AVR-DB: emonTx V4, new hardware in progress

The DS3231 uses a Temperature Compensated Crystal Oscillator. (TCXO)
Drift is ~15 seconds per year, IIRC.


Unfortunately, they’re not as cheap as the 7940. :frowning_face:

Thanks @Bill.Thomson yep! very steep for that one at the moment…

Having been doing the OpenEVSE WiFi module firmware I would probably agree, the ESP IDF integration looks like it has some great tools.

If you are going to re-use the EmonESP code this should be fairly trivial to port, basically the same code as the OpenEVSE and I have done many syncs of features between the two, I have even started a port back to AsyncTCP on the OpenEVSE. Let me know if you want a chat about it.

Also on the RTC, we did look at this on OpenEVSE. I think it just needs a battery and maybe a MOSFET to use the low power processor on the ESP32. Can dig up my notes if you want

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I suggest this conversation / discussion goes into it’s own thread as it is a fundamental change.

Thanks @jeremypoulter , yes it would be great to chat about this. Will be in touch soon.

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@glyn.hudson and I have been at https://www.cassindustries.com/ today testing the emonTx v4 and voltage sensing power supply for CE compliance (EMC & LDV). A couple of modifications required to the power supply (common mode choke and capacitor) to pass conducted emissions, similar to what was required for the EmonEVSE, radiated emissions looks fine, part way through immunity testing to be continued on Monday.


Here are a few photos from the EMC test lab, it’s always an interesting experience


I’m late coming to this thread, but interesting news, thanks. I’m now living somewhere with european wiring, no ring main but lots of radial circuits to monitor, so more CTs would be welcome.

One small point - the only problem I’ve had with a 2015 vintage V3 has been the 3.5mm CT sockets not making good contact with the plugs (several times!) Cleaning and rotating them fixes the problem. In an ideal world they’d be gold plated or have a wetting voltage. Are the blue sockets in the photos a better quality than the V3 black ones?


Yes they are. The new voltage output CTs are also higher quality including better jack plugs. Although to be fair, our current YHDC jack plugs have improved and 2015.

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Is there any update with the new hardware?

I am happy to fabricate my own PCBs or test existing.

Hello @Swwils, thank you for your interest. We are making progress, in the process of a few final design changes to improve measurement accuracy at low power levels. I will update with more detail soon!


I have recently had my consumer unit changed and have seen my inverters accuracy drop slightly - its amazing how much CT clamp installation can make a small difference.

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Progress update time :slight_smile:

Our latest set of boards with changes resulting both from our EMC testing and improving performance at low power levels have come through. Results are all looking good from the testing that we’ve done on these so far and so we are happy to now proceed with getting the first batch built up.

The unit on the left is the 3-phase voltage sensing unit and combined power supply (This replaces the current use of AC-AC adapters and separate USB power supplies). The right hand unit is the new EmonTx v4:

The main changes from the EMC testing were the addition of common mode chokes to the voltage sensor to reduce conducted noise from the power supply and also improve immunity from noise injected e.g down the Ethernet of a connected RaspberryPi.

The main change to improve performance at low power levels was to change the way the CT sensors and AC voltage input’s were biased. These now have one side connected to ground and the other side connected to the bias via a DC-decoupling capacitor, this provides a better path for noise to get to ground. The DC ground is also connected to the mains ground. The result at low power is much better.

With enclosures:

ESP32 WiFi extension board with I2C LCD:
As discussed above the plan is to have the option to connect the EmonTx v4 directly to WiFi via an ESP32 module - other options include rfm69cw radio link to an emonBase or emonPi, or a direct wired USB connection. Here’s a picture of the ESP32 WiFi module with a connected I2C LCD to give feedback on connectivity etc. It’s amazing how short a time it takes the ESP32 to connect to a WiFi network! The IP address pops up almost immediately!

Next steps
We should get the assembly quotes and timelines back from the company we use to do our manufacturing soon and pending no large surprises that should give us a clearer ETA for when we will have stock.

I look forward to updating with more on all this soon!


In addition to the emonTx v4 and voltage sensor design above, we also have a few other designs in the works including an upgraded emonPi design and an upgraded rfm69pi. We are getting the first prototype boards back for these next week so I dont have pictures to share of the physical boards yet.

emonPi v2
An upgraded emonPi design with 3 CT inputs and the RJ11 voltage sensor to support 3 phase voltage sensing. The intended aluminium enclosure for this design is a little less expensive and the assembly of the LCD display and push button should be simpler reducing assembly time.

A RaspberryPi + RFM69Pi (emonBase) is often used in combination with an EmonTx as an alternative configuration to the emonPi. One disadvantage with the emonBase is that it does not have an LCD display that can be used to find the IP-address of the Pi. The following upgraded RFM69Pi design includes a 128x32 I2C LCD and push button to provide this useful feedback during system setup.

I’ve also at least added the option to use the RFM69Pi as a mini energy monitoring board in and of itself. The additional circuitry and components is relatively minimal. Im getting two variants of this board made at this prototype stage, the first is an option with terminal blocks that should support a wider range of off the shelf enclosures and the second uses the standard RJ11 voltage sensing input and CT sensor jack sockets.

Terminal block option:

RJ11 Voltage sensor and CT jack socket inputs:

Here are the github repositories with the schematics and board files for all of the above for anyone interested in looking at these in a bit more detail:

Hi Trystan,
Thanks very much for posting all the github files.
What package opens the .sch files? I’ve tried importing the emonPi_v2.0.0-beta.sch file into the Kicad standalone schematic editor and it doesn’t like it! Alternatively could you include .png schematic files?
Also, I’ve tried opening the emonTx4 schematic.png and board2.png links at the bottom of the emonTx4 readme.md and get error 404.
Am I doing something silly?

You need Eagle CAD. There’s a free version that’s good enough for OEM projects.

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Thanks for the update @TrystanLea ! Is it expected that the emonTx V4 will be powered by the separate 3 phase voltage sensing unit, even on single phase? Otherwise, I can’t see how it’d be able to sense the voltage.

Any update on the CT sensor expander board and firmware?

Yes. The unit (when you look at the schematic) has 4 separate parts: a d.c. supply for power, and 3 isolated voltage sensing circuits for the (possible) 3 phases. The d.c. supply comes from L1.

A cheaper single phase version of the voltage sensor will be available without the 3phase components populated on the board - but otherwise it’s the same PCB. The single phase version will be available with a 3pin plug or without for fixed installation