EmonTx V3.4

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emonTx V3.4

The emonTx V3.4 is the latest version of the emonTx Low Power Wireless Energy Monitoring Node designed for monitoring AC electrical power on a maximum of 4 separate (household/building) circuits using non-invasive clip-on CT sensors and an AC-AC Voltage adapter to provide a voltage signal for Real Power calculations. The emonTx V3.4 is a minor update to the emonTx_V3

emonTx V3.4


New features on emonTx V3.4 over V3 shown in bold

  • Measure AC Apparent Power, AC Real power* and AC RMS voltage*
  • 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 input for connecting DS18B20 temperature sensors
  • Single AC-AC adapter can power the unit and provide AC voltage measurement
  • An on-board 3x AA battery option with remote monitoring of battery voltage
  • Terminal block access to power rails, digital and analogue I/O and IRQ port for connecting pulse counting sensor / DS18B20 temperature / Aux sensors
  • DIP switch selection of RF node ID and 240V/120V AC adapter selection, see #DIP Switch Config

* when AC-AC voltage adapter is connected

Port Map

emonTx V3.4

emonTx V3 Port map

emonTx V3 Ports

A schematic diagram and PCB layout (Eagle CAD format) can be found on GitHub.

Note: The FTDI connector Tx and Rx pins are reversed on the PCB legend and on the Schematic. Data is received by the emonTx on the Tx pin and transmitted by the emonTx on the Rx pin.


Using an ATmega328 microprocessor, the emonTx V3.4 runs standard Arduino sketches. It is easy to customise and upload code using the Arduino IDE and a USB to UART cable.

The data from the emonTx V3.4 is transmitted via an RFM12B / RFM68CW 433/868 MHz radio 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. http://emoncms.org) for logging, processing and graphing.

DIP Switch Config

The emonTx V3.4 has two DIP switches enabling selection of RF node ID and setting UK/EU or US AC-AC adapter choice without having to change the firmware. The unit must be restarted after changing DIP switch positions. Default DIP switch selection (both off) is node ID 10 and 240V AC.

Note: newer emonTx V3 firmware (V2.3+) use node ID 8 and 7, instead of 10 and 9 (shown below) see emonhub config guide for more info

Note#2: RF node ID can now be set using serial config. See forum post

EmonTx V3.4 DIP Switch.jpg

Power Supply Options

There are four ways to power the emonTx V3:

  1. USB to UART cable - only recommended for short periods while programming, it is recommended to remove all other power sources
  2. 5V DC Mini-USB cable - remove jumper JP2 when powering via DC if AC adapter is present
  3. 3 x AA Batteries - remove jumper JP2 when powering via DC if AC adapter is present
  4. 9V AC-AC power adapter - with jumper JP2 closed (If jumper 2 is left open then the AC-AC adapter will be used for power sampling but not to power the emonTx V3 - see Powering via AC below)

Powering via DC (1-3)

When powering the emonTx V3 via DC (1-3) and running the default emonTx firmware the emonTx V3 will take Apparent Power measurements, measuring only current, assuming the Power Factor of 1 and Vrms to be 230V, power saving mode will also be implemented, going into sleep mode between readings (default every 10s) to save battery power.

The red LED indicator will illuminate for a couple of seconds then extinguish never to flash again if powering from a DC source. If the LED flashes multiple times at startup this indicates the emonTx is detecting an AC waveform - this can happen if an AC adapter has just been removed, hit the reset button if this occurs. When powering the unit via 5V USB/battery it is advisable to remove the JP2 jumper if an AC-AC adapter is present.

Powering via AC (4)

A nice feature of the emonTx V3 is the ability to use an AC-AC adapter to power the unit while simultaneously providing an AC voltage sample. If the emonTx V3 detects the presence of an AC adapter at startup, it will automatically implement Real Power and Vrms measurements by sampling the AC voltage. Real Power is what you get billed for, and depending on the appliances connected to the circuit being monitored, can vary significantly from Apparent Power - see Building Blocks for more info on AC power theory. For best energy monitoring accuracy, we recommend powering the EmonTX with an AC-AC adapter whenever possible.

Using the AC voltage for reference also enables the emonTx to monitor the direction of current flow. This is important for solar PV monitoring. If you notice a negative reading when you were expecting a positive one, reverse the orientation of the CT on the conductor. The red indicator LED will flash at startup then flash once every 10 seconds to indicate an AC waveform has been detected. Sleep mode will be disabled to keep a more consistent power draw on the AC circuit.

Important note regarding powering with AC: powering via AC is recommended only for standard emonTx operation without auxiliary sensors (apart from a maximum of 4 DS18B20 temperature sensors) or equipment (e.g. relay modules) connected. Correct operation via the AC supply is critically dependent upon using the correct AC-AC adapter. If you are using the recommended AC-AC adapter and the current draw exceeds 10 mA and the mains supply is below the minimum allowable, then circuit operation will be impaired, adversely affecting the accuracy of the emonTx. To avoid damage to the emonTx V3 circuits, the current drawn from the AC circuit should never exceed 60mA - see the technical wiki for more info. If more than 10 mA of current is required, it is recommended to remove jumper 2 (JP2) and power the emonTx via the 5V mini-USB connector. When JP2 is removed, the AC-AC adapter (if connected) will be used only to provide an AC voltage sample, i.e. it will not power the emonTx.

Uploading Firmware

Latest firmware source code and compiling instructions can be found in the Firmware section of emonTx 3 Github:


Also see compiling instructions are part of the user guide:


Flashing Pre-compiled .hex

The main emonTx v3.4 RFM69CW firmware as pre-loaded onto pre-build emonTx, is also supplied as a pre-compiled .hex file. Download from:


Linux : use AVRdude

$ sudo apt-get install avrdude

$ avrdude -v -c arduino -p ATMEGA328P -P /dev/ttyUSB0 -b 115200 -U flash:w:emonTxV3_RFM69CW_DiscreteSampling_433.cpp.hexx

Specify the port your USB to UART programmer is connected to, and the path name of the .hex file

Windows :you might be able to use the the XLoader application to upload .hex files (we have not tested this, please update the wiki if you have)

Flashing the Bootloader

Not normally required

The OptioBoot Arduino Uno serial bootloader should be ready loaded onto the ATmega328 on the emonTx from the factory. If however, for any reason you need to reload the bootloader and set the fuses, here is the procedure:

The stock emonTx firmware including the bootloader, can be downloaded in pre-compiled .hex form from github. Look for a .hex file including the word "bootloader" in the title, on the emonTx V3 GitHub page.

The .hex file can be flashed, and the fuses set, on the ATmega328 using the following avrdude command:

$ sudo avrdude -v -p atmega328p -c avrispmkII -P usb -e -Ulock:w:0x3F:m -Uefuse:w:0x05:m -Uhfuse:w:0xDE:m -Ulfuse:w:0xFF:m -U flash:w:emonTxV3_4_RFM12B_DiscreteSampling_bootloader.cpp.hex:i  -Ulock:w:0x0F:m

or, to speed things up:

$ sudo avrdude -V -u -p atmega328p -c avrispmkII -P usb -e -Ulock:w:0x3F:m -Uefuse:w:0x05:m -Uhfuse:w:0xDE:m -Ulfuse:w:0xFF:m -U flash:w:emonTxV3_4_RFM12B_DiscreteSampling_bootloader.cpp.hex:i  -Ulock:w:0x0F:m

If, for any reason e.g to make a backup, you want to read the data from the ATmega328 this can be done with:

 $ avrdude -v -p atmega328p -c avrispmkII -P usb -U flash:r:test_intel.hex:i

Arduino Firmware Info

See the GitHub readme for firmware info:

All emonTx V3 firmware is available on Github:emonTxFirmware

All emonTx V3s sold pre-assembled with a radio module are shipped with Discrete Sampling firmware. This is the main firmware for the emonTx V3 and has been tested and calibrated based on nominal component values. If better accuracy is required, and facilities are available, the unit should be re-calibrated to take account of variations present in the components used. You will need access to a programmer to reload the sketch. If you ordered an emonTx V3 without a radio module, no firmware will be loaded, although there will be a bootloader present. You will need access to a programmer to load a sketch.

The Discrete Sampling firmware has the following features:

  • Detection of AC-AC adapter sets Apparent Power / Real Power Sampling accordingly
  • Detection of battery / USB 5V or AC-DC power method and sets sleep mode accordingly
  • Detection of CT sensors. Samples only from connected channels
  • Detection of remote DS18B20 temperature sensor

This firmware is suitable for real power measurements in a single-phase installation only. If you have a three-phase installation and need to measure real power, you will need to replace the sketch with the equivalent three-phase firmware.

Note: Detection of the connected senors use takes place at power-on. The sketch must be re-started (press the reset button) if any sensors are connected or disconnected after power-on.

The emonTxV3_RFM12B_Discrete Sampling firmware samples 10 full AC cycles, i.e. 200 ms per measurement from each CT at a rate of approximately 2500 samples per second. It is assumed the power does not fluctuate significantly during the time the microcontroller is asleep, which is usually good enough for typical household monitoring. If greater precision is required (as is the case with PV energy Diversion it is recommended that you use a PLL or continuous sampling sketch. See the RFM12B firmware examples folder on GitHub.

Standard Operation

CT Energy Monitoring

Extended Operation

Temperature Monitoring

A DS18B20 digital temperature sensor can easily be connected to the emonTx V3 by connecting the sensor to the emonTx V3 screw terminal block or RJ45 connector. The default firmware (discrete sampling) supports auto-detecting one DS18B20 sensor. Multiple DS18B20s can be daisy-chained, but this will require changes to the pre-installed emonTx firmware - you may also run into power draw issues depending on how you are powering the emonTx (see Power Supply Options above).

In order to save power when running on batteries, the emonTx V3 supports switching off of the DS18B20 in-between readings and performing the temperature conversion while the ATmega328 is sleeping. To do this, power (3.3 V) is supplied to the DS18B20's power pin from Dig19 (ADC5), this digital pin is switched off between readings. (This facility is available only if the temperature sensors are connected via the terminal block.) The data connection from the DS18B20 is connected to Dig5, this I/O pin has a 4K7 pull-up resistor to 3.3 V on-board the emonTx V3's PCB, as required by the DS18B20.

RJ45 Connection

A DS18B20 sensor can be connected directly to the RJ45 socket, or an RJ45 to terminal block breakout can be used to connect multiple sensors:

EmonTx RJ45 DS18B20.jpg

Note: The RJ45 socket does not support power supply switching via Dig19 (ADC5) as described above.

Shop Links

DS18B20 sensor on RJ45

RJ45 to Terminal Block Breakout

Encapsulated DS18B20 temperature sensor

Additional RJ45 breakouts are available from Sheep Walk Electronics

The emonTx V3.4 uses a standard RJ45 DS18B20 pin-out as used by Sheepwalk Electronics and others:

EmonTxV3 4 RJ45-Pin-out.png

Terminal Block Connection

To connect an external DS18B20 to the emonTx V3 screw terminal block connections are as follows

DS18B20 emonTx V3 screw terminal connections
Screw terminal pin DS18B20 Connection
3 - GND GND (Black)
5 - Dig19 Power (Red)
6 - Dig5 Data (White)

Utility Meter Interface

Optical Pulse Counting

The OpenEnergyMonitor optical pulse sensor can be connected directly to the emonTx V3 via its RJ45. As of March 2015, pulse counting has been integrated into the main emonTx V3 firmware and shipped as standard

Many meters, including single phase, 3-phase, import and export electricity meters, gas meters and water meters, have pulse outputs. The pulse output may be a flashing LED or a relay (typically solid state) or both. We recommend using the optical interface if one is available, as this decouples the monitoring equipment from the mains.

In the case of an electricity meter, a pulse output corresponds to a certain amount of energy passing through the meter (kWh/Wh). For single-phase domestic electricity meters e.g. Elster A100c, each pulse typically corresponds to 1 Wh (1000 pulses per kWh). Water and gas meters will usually be marked to show the quantity of water (litres/gallons) or gas (cubic meters/cubic feet) that each pulse represents.

The emonTx V3.4 has one spare interrupt input (IRQ 1, Dig3) which can be used for pulse counting. This is broken out on terminal block port 4 and RJ45 socket

Pulses and the most recent pulse count are transmitted via RF, as the final variable in the JeeLib packet structure. The emonCMS wh_accumulator input process can be used to log the pulse count. The wh_accumulator input process detects when the pulse count gets reset to zero (after the emonTx is reset) and continues to accumulate, ignoring the reset. A scaler input process can be used to convert number of pulses to kWh. For example: my utility meter outputs 800 pulses per kWh, so each pulse is 0.8Wh. I can multiply the number of pulses by 0.8 to get the number of Wh, or by 0.0008 for the number of kWh accumulated.

Read more about pulse counting pulse sensor documentation page.

emonTx V3 Hardware Connections

Connect the pulse input into emonTx V3.4 terminal block port 4 (IRQ 1 / Digital 3) or RJ45 socket pin 6. Terminal connections shown below. If you are connecting a hard-wired pulse output you may need to add a pull-down resistor. If you are using an optical counter (e.g TSL257), connect the power pin to the 3.3V or 5V rail. (if powered from the 5V mini-USB conector)

emonTx V3.4 pulse-counting screw terminal connections
Screw Terminal Connection
1 5V (if powered via 5V USB)
2 3.3V
4 IRQ 1 / Dig3

There is an pull-up resistor connected to the pulse (IRQ) input that is enabled in the standard sketch. Therefore, you can connect a volt-free contact or an SO output between screw terminal 4 / RJ45 pin 6 (IRQ input, SO+) and screw terminal 3 / RJ45 pin 5 (GND, SO-) without the need for an additional resistor. If you must connect your contacts between VCC (screw terminal 2 / RJ45 pin 2) and screw terminal 4 / RJ45 pin 6, then you must add a pull-down resistor of resistance low enough to overcome the internal pull-up resistor, or you can use a higher-value resistor and modify the sketch to disable the internal pull-up.

We recommend powering the emonTx v3 from either a 5V USB or AC-AC adaptor when used for pulse counting operation. Due to the additional power requirements of the optical pulse sensor, battery life will be reduced significantly compared to an emonTx powered by 3 AA batteries for CT operation only.

Direct Optical Interface

if you have an Elster meter (tested with Elster 100C) the emonTx V3 with an IR TSL261R sensor can be used to interface directly with the meter protocol to read off the exact accumulated watt hours that you have generated or used. This reading can be used on its own or to cross-check and calibrate CT based measurement. See here for original blog post


RF Link

The emonTx transmits data to a web-connected base station (Raspberry Pi with RFM12Pi / RFM69Pi) via a low power 433MHz / 868 MHz radio.


See the RFM12B Building Block documentation:

RFM12B - Part 1 - Hardware Overview

RFM12B - Part 2 - Sending Data Between Modules


Using the latest JeeLib library the RFM69CW is backward compatible with RFM12B

See RFM69CW Building Block documentation for genral info on RFM69CW

To enable RFM69CW, ensure that define RF69_COMPAT at the begging of the sketch is set to '1'


use a pre-compiled .hex file with RFM69CW in the title. See emontx V3.4 GitHub repo

Emontx34 RFM69CW.jpg

RFM69CW module orientation as installed on an emonTx v3.4

Wired Connectivity

Whilst the system was designed to transfer data via radio it is possible to link the emonTx and Raspberry Pi by wires.

It should be noted that this is NOT the standard setup.

Direct connection emonTx V3 > Raspberry Pi GPIO

Both the Raspberry Pi and emonTx v3 run at 3.3V so the serial Receive and Transmit lines can be connected directly. The 5V power rail from the Raspberry Pi can be supplied to the emonTx which is then stepped down to to 3.3V by the emonTx voltage regulator. 5V is provided by the red wire (see photo). The ground connection is the black wire and the serial data going from the emonTx to the Raspberry Pi is the green wire. Wire for serial data going the opposite direction (Pi to emonTx) has not been connected in this example but could be added if two-way communication is required.

Note: on the PCB and schematic, the Tx and Rx pins are labelled according to the connections on the Programmer, meaning that data is received by the emonTx on the Tx pin, and transmitted by the emonTx from the Rx pin.

Emontx to pi serial emonTx Pi direct connection

For emonTx V3 firmware V2.5+ then no firmware change is required, the directserial-serialtx3e emonhub interfacer can be used to decode the serial output see directserial-serialtx3e emonhub interfacer config guide

If you wish to disable RF connectivity completely, the emonTx V3 no RF FW can be used together with emonhub DirectSerial interfacer .

Electrical Characteristics - Absolute min / max

emonTx V3.4 Datasheet
Function Parameter Min Recommended

Max / typical

Absolute Max

if exceeded, damage may occur

CT 1-3 Monitoring Power @ 240V 23kW / 95.8A 60kW / 250A Using 22R burden and YHDC SCT-013-00 with 22 turns
CT 4 Monitoring Power @ 240V 4.5kW / 19.2A 4.6kW max measure – 19.7kW / 82A Max dissipation Using 120R burden and YHDC SCT-013-00 with 22 turns
DC half-wave power supply Current output 20mA 60mA Current draw above 20mA AC sample signal will be affected
3.3V Rail current output When powered via 5V USB connector 150mA 168mA Limitation SOT22 MCP1700 Ta=40C Vi=5.25V
Operating Temperature Temperature -40C +40C +85C
Storage Temperature Temperature -50C +150C
5V Input Voltage USB/FTDI/5V Aux +3.4V +6V +6.5V (see note 1) See note 1
3.3V Supply Voltage on 3.3V supply rail 2.6V * 3.3V 3.9V RFM69CW *ADC readings will be incorrect if Vcc is not 3.3V

See emonTx V3 Wiki for Design Calculations


  • The emonTx V3 PCB is 100mm x 80mm and enclosed in an EBS80 enclosure sourced from Lincoln Binns, see data sheet

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