Basic concepts


I’ve been reading in for a while and I want to confirm what I understood is correct and clarify some doubts.

  1. If I buy the “emonPi, Assembled PCB Only - No Enclosure” I can connect to my Raspberry Pi 2 Model B? And if I add adapters voltage and current transformer I get the equivalent of emonPi (whitout display)?

  2. With the device assembly in point 1 is possible to monitor current
    (two current transformer only), voltage and connect to my WiFi to
    transmit information to emonCMS?

  3. If I buy the “emonTx Arduino Shield SMT” the 433Mhz RFM69CW
    included and connect to my Arduino and added the four adapters and
    currente transformer, I get the equivalent of emonTX?

  4. With the device assembly in point 3 I can transmit information to
    the assembled device in point 1 and then to see the information in

  5. How many emonTX can be connected simultaneously to each emonPI or emonBase?

  6. If I buy “RFM69Pi 433Mhz Base Station Raspberry Pi Board Receiver” I
    can connect to my Raspberry Pi 2 Model B kit and get an equivalent to

  7. I guess it’s necessary that all the devices are operating at the same frequency 433 Mhz or 868 Mhz.

  8. What is the “OOK (On-Off-Keying) RF Transmitter - 433Mhz” and where you should connect?

  9. Where emonCMS data resides ? ¿Residing on the site “Emoncms - site home” or reside locally?

  10. I can use 200 amps or greater current transformer with the emonPi or the emonTx ?

  11. What it is the accuracy of the information from these devices?

Best regards.

  1. Yes - but the ac adapter and current transformer(s) are also required for the emonPi to work as a monitoring node.

  2. Your Raspberry Pi will run emonCMS, and you can see the data on your LAN via WiFi if you add the WiFi dongle. You can at the same time send the data to via the same WiFi dongle and your router.

  3. Yes.

  4. Yes.

  5. Many. There is a limit of 30 separate “Node IDs” that identify each emonTx, but because all use the same radio frequency, you might have problems with two transmitting at the same time and blocking each other. That could happen with just 2, and the probability of blocking increases as you add to the network. The probability of blocking decreases if you increase the time between radio transmissions.

  6. Yes. You then have an emonPi that cannot measure voltage or currents.

  7. Yes. 868 MHz devices are no longer stocked by the shop.

  8. What it says. Rather than transmitting numbers in a message, it turns the transmitter on and off. The emonTx does not use OOK. I am open to correction, but I think the emonBase/emonPi cannot handle OOK.

  9. Either or both. See 2.

  10. Yes, but you will almost certainly need to change the burden resistor. See the N.America page in Resources> Building Blocks.

  11. See Resources > Building Blocks. There’s a page where everything regarding accuracy is explained.

  1. yes, pretty much, just the Pi spec will differ as the emonPi is now a Pi3
  2. yes, if you plug in a usb wifi dongle. (Pi2 has no wifi in-built)
  3. almost. The emonTx has a RJ45 breakout and the 4th CT is a high sensitivity.
  4. yes
  5. there are 30 available node id’s on any one rfm network, each device must have it’s own node id. You will however most likely have traffic issues way before you use all 30 node id’s unless you are sending short packets infrequently or introduce some synchronization.
  6. yes, a Pi with a rfm board IS an emonbase.
  7. yes
  8. There is a specific place on the emonPi board to attach the OOK tx, the emonPi board doesn’t actually control the OOK though that is done by the Pi via the GPIO so you could also attach it to the relevent GPIO pins of a Pi (ie emonBase) directly if not using an emonPi, the same SD card image would give you the same functionality.
  9. either or both. There is an instance of local emoncms included in the sd card image if you use that, but you can also post to one or (several) more emoncms(s) local or remote. There are many options here, you could also add your own hdd for a local server instance, which is where the emonPi idea originated even though it no longer had a hdd come it’s release.
  10. Yes, probably, you will need to be more specific about the model you have in mind to be sure or seek the advice of Robert or Bill who are better informed in these matters than I if you are needing a recommendation
  11. There are far too many variables to even think about quantifying the accuracy. it will vary depending on how it is installed and whether you calibrate it etc. but it has the potential to be extremely accurate if you wish it be and generally speaking you would probably need to spend a small fortune to find something with greater accuracy.

I hope this helps :slight_smile:

DOH! I see while I have been hammering away at the keyboard RW has slipped in there ahead of me, but I will still post as there may still be something of use or interest to you.

Wow very fast and very clear answers.

I will read the material in Building Blocks. However, I do not yet understand that it serves the OOK what is it that turns off and on?

Thank you very much Robert.

Thank you very much Paul, you have my doubts cleared up very well. However, do not turn on the operation of OOK.

Very fast and clear answers.

There, FTFY! My turn to beat you this time.

And you’d need to spend a small fortune on test gear to calibrate to the accuracy that the basic emonTx is capable of. In the past, users who have carefully adjusted the calibration so as to match their supplier’s meter (which by definition is 100% accurate, even when it isn’t!) report agreement afterwards to within 1%. The weakest link, following tests on the latest batch of CT, is the phase shift variation in the a.c adapter due to voltage changes, and that’s likely to be significant only with low power factor loads.


A simple example of OOK is a radio door bell. Press the button, the transmitter turns on. Take your finger off, the transmitter turns off.

I’m not sure I’m in the right topic but questions and answers above are interesting thus I raise this question here:
What I have now
-Raspberry PI2 running Raspbian Jesssy

  • Arduino mega 2560
  • Ethernet shield W5100
    All together connected on a local network.
    What I try to achieve:
    Logging my PV’s generation and houshold’s consumption to emoncms.
    My question:
    Is it possible to do so with an extension of my current RPi/arduino setup with an emon tx arduino shield + 2 CT-s + AC-AC power supply?

I don’t know the Arduino range so I am not able to say whether the emonTx Shield is suitable for use with the Arduino Mega 2560.
The second question is, assuming the Shield will work with it, what is the Arduino Mega doing now, and does it have sufficient spare processing capacity to do the power calculations in addition to what it’s doing at present?
If the answers to those questions are satisfactory, then yes, you should be able to do as you suggest.

Thank you, Robert!
The Mega is just lying around, the project what it was purchased for has been finally realized with a script running on RPi.
I guess, I will give it a chance! :slight_smile:
Is there somewhere a specific setup guide for Emon Tx Arduino shield?

Sensor installation is no different, and the calibration instructions also cover the Shield. There are specific links to set, the Wiki has the details. For a reason that totally escapes me, there’s no link to the hardware Wiki for the Shield here, you have to go via Building Blocks on the “old” site:

Resources > Scroll all the way down to Fundamentals - Building Block Resources > Resources (in the banner) > Hardware Wiki (in the dropdown) > emonTx Arduino Shield (in the contents list) > emonTx Arduino Shield (in the table).

If you can’t find your way through that maze, here is the direct route.

Tank you again, Robert.
That’ a great help, I’m not sure I would have been able to find it without your guidance.