Using EmonTx Arduino Shield SMT with Arduino Zero

Hardware
1

Hello Everyone,

I was told that EmonTx Arduino Shield SMT is not compatible with Arduino Zero, since Arduino Zero works with 3.3 volts.
First, I wanted to make sure if this is true and if it is, is there a way to make an interface between this Shield and Arduino Zero to make it compatible?

I’d really appreciate it if you could answer this question.

Sincerely,
Mehrdad

2

While it is true that the emonTx Shield is designed for a 5 V supply, one of the principal functions of the Shield is to provide an interface between the 5 V Arduino and the 3.3 V RFM69CW.

So if you are prepared, and have the necessary skills, to modify your Shield, a conversion to 3.3 V operation should be relatively easy. I have not checked that the pin allocations for the Shield are identical, but other than any changes to accommodate any differences there, a cursory glance suggests that the only other change required would be to remove 3 resistors and possibly short-circuit another 3 - effectively removing (rather than adding) the conversion from 5 V to 3.3 V.

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Thank you for your reply, I think I got the hang of it. I almost figured it out based on your helpful suggestion and the board schematic in GitHub.
Thanks again.

4

You will want all the points on the schematic labelled “5 V” to actually be 3.3 V, so that the input biassing is correct (i.e. the quiescent input is mid-way between 0 V and 3.3 V, the temperature sensor (if you have one) is OK down to 3.0 V, so the resistors to remove are R7, R9 & R19 and you may or may not need to short out R20, R8 & R18.

If you intend to use the full input range, check the value of burden resistor that you need (as the peak-peak voltage across it needs to be lower). The emonTx uses a 22 Ω burden, so 68 Ω in parallel with the existing burden will be close enough to give you a measuring range on 100 A with our standard CT. (It would have been 66 A maximum without the parallel resistor).

Having now had time to check a schematic and layout of an Arduino Zero, it appears that the Arduino will supply both 5 V and 3.3 V to the Shield, but as you report, it also appears that the Arduino ADC runs on 3.3 V. Therefore, it would seem that you need to cut (or not fit) the 5 V pin on your Shield, and link the track to that pin to the adjacent 3.3 V pin. That will feed all the points labelled 5 V and the points labelled 3.3 V on the Shield with 3.3 V from the Arduino Zero.

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