Which EmonTx fits me best


I have a confusion about what product would fit me the best for my kind of project. The thing is that I need to take several measurements of real power consumption, apparent power and power factor for a series of research experiments with an ultrasonic generator capable of 2kW; recording them locally and analyze the data for making conclusions. I was reading that the EmonPi can record them locally but I need live data acquisition and also would more time consuming; I was hoping that I can achieve this with an Arduino MEGA hooked up to an emonTx V3 - Electricity Monitoring Transmitter Unit without the RF module, reading the values from the Serial Monitor with the USB to UART programming cable. But I haven’t found any concrete answer. There is also the emonTx Arduino Shield SMT but I’m not very conviced about it.

Thanks in advance.


If you require serial measurements and the ability to easily modify the code to your needs both the emonTx V3 or emonTx Arduino Shield will work.

The emonTx Shield is the cheaper option if you already own an Arduino.

You will require a USB to UART cable (sold via the shop) to connect to the emonTx V3 to view serial output and upload new firmware if required. The emonTx comes pre loaded with firmware.

We currently don’t have any stock of emonTx V3 with no RF. For the next three weeks purchasing an emonTx v V3 with Rf module is the only option.

I would recommend an ac-ac voltage adapter for accurate Real Power measurements. The emonTx V3 can be powered by this adapter while the emonTx Shield will require an additional DC power supply for the Arduino. The is probably not an issue if the unit is working on your workbench.

Another consideration is the emonTx V3 has a ‘high sensitivity’ CT channel. This channel would be more suited to monitoring low power loads (e.g 2kw). However, the emonTx Shield could be modified by changing burden resistor to achieve the same performance. See the building blocks section of the main .org website

Thanks for the explanation, one last thing that I want to ask. I really convinced that the EmonTx V3 is my best option thinking that is more convenient to have an enclosure since its makes it more robust; but I need to add an RTC module to the Arduino take measurements on specific times. My question is, does the EmonTx V3 has this type of flexibilty?

An emonTx V3 is basically an Arduino (ATmega328) with added energy monitoring components. It would be possible to wire an RTC to the emonTx using the ISP connector. See the emonTx hardware wiki for pinout showing what digital IO is free.

However, connecting an RTC is probably easier using an emonTx shield since access to IO is easier

The “no RF” option doesn’t seem to be available via the shop site, can we pre-order?

By “no RF” do you mean “no RFM fitted” allowing a choice of module to be retro fitted (or not) or can they come without the SMA antenna and connector too?

There is plenty of room inside the case for a RTC, (more than enough if you are not using and remove the battery tray.)

If you wish to retain the use of the serial port for uploading sketches and for passing data out, You could hang on for the “No RF” option and use a SPI RTC (eg) and plug it on to the 2x3way ISP header as the RFM will not be there to interfere with the SPI lines.

You won’t be able to use a I2C RTC (easily) as the 4th CT channel uses AIO4 which is also one of the 2 designated I2C lines.

We should be getting some stock of emonTx with no RF tomorrow, sooner than expected :slight_smile:

The no RF option has been added back. See emonTx V3 shop item.

Either, please leave note at checkout. Yes they will come with SMA connector and without antenna.

Ok, so you’re saying me that if I buy the noRF EmonTx v3 there could some space for the SPI RTC module from Sparkfun, interesting. And for the software, I should be able to add the RTC programming with the serial monitor measurement to the EmonTx V3 firmware easily without interfering with any energy measurement right? I saw at wiki that the RF module uses digital pins 10,11,12 and 13; so I should easily connect the SPI RTC module there if I buy the noRF EmonTx v3.

There would be space for a RTC in any emonTx v3 not just the “no RF” version.

The Sparkfun RTC I linked to was just an example of an SPI type RTC. I see no reason why it wouldn’t work but have no experience to recommend it.

When fitted the RFM module does use pins 10,11,12 & 13, it shares them with the In-circuit Serial programming header. That 6pin header would probally be the easiest way to connect the RTC to those pins freed up by not having the RFM.

I’m not sure I understand your question but the energy monitoring code needn’t be affected by outputing to serial rather than RF and I expect the use of the RTC would just replace the existing interval timing. So whilst any sketch you choose will need considerable editing, you shouldn’t need to alter the energy calcs.

Do you really need the RTC if you are passing data via serial not RF? Another option could be to rely on the time-keeping of the device (pi ?) receiving the serial data and trigger the datapoints in response to a serial command.

Yes, I’m kind of moving also to the EmonPi just to avoid soldering and troubleshooting issues. Well I’m very clear about all the potential of the EmonTx

Thanks Paul