EmonTX Shield assembled now the questions

I have searched but could not find answers.

What are they for?


The four Header Connectors next to the DS18B20?

Are these just user optional?

Is Tx Rx used by the emonTxShield?

How does one display data? I am not going to stay connected to a PC.

Thanks for the help,


They are used for programming the firmware.
UART = Universal asynchronous receiver-transmitter.

You may wish to read the user guide, and also the specific emonTX wiki to gain a better understanding


Thanks Paul, I have read the WiKi (often) it was the User Guide that I had not found.

More questions after I read (a couple of times) the User guide!


1 Like

Read the User Guide, still have questions.

I have the emonTX version 2.4.

I have it running on my Arduino at the moment.

I am using Arduino Serial Monitor.

I am running the Voltage and Current.ino and seeing 5 columns of numbers.

What would be the columns headers? What are the numbers I am seeing?

Is this documented somewhere?


If you look at the source code for the sketch that you are running, the names of the variables will give you a good clue. EmonLib calculates:

  • realPower,
  • apparentPower,
  • powerFactor,
  • Vrms,
  • Irms.

Those can be (but aren’t for that particular sketch) calculated for each of the 3 c.t. inputs.

I should point out that that is a demo sketch for emonLib. You’ll find fully engineered sketches for the emonTx V2 in a separate section (“emontx2”) of Github.

Thanks Robert,

emonTx V2 has been superseded by the emonTx V3

So should I use V2 or V3?

Is there any actual documentation specifically for emonTX Arduino Shield? The User guide is kinda jumbled and not specific, or at least my reading of it.


Indeed - but I have one, and there are many in service!
The emonTx V2, V3.2 & V3.4 and the emonTx Shield V1, V2.1 & V2.5 are all basically the same - their objective is the same and they all use the same processor - but they are all different in detail. It should go without saying that the emonTx has the processor on-board, whereas the Shield uses the processor of the Arduino it’s piggy-backed on.

I suspect the Wiki pages are a better source for you. The index page is OpenEnergyMonitor Wiki
and the emonTx Shield page is EmonTx Arduino Shield - OpenEnergyMonitor Wiki

This https://community.openenergymonitor.org/uploads/default/original/2X/d/d3fedc62d1e881cfefae0abfe7ea3635850ce6b4.txt (rename it to .html) gives the software differences.

Thanks again Robert!!

Now this is what causes me problems, you refer to the emonTx but you are not talking about the emonTX that I have. I have the emonTX version 2.4 I would think that someone could have shown a little creativity and come up with different names instead of being lazy, to much trouble.

Moving on!!

I have my emonTx shield operating and getting strange readings:

emonTX CT123 Voltage example
Node: 10 Freq: 868Mhz Network: 210
67 -1 5 1.34 2856
-2 -49 3 1.35 2856
-3 60 2 2.08 2862
-3 -32 1 2.10 2862
-3 -5 1 2.21 2862

First the CT’s have no current, they are plugged into the emonTX but no wire is going through them.
Should’nt that be 0.

Then the VRMS I have the 9 volt AC power supply but seeing 1.34, 1.35, 2.08, 2.10, 2.21.

Why is the Temperature multiplied by 100?

Now my emonTX will accept FOUR CT’s why do I have to change the code to process FOUR CT’s?
I “assume” I can just replicate CT3’s code?


Ah, so you really have an emonTx SHIELD, not an emonTx, after all. You have been confusing me by referring to it as an emonTx, and I thought the “2.4” was a typo or a misread, because I think not very many V2.4 shields were produced.

Take a look at the “Shop” website, where you’ll see that in their appearance, the two are quite different beasts.

So you need to be using the sketches for the emonTx Shield, not those for the emonTx. I think you’ll find that when you have the equivalent sketch for the emonTx Shield, things might change. For example, you’ll find there is the code for 4 c.t’s.

and where would I find the sketches for the emonTX Shield?


From the Shop page for the shield (https://openenergymonitor.com/emontx-arduino-shield-smt/), you should find this section:

Firmware Examples

EmonTx Shield example code can be found on github here:
emonTxFirmware/emonTxShield at master · openenergymonitor/emonTxFirmware · GitHub

You should be able to click through the various links and redirects that sends you on, but you can avoid all of that by going straight here:

Thanks Greebo, and thanks again Robert.

I was using the “Internet” before there was a World Wide Web and learned, especial in a HELP situation
one should always give a link when making a reference, even when you assume the correspondent “should” know the links.

“Take a look at the “Shop” website” and “So you need to be using the sketches for the emonTx Shield”
were meanigless to me.

Lucky for me Greebo furnished the links.

Now I can really start asking questions.


Ok so now I have:



I “assume” I will load Shield_CT1234.ino on my Arduino but what do I do with
emontx_lib.ino and platformio.ini?

I “think” I might have seen a reference to a different platform that the Arduino but …
I will look at the code possible that will give me a clue.

Thanks everyone for the much needed help.


Can I use a extension cable for the CT’s?

I need to have two CT’s 12’ away from the emonTX Shield.


Can I use the emon ESP8266 Wifi HUZZAH with the emonTX Shield?

If not , WHY?



BTW. If you haven’t noticed elsewhere, note the relationship between calibration factors and supply voltage for the Tx SHIELD.

Thanks danBates, now how about the extension 3.5mm cable?

Also where is the emon8266 source code?


Thanks, re: note the relationship between calibration factors and supply voltage for the Tx SHIELD

I have no comprehension of what/how the 9 AC is utilized to say nothing of calibration factors.

Hopefully I will get there.



It’s a not-so-well documented fact that calibration of the Tx SHIELD is related to the supply voltage, hence calibrating of the FTDI supply might give a different result than calibrating from a USB supply through the arduino.

Go have a look around the forum first, everything else is there, or github for the code.

Thanks for the replies.

Gotta take it slow I am new at this, I have been looking at the forum and looking at the code.

“calibration of the Tx SHIELD is related to the supply voltage” I really have no idea what you are talking about.

CT reading? 18B20 reading?

I understand how different voltage sources could give one different readings but I do not know what reading to look for.

And then what to do about it.

In use I would not be using a FTDI cable so do I care?

In setting things up I probably will use a FTDI cable but my 3.3volt FTDI cable has 5 volt Vcc so I would be using a separate 3.3volt

Vcc not the FTDI.