868MHz emonPi working with 433MHz emonTH?

I have an old 868 MHz emonPi from the kickstarter launch in 2015. I retired it a few years ago and am currently using a 2017 vintage 433MHz emonBase. I needed another Raspberry pi so I put a new SD card in with the Nov22 emonSD image. I was surprised to see my three 2017 vintage 433MHz emonTHs appear in the feeds and even more surprised that they reported a better RSSI than my 433MHz emonBase.

Presumably the emonSD image reconfigured the old RFM12B module to 433MHz?

Has anyone any exprerience of using RFM12Bs on the “wrong” frequency?

If I wanted to use my old 868MHz emonTHs could I reconfigure the RFM12B back to 868MHz?
Or could I reflash the old emonTHs to work at 433MHz?

The answer is yes to most of the questions. The “problem” with the RFMs (Radio Frequency Modules) is the hardware is tuned to the specific band, so while they work on the wrong frequency for the hardware, the efficacy, i.e. signal strength and range, should be worse, not better! I’d look at ‘Docs’ and compare pictures of the modules with what you’ve got, just to make sure you have what you think you have.

When it comes down to the software configuration, choosing the frequency is a matter of sending the right value to the right register in the module, and that can be done at any time. So unless you want to dive deep into the software, all you need do is load the correct sketch - either in the emonTx or the emonPi/Base.

Here’s a fairly typical line from the sketch for an emonTH V1.5 with the RFµ 328 module and a RFM12B:
#define RF_freq RF12_433MHZ // Frequency of RF12B module can be RF12_433MHZ, RF12_868MHZ or RF12_915MHZ. You should use the one matching the module you have.

Thanks Robert,

I can remember editing sketches for Nanodes, emonTXs and emonGLCDs a long time ago and I understand that the same must apply to the Arduino device in the emonPi but I don’t know how my emonPi changed from 868 to 433MHz.

Was a new sketch uploaded automaticaly from the pi when I booted from the SD card with the new pi firmware? I first tried the February 2024 load which I discovered did not work as my old pi is limited to 32 bit. I then loaded the November 2022 version.

Is there any way to determine which vesion of firmware is loaded on the Arduino?

If the emonPi itself was originally working at 868 MHz and it has an 868 MHz radio inside, and it’s now operating at 433 MHz, this is almost certainly what’s happened. You’ll need to find the front-end Arduino sketch inside the emonPi, extract it, edit it to change the operating frequency and put it back – easiest with the SD card in a full-size computer, but if you’re a RPi expert, doable inside the RPi, or so I’m told.

There are two possible methods: either connect to the FTDI port and watching the message it sends at start-up, and hope the sketch identifies itself (you can’t download the compiled sketch via FTDI port), or get a sample of the data it outputs and try to match this to the known ‘standard’ sketches. I’ve got a spreadsheet that helps with this.