emonTx V2.2 Build Guide 

Author: Ian Chilton

If you run into problems please post on the forum.

The PCB is built in an orderly sequence starting with the resistors and small components that are low on the board and then working up to the bulkier parts like connectors. After soldering each component, you need to cut off the excess component leg from the bottom using some cutters.

It is very easy to cause shorts which will make the board or specific parts of it not work, take your time during the build, be precise, use minimal solder and carefully check each joint after soldering it. If you suspect any short, check it with a multimeter and fix any shorts there and then, before continuing.

If you're new to soldering or looking to improve your technique we highly recommend these videos:

It's recommended you read through the whole of this guide and familiarise yourself with the steps before you start the actual build.

The emonTx schematic and PCB board file can be viewed on github or downloaded in PDF format

Step 1 - Identify Components and Kit Contents

Layout and identify the components from the kit.
 

From OpenEnergyMonitor - emonTx Build

Here are all the components required to build your emonTx:

  • The emonTx PCB
  • 3x 18 ohm 1% Resistors (brown, grey, black, gold, brown on blue)
  • 2x 10k 1% Resistors (brown, black, black, red, brown on blue)
  • 1x 100k 1% Resistor (brown, black, black, orange, brown on blue)
  • 8x 470k Resistors (yellow, violet, yellow, gold)
  • 1x 4k7 Resistor (yellow, violet, red, gold)
  • 1x 100 ohm Resistor (brown, black, brown, gold)
  • 4x 100nF (0.1uF) Ceramic Capacitors
  • 5x 10uF Electrolytic Capacitors
  • 2x 22pF Ceramic Capacitors
  • 1x Atmel ATMega328 - the main microcontroller
  • 1x 28-Pin DIL Socket
  • 1x RFM12B Radio Module - available in 433Mhz or 868Mhz versions
  • 1x 165mm (433Mhz) or 82mm (868Mhz) Wire for RFM12B Antenna
  • 1x 16Mhz Crystal - for the ATMega328.
  • 1x USB Mini B Socket
  • 1x Microchip MCP1702 3.3v Voltage Regulator
  • 1x 3mm Green LED
  • 1x 2.1mm Power Socket
  • 5x 3.5mm Jack Sockets
  • 1x 6pin Header (FTDI port)
  • 100A CT Clamp - note : to maintain flexibility the emonTx kit does not include a CT, compatible CT's can be purchased from the sensors section of the shop

From OpenEnergyMonitor - emonTx Build

Note that the photo shows two different types of the 22pF capacitors - brown ceramic disk (labelled 22 and with black tops) and small blue ones (labeled 22) - only one of these pairs are required/included and they are equivalent.

 

Ok, we're ready to start. Here is the PCB ready:
 

From OpenEnergyMonitor - emonTx Build
From OpenEnergyMonitor - emonTx Build

If not already marked we recommend you place a mark in the correct box to o the rear of the PCB to indicate what frequency RFM12B is soldered onto the board as it can be very difficult to tell later! 

This is where everything should go:

From OpenEnergyMonitor - emonTx Build

Please note that despite the diagram above and the photo below, we do not fit the 10k resistor under the pulse counting socket at the top of the board as this can cause problems.

Please note the above photo is from an older version of the board and therefore is slightly different (eg: some of the resistor values have changed) - it's just here to show the various parts of the board.

 

Step 2 - 18 ohm Resistors

Start with the resistors. Bend each leg of the resistor over 90 degrees, right next to the body. It's recommended that you align them all in the same direction.

First we add the 3x 18 ohm resistors - they are blue and their colour code is: Brown, Grey, Black, Gold, Brown.
 

From OpenEnergyMonitor - emonTx Build

 

 

Step 3 - 10k Resistors

Add the 2x 10k resistors.

Unlike the picture below these are usually cream coloured and with Brown, Black, Orange strippes.

(If you have blue 10k resistors their colour code is: Brown, Black, Black, Red, Brown - but these are not usually shipped with the kit)
 

From OpenEnergyMonitor - emonTx Build

Note that the diagram above shows a 3rd 10k resistor - under the pulse counting input (R8). This is an optional pull down resistor for the pulse counting interrupt input / general digital I/O. More often than not it's not needed so is not included in the kit as if fitted, it will do more harm than good e.g if 10K resistor is soldered into R8 an optical sensor plugged into this port to detect the LED pulses from pulse output meters won't work.

R8 is only actually useful when connecting directly (via wire) to the pulse output from a utility meter, we're not recommending this method since it's not galvanically isolated and to connect the wires you have to work very close to high voltage, not ideal. Optical pulse detection is much safer.

See pulse counting section of building blocks for more information

 

Step 4 - 100k Resistor

Add the 100k resistor - it's blue and it's colour code is: Brown, Black, Black, Orange, Brown.
 

From OpenEnergyMonitor - emonTx Build

 

Step 5 - 470k Resistors

Add the 8x 470k resistors - their colour code is: Yellow, Violet, Yellow, Gold.
 

From OpenEnergyMonitor - emonTx Build

 

 

Step 6 - 4k7 Resistor

Add the 4k7 resistor - it's colour code is: Yellow, Violet, Red, Gold.
 

From OpenEnergyMonitor - emonTx Build

 

 

Step 7 - 100 ohm Resistor

Add the 100 ohm resistor - it's colour code is: Brown, Black, Brown, Gold.
 

From OpenEnergyMonitor - emonTx Build

 

 

Step 8 - 22pF Ceramic Capacitors

Add the 2x 22pF ceramic capacitors - they are either blue (as per the photo) or small brown discs with black tips.
 

From OpenEnergyMonitor - emonTx Build

 

 

Step 9 - 100nF Ceramic Capacitors

Add the 4x 100nF ceramic capacitors - they are labelled 104.
 

From OpenEnergyMonitor - emonTx Build

 

 

Step 10 - 10uF Electrolytic Capacitors

Add the 5x 10uF electrolytic capacitors. These must go the correct way round. The PCB is marked with a + symbol to indicate positive lead. The capacitors are marked with a white strip and a short leg to indicate the negative lead and a longer leg to indicate the positive lead.
 

From OpenEnergyMonitor - emonTx Build

 

 

Step 11 - 16Mhz Crystal

Add the 16Mhz crystal - it can go either way round.
 

From OpenEnergyMonitor - emonTx Build

 

 

Step 12 - Green LED

Add the green LED. The PCB is marked with a + symbol to indicate positive. The LED has a short leg (and flat edge) to indicate the negative leg and a longer leg to indicate the positive leg.
 

From OpenEnergyMonitor - emonTx Build

 

 

Step 13 - MCP1702 3.3v Voltage Regulator

Add the MCP1702 3.3v Voltage Regulator - the flat edge goes towards the bottom of the board as per the markings.
 

From OpenEnergyMonitor - emonTx Build

 

 

Step 14 - Mini USB Socket

Add the mini USB socket.
 

From OpenEnergyMonitor - emonTx Build

 

 

Step 15 - 3.5mm Sockets

Add the 5x 3.5mm sockets.
 

From OpenEnergyMonitor - emonTx Build

 

 

Step 16 - 2.1mm Power Socket

I added the 2.1mm power socket next, but it would actually be better to add this after the DIL socket (next step) as the DIL socket will fit flush on the desk while you solder it.
 

From OpenEnergyMonitor - emonTx Build

 

 

Step 17 - DIL Socket

Add the DIL socket. If you are observant, you will notice that I put it the wrong way around on the photos! - the notch on the socket should go to the left of the board, as per the markings on the PCB.
 

From OpenEnergyMonitor - emonTx Build

 

 

Step 18 - Programming Header

Add the 6-pin programing header at the bottom of the board.  Solder one pin first so you can adjust the alignment if you don't get it at right angles to the board first time.
 

From OpenEnergyMonitor - emonTx Build

 

 

Step 19 - Voltage Check

We now need to check the voltage is correct before we add any of the expensive components. Power the board with either an FTDI programming cable or a mini USB and measure the voltage between the GND and VCC pads on the RFM12B socket. You should get 3.3v. If you get more than this, you should work out what has gone wrong before you continue.
 

From OpenEnergyMonitor - emonTx Build

 

 

Step 20 - RFM12B

Add the RFM12B module. You can see the shapes of the crystal on the PCB. Position the module in place over the pads and gently apply heat and solder the opposite corners to hold it in place so you can solder the remaining pads.
 

From OpenEnergyMonitor - emonTx Build

 

 

Step 21 - Antenna

Add the antenna by soldering the end to the bottom right pad on the RFM12B module.
 

From OpenEnergyMonitor - emonTx Build

 

 

Completed Board

We can now add the ATMega328 chip. You will need to bend the pins in slightly by resting the chip sideways on the desk and rolling it forwards to bend the pins in. Do the same with the other side.
 

From OpenEnergyMonitor - emonTx Build
From OpenEnergyMonitor - emonTx Build
From OpenEnergyMonitor - emonTx Build

 

 

Voltage Selection Notes

On this latest version of the emonTx PCB, two solder jumpers have been added by the IRQ I/O and temperature (DS18B20) ports. They allow the user to decide what voltage to use on these ports. The emonTx runs at 3.3V. More explanation can be found in this blog post. Connection is made by joining the middle pad the either of the outer pads with solder.

If you look careful the solder jumper above the temperature port is connected to 5V (small PCB trace). This was an error, but actually work out well. The temperature sensors always use 5v (which is good) and when the emonTx is powered by 2 x AA's the the 5V rail becomes 3.3V anyway. Therefore, you can ignore this solder jumper, it will disappear in the next generation of emonTx PCB's. However the pulse counting voltage selection jumper (pictured) above must be connected to either 3.3V/5V.

See DS18B20 temperature sensing section of building blocks for more information on temperature sensing. 

Firmware Upload

ATmega will come with Arduino Optiboot 4.4 bootloader (select Uno in IDE) but with no sketch uploaded. The correct sketch must be uploaded using Arduno IDE.

emonTx Arduino firmware can be downloaded from: https://github.com/openenergymonitor/emonTxFirmware.

The EmonTx v2 firmware examples are in the EmonTx v2 folder, there is a github readme that lists all the examples and the Arduino libraries required here:

https://github.com/openenergymonitor/emonTxFirmware/tree/master/emonTxV2

The best firmware for power monitoring with a CT sensor and an ACAC Adapter for voltage measurement is the emonTx_CT123_Voltage example.

Connections

See emonTx Design Reference page.

 

Testing

Compare your results with the EmonTX accuracy test

 

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EmonTx_v2.2_schematic.pdf33.33 KB
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