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emonTx SMT

The emonTx is the data-gathering heart of the system. In the most basic configuration, it uses just one current transformer (CT - clip on current-sensor). This can be added to with more current transformers, a voltage monitor, a pulse input and temperature sensors. It reads the data from the sensors, does calculations to convert the raw data into useful values and transmits the results via wireless to either the emonBase (web-connected base sation) unit or the emonGLCD display, or both.

  1. Basic configuration: -one current transformer. Every few seconds, the program in the emonTx samples the mains current waveform. It takes over 50 readings for each cycle, and from that it calculates the "RMS" average value of the current. Assuming a standard voltage, it goes on to calculate the apparent power and transmits this to the base unit and the display. The apparent power can be significantly higher than the Real Power (what your billed for). To monitor Real Power (recommended) use an AC voltage sensor, see below.
  2. Voltage and current: By adding a voltage sensor, it can read the true voltage and calculate a more accurate value for power (Real Power). This also allows the direction of power flow to be determined, which is useful if you have a PV installation that generates and exports power back into the grid. If you wish, you can also calculate more advanced data, i.e apparent power, power factor and RMS mains voltage.
  3. Voltage and 2 or 3 current sensors. Adding extra current sensors allows you to monitor different loads around the house, e.g. how much power is used for heating water. If you have a PV installation, you could also measure the power being generated at any instant.
  4. Pulse Input: Adding a pulse input from your meter gives you a direct feed of (Kwh increment) data that will ultimately be used to calculate your energy bill. However due to the long time periods in-between pulses at low power levels this method is not sutiable for reliable instantaneous power readings.
  5. IR Sensor Input: Adding an Infra Red (IR) sensor allows direct interfacing with an Elster A100C utility-meter to obtain a real-time instantaneous power feed of billing quality.
  6. Temperature Sensors: Adding temperature sensors (maybe inside and out) allows you to monitor the temperatures to correlate your energy consumption with the weather and internal environment.

Tech Spec

Powering the emonTx

From USB 5V Adapter

The on-board 3.3V voltage regulator (MCP1702 SOT-89) can supply 500mW on 3.3V line when supplied from USB 5V. Up to 14.5V can be connected to the Aux power input, however the amount of power the regulator can supply reduces as input voltage increases (eg 17.5mA max with 12V input). See here for voltage regulator thermal dissipation calculations.

From AC-AC Adaper

The AC-DC power supply circuit has been designed to supply about 20mA @ 3.3V, this is equivalent to the basic setup minus the LED's. If more current is drawn the AC voltage sample will be effected. Use of LED's should be limited to quick flashes one LED at a time when powering from AC-AC adapter.

The on-board AC-DC half wave rectifier circuit has an absolute max supply current of 40mA @ 3.3V, PTC fuse will blow at 150mA - AC voltage sample will be effected if more than 20mA is drawn

Quick headcount approximate Max power consumption of various emonTx Setups:

Basic Setup Max

  • ATmega: 7.5mA
  • RFM12B: 14mA
  • Op-amps + switches: 1mA
  • 4 x LED's: 40mA

Total: 63mA

Advanced Temperature setup

  • Basic Setup: 63mA
  • 8 x DS18B20: 18mA - although temp sensor are powered from 5V line so not drawing from 3.3V MCP1702

Total: 80mA

Advanced RF Setup + Advanced Temperature

  • Xbee: 50mA / RN-XV Wifi:38mA /XRF:36mA
  • Advanced Temperature setup: 80mA


Ultimate setup: as above but plus Wiznet Ethernet (120mA) = Grand Total: 250mA

Note: RF module power consumption was when they are Tx'in, this won't be happening all the time.

CT Energy Monitoring

Current Only (Apparent Power)

Voltage and Current (Real Power)

Reading Directly from Utility Meters

Pluse Counting



IR Interface

Elster a100c

Temperature Monitoring

The emonTx has inputs to connect multiple DS18B20 one-wire digital temperature sensors. By default these sensor are powered from the 5V line (supplied from the USB) if powering the board from the 3xAAA battery holder the 3V and 5V power rails should be connected together to enable the sensors to be powered from 3.3V. This can easily by done by jumpering the 5V and 3.3V on the JeePort.

Caution: This jumper should be removed before a usb connection is plugged into the board. No voltage greater than 5.5V should be connected to the Aux input when DS18B20 temperature sensors are connected.

emonTx SMT Prototype V0.6 rev 1 Testing

Questions to Answer:

  • Can a 0603 resistor supply enough current to the LED's 1/16W=62.5mW = 18.9mA MAX @ 3.3V. Give a 1.8 Vf LED 10mA with a 160R resistor 16mA through Resistor = safe but will LED be bright enough?

Changes to make.



RFM12B (Default)



Simple ASK RF

Wifi - RN XV


Wired Serial Network

Open-Source Design