Loading... Please wait...

emonTx - Energy Monitor Transmitter

  • Image 1
  • CT sensors and AC-AC adapters NOT included - can be purchased separately
  • Image 3
  • emonTx V3.4 PCB
  • DS18B20 on RJ45 temperature sensor connection
  • Image 6
  • Image 7
  • emonTx V3.4.3 with ESP8266 UART header
£71.99 (inc VAT) £59.99 (exc VAT)
300.00 Grams
Calculated at checkout

Sorry but this item is currently unavailable.

Please check back at a later stage.

The emonTx V3.4 is a low power wireless energy monitoring node designed for monitoring AC electrical power on (up to) 4 separate (household/building) circuits using non-invasive clip-on CT current sensors and an AC-AC Voltage adapter to provide a voltage signal for full Real Power calculations and AC VRMS readings.

The emonTx is designed to be used in conjunction with an emonPi or emonBase web-connected base-station to post data online to Emoncms.org and/or log data locally to the Raspberry Pi's SD card. Alternatively it is possible to add Wi-Fi directly to the EmonTx with a ESP8266 WIFI adapter.


To get started visit the OpenEnergyMonitor Setup Guide.


Required for Operation

  • One or more clip-on CT sensor(s)
  • 9V AC-AC adapter power supply (UK Plug), (Euro Plug) or (US Plug) - enables accurate Real Power measurements. (Alternatively 3 x AA batteries / 5V DC USB Mini-B Power Supply can be used for less accurate Apparent Power measurements.)



  • Measure AC Apparent Power, AC Real power* and AC VRMS voltage (when AC-AC voltage adapter is connected)
  • 3 x single-phase CT current sensor inputs (100A / 24KW @ 240V max)
  • 1 x high sensitivity single-phase CT current sensor input channel (18.8A / 4.5KW @ 240V max)
  • 1 x RJ45 I/O - connect chain of 3x DS18B20 temperature sensors or pulse counting IRQ sensor
  • Terminal block access to power rails, digital and analogue I/O, IRQ port and PWM for connecting pulse counting sensor / 3x DS18B20 temperature / Aux sensors
  • Single AC-AC adapter can power the unit and provide AC voltage measurement
  • RFM69CW 433Mhz Low Power RF Module with SMA antenna
  • DIP switch selection of RF node ID and 240V/110V AC adapter calibration selection
  • Arduino compatible ATmega328 8-bit microprocessor
  • Aluminium wall mount enclosure with recycled acrylic fascia
  • CE Certified
  • Made in UK with RoHS components free of conflict materials
  • The unit is manufactured and assembled with care in the UK.

Optional: The EmonTx can be optionally powered by 3x AA batteries. Select the 3x AA battery holder option above if you wish to use this option. In order to preserve battery life the EmonTx needs to go to sleep between readings, we upload our discrete sampling firmware to the EmonTx to achieve this rather than the standard continuous sampling firmware. The discrete sampling firmware is less accurate than the continuous sampling firmware so its worth only selecting this option if you need to power the EmonTx from battery power.


The emonTx with the default firmware is suitable for use on a single-phase supply, or on a split-phase supply, or on one phase of a multi-phase supply. Firmware is available which will allow for 'approximate' 3-phase monitoring (described here), which we can upload prior to shipping if it is requested.


Includes (as standard)

  • emonTx V3.4 PCB fully assembled
  • SMA Antenna 
  • Aluminium enclosure & custom fascia - assembled
  • Firmware uploaded and tested


Does Not Include (as standard)

Note: CT sensors give a much better reading of instantaneous power than pulse counting, we recommend using a CT wherever possible. When using the pulse counting method to monitor power from a utility meter the sample rate is limited to the number of pulses; at low power values the interval between pulses can be extremely long.


RF Node ID

Multiple emonTx unit's can operate on a single RF network posting to a single emonPi / emonBase web-connected base station (Raspberry Pi + RFM69Pi), each emonTx on the same network must have an unique node ID. The nodeID can be selected at time of purchase or set using the on-board DIP switch to toggle. If more than two emonTx's are required on the same network then further nodeID values can be set via RF node ID serial config.  


Temperature Sensors 

Up to 3x DS28B20 Temperature sensors can be connected to either the RJ45 sonnector or wired into the terminal block. Only genuine Maxim DS18B20 sensors should be used (as sold via the shop), incompatabilty with non genuine sensors has been reported. DS18B20 can be wired into the terminal block using screw terminals:

2 - 3.3V Power  

3 - GND

6 - 1-wire data 



The emonTx is Arduino compatible: updated/customised firmware sketches can be uploaded using Arduino IDE and a USB to UART cable (available separately)

The emonTx comes pre-loaded with the EmonTxV3CM Continuous Sampling firmware as standard for single phase operation. If the battery holder is selected we will upload the low power discrete sampling firmware see note above.

Customised firmware sketches and be easily uploaded from the PlatformIO / Arduino IDE and a USB to UART cable (not included).

Firmware repositories:


Note: even though the unit is pre-programmed we recommend a USB to UART cable to keep at hand for debugging and updating / customising firmware using Arduino IDE.




Customers Who Viewed This Product Also Viewed