Monday, 11 June 2018

MMDVM based digital modes hotspot

Now that there are so many different digital modes around for the V/UHF bands I decided that it was time I had a hotspot that could cover all the modes at the same time.

I've been following G4KLX's MMDVM project since it's initial inception and have even donated one of my NQSMHS modems to the project (it started out as a D-STAR project). Today the project has gained quite some momentum and is now able to run on a Raspberry Pi Zero W along with other hardware addons such as screens and buttons etc. Significant work has also gone into creating a RPi distro explicitly for the MMDVM.

My hotspot is based on the RPI0W and ZumSPOT hotspot board that you can get from HRO here in the US. I followed this HowTo which said to download the latest image from the PiSTAR site and transfer it to an SD Card (just like any other RPi distro).

And this is the finished result. The 3D printed case is by George, M1GEO and is available from Thingiverse. It only supports a 3.5" Nextion LCD display but having also tried a 2.8" display I can tell you that you want the bigger screen! I have asked George to knock up some designs for other screen sizes and also to add some cooling holes into the box as it can get quite hot in there after a good round table QSO!

The Nextion screen is quite clever in that you upload a handful of screen images to it when setting it up. These images also have various text fields that will be populated with data every time the MMDVM host sends some info to the screen (eg, which screen image to display, what frequency, RX Callsign etc). Below is a selection of the screens that mine does.

The bootup screen. I decided to abandon this particular one as I couldn't really see it properly.
Traffic arriving from the Internet. This will be broadcast via the built in 20mW TRX. Note the RPi3 mounted next to it on the left. That's an Echolink system.
It's listening for my local input. It'll do this for 10 seconds. If I do not respond in YSF (in this case) it'll drop back to listening for any mode.

Receiving my YSF signal from my HT in the shack. Note the BER. This is quite high and can be attributed to overload in the RX. When I'm on the other side of the room the BER is down to almost 0%.

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