Hi Brian, the one used above is just a cheap Chinese Nano, the main point to watch is they are 5v so when hooking up RFM’s etc you need to be voltage level savvy.
I use these for dev work as they are ATmega328p@16MHz with an optiboot bootloader so USB uploads are easy and they are quite similar to the emon devices.
There are also Arduino ProMini’s that operate at 3.3v but they are often 8MHz and the 5v ones 16MHz, With some digging you can find 16MHz ProMini’s that are voltage selectable, they are the ones I sometimes opt for so I can run 3.3v 16MHz, But the ProMini’s do not have USB and the 6pin programming header is not exactly the same as the emon 6 pin header/programmer, so a simple adapter lead is needed to program using the emon programmer.
Are you using an RFM? are you using battery power?
I used the usb Nano in this instance so it could be easily powered via a 5v mini usb wall wart.
It’s worth noting the particular hall sensor I used will not work at 3.3v (3.8v Min)