Raspberry Pi + emonSD + Arduino?

Hi all, noob here. Some basic questions that show my ignorance on the hardware side.

First of all, I’m from the US, so I’m looking to see if I can reconstruct the proper hardware without the high prices of buying and shipping from the UK which is really what drives my question.

I see the emonPi contains a case, lcd screen, raspberry pi 3, along with a shield to interface with the CT’s and AC-AC adapter. What is the proper name for the shield? Is that the emonPi or is there a technical name for it?

What I’m looking to do is just buy the raspberry pi 3 on amazon over here which I can get for roughly 23 pounds. I’ll also snag an 8GB SD card for put the OS on.

So what is it that the shield (or whatever it’s technical name is) does? I see it’s got an Atmega328 on it. Does that do the ADC sampling from the CT’s and then TX/RX it over to the Pi? Anything else other than that?

I’m interested to know if I could just use an arduino in its place. I’ve got a few Uno’s lying around and a Zero. Personally I’d love to use the Zero for it’s higher quality ADC. Or shoot, can I just hook a CT up to an ADC and interface that with the Pi and have the Pi read via an SPI?

My overall goal is to get a somewhat easy, not super customized hardware setup since I’m mostly focused on pulling the data from emonCMS or possibly locally and doing stuff with that data. Since I’m a little low on cash, straight up ordering an emonPi to roll out of the box isn’t ideal. Possible, just not ideal.

Thanks all in advance for any advice!

The “Shield” is, to all intents and purposes, an emonTx V3 but with only two current input channels. It communicates serially with the RPi, receives by radio data from other emonTxs or emonTHs. So if you look at what the emonTx does, it does the same. (And before somebody takes issue with that, the emonTx can receive, but normally it doesn’t.)

You can certainly connect an Arduino/emonTx serially to your RPi - that is documented. You can also go via the ADC route into your RPi.

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Since a shield is commonly understood to be a board attached to an Arduino, (similar to the way a Cape is understood to be an add-on for a Beaglebone) the board you’re speaking of could be considered a variant of an RPi HAT.

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Thanks guys. One last question for the time being. Is the Ac-Ac used to get an exact measurement? Would there be a problem just hard coding my Vrms to ~170v being in the US? I’ve got a few CT’s on hand and want to get up and running as soon as possible.

As far as the emonPi is concerned, the a.c. input serves as an accurate measure of the voltage, but it also allows the direction of power flow to be determined, and that is important if you both import and export. Without it, the emonPi (and the emonTx) behave only as an ammeter - they measure current, but cannot tell you the direction of power flow.

I think you’ll find your nominal voltage is 120 V rms line-neutral, or 240 V line-line; not 170 V.

Oh man, good catch. I meant 120Vrms, 170 peak.

For my instance direction doesn’t matter since I’m only looking at consumption.

With no AC-AC adapter, you’ll be measuring apparent power vice real power. (which is what you get billed for) It makes no diference with resistive loads, but reactive load measurement accuracy will suffer.