AVR-DB: emonTx V4, new hardware in progress

Thanks @abbbbbottttt out of the box only when used with the emonBase (raspberry pi Wi-Fi) at the moment.

We will also be making the EmonTx4 Pi Pico, Pi Zero, ESP8266 and ESP32 extension boards available as bare PCB development boards for those happy with a bit of soldering and code uploading, is that what you are after? Here are the links to the brief documentation pages for these so far:

The Pi Pico option is looking like the direction we are most likely to be going at this point, in terms of focusing our efforts longer term, for the non full Pi application that is.

@borpin has also been doing some good work on an ESPHome integration currently tested on an ESP8266 though possibly soon also available to run on a Pi Pico by the sound of things!


This is great news, I am happy to jump in and be an early adopter!

Question on power though, I don’t really want to buy the expensive PSU if I can re-use one of my 3 existing ac/ac bricks. On a mobile device so hard to scroll through the detail but was there an adapter board for voltage, is that available yet?


Hello Trystan,
just want to double check: the 3-phase version is just a firmware change, i.e. not different hardware? So if I want one 3-phase unit and one single phase I can just order two now and do a firmware update later. Is this straightforward with a PC ? I would like to order both if possible because I am in Germany would save on postage.

Do I need a 3-phase device if its only being used on a AEG 3-phase oven-grill, i.e. no motors ? The device can use a bit of power but I don’t know how it balances load.

Very pleased you guys got this pulled together!



I think there will be an adapter cable for the round 9V to the RJ45 socket.

thanks @borpin yes that’s correct, coming soon!

Ah found it, so I think what I need is the single phase precision option - I’m in so deep at this stage it probably makes sense to go for the more accurate reading.

The RJ11 just looks like a physical adapter, are there components on that board, or could something be made up as an interim solution?

Does the EmonPi benefit from the accurate voltage reading, or is it only of benefit on the circuits monitored by the EmonTx4? Wondering if I put the solar into a channel on the EmonTx4 and move it off the EmonPi if so.

Thanks Marc!

There is a hardware difference but we can send you the version that will be ready for 3-phase monitoring. I will double check with @glyn.hudson how we can administer that in terms of the shop, the cost for the 3-phase version is about 10% more than the single phase version (It includes two additional ZMPT precision voltage transformers).

Yes, if you have an emonBase or existing emonPi it’s easy to do this by connecting the emonTx via USB, see the example here for upgrading the current firmware: https://openenergymonitor.org/guide/emontx4/firmware.html#updating-firmware-using-an-emonbase.

I cant say, my guess would be that it might put the oven, grill and hobs on different phases? Are the other circuits in your house all on one of the 3 phases?

Great, yes we’ve already prepared quite a few of these, just need to get the shop item up, which we will likely do tomorrow. When using the single phase voltage sensor only, the emonTx4 does require a separate USB power supply. Will the emonTx4 be located next to your existing emonPi? if it is you could power the emonTx4 from the emonPi, you could also transfer the measurement data to the emonPi over the same connection.

The emonPi cant benefit unfortunately from the emonTx4 voltage sensor.

Yes the EmonPi is already next to the CU, so I plan to hard wire the emontx4 with usb to the EmonPi for power, data and the ability to flash FW.

Sounds like I need to be patient another day or so :smile:

I need to do some head scratching on what circuits get the honour of measurement and what CT to order, I have to get the immersion on one but then I have 3 rings, want to track the oven and then there is lighting.

Will need to compromise in some way before the expansion board is available… then I can really lecture the family with graphs on their energy usage!

I think the only way it could present a balanced load would be it works by induction and all the elements were controlled electronically - power being derived from all three phases to feed a d.c. bus, or something like that.

If not, I think it’s a near certainty that individual heating elements will be arranged to spread the load across the three phases, rather than balance the load. but exactly which it does Marc would have to find by experiment or consult the manufacturer.

I’ve added a hidden shop option for emonVS 3-phase:

Can I just clarify that this can be used with an existing emonPi, and if so how? The documentation and the shop talk about an emonBase, but doesn’t appear to explicitly say it can be used with an emonPi. Is the connection over wifi?


In fact I’m a bit confused what I need now.
I have an emonPi which is doing some ct sensing and some temperature sensing for my heat pump.
I need to expand that to include some further ct sensing elsewhere (immersion heater), some temperature sensing (hot water tank), but mainly modbus reading of a heat flow meter. I’m unclear what I need. There are three separate locations involved (maybe 2, depending on how what kind of wiring you need for the modbus). I can’t get a wire from the flow meter to the existing emonPi, though I could probably replace the emonPi with an emonTx and relocate the emonPi to where I can get a modbus wire connector.

So either way it looks like I need an emonTx, what I’m unclear about is whether I need an emonBase as well, or indeed anything else, and whether my existing emonPi stays, or gets replaced.

Thanks (sorry for the confusion).

If you have an EmonPi you will not need an EmonBase as your running EmonCMS on the Pi.


@TrystanLea @glyn.hudson I think you need to be much clearer on the various options as they are quite extensive now!

For instance, I think there are 3 different options for the AC sensor (EmonVS with power, EmonVS AC only and AC/AC adapter). Will the RFM comms work with an EmonPi?

It is all getting a little confused :frowning:

The purpose of the emonVs is what, exactly? Is the point that in order to accurately measure current via the ct clamps you need a precision reference? So if you are doing temperature monitoring only, you can just use a normal (and much cheaper) ac adapter??

I’d suggest it would be a total waste of money to use an EmonTX4 for temperature only!

Hello @tomq42

Yes it can, we will definitely make that clearer. I think it needs a dedicated documentation page to cater for interfacing with an existing system as there are a couple of things to watch out for.

There are two ways the emonTx4 can be used with the existing emonPi:

  1. The emonTx4 can be connected to an existing emonPi via a USB-C to USB-A cable, measurement data can be transferred over USB, for a simple and reliable setup.

  2. Data can be transferred between the emonTx4 and emonPi via 433Mhz radio. A key thing to watch out for here is that the default radio packet format for the latest emonTx4 is based on the LowPowerLabs RFM69 code which is not compatible with an emonPi running the original JeeLib code, I will go into the why in more detail soon. It is however easy to change a new emonTx4 to transmit using the original JeeLib format so that compatibility is maintained. I will explain how that works in more detail in the documentation page so as not to make this post too long :slight_smile:

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Is it an option to run a cable from the tank location to your emonPi? both for the modbus and the temperature sensors? It may be more cost effective to get a second modbus SDM120 electric meter for the immersion heater if you only need a single additional CT measurement?

@tomq42 I should say the emonTx4 cant read from modbus, that needs to be a emonBase/emonPi/raspberryPi of some sort. You could locate another of these next to the modbus meter and send the data from that to the main emonPi via WiFi, but my recommendation would be to run a cable if you can to a modbus reader located at the main emonPi as relying on the WiFi data connection with router issues etc can be more trouble than it’s worth. Hope that’s helpful.