and software) are constantly changing, documentation (including build guide and instructions) may not be complete or correct.

If you feel uncomfortable with assembling or using any part of the kit, simply return it to us for a full refund.