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%.

INADVM Rev1 build

I bought this http://inadvm.com/inadvm/inadvm-index.html Its a multi mode modem for use with Fusion, DMR and D-STAR radio systems. One uses it with MMDVM software to create a multi mode hotspot.

The board I bought is a development board. It is already in Rev2 but much of the board is the same. The difference between the revisions is simply RSSI facilities added to Rev2.

There being currently no construction and test details I documented my construction and initital testing of the modem.

On a clean workbench assemble your tools. You'll need a 15W soldering iron, solder, the kit, your Arduino Due, test meter, chocolate cookies and a Brownian Motion Producer such as a nice hot cup of tea.

Dump out the contents of the anti static bag and inventory your kit per the parts list found here.

Conventional wisdom dictates that we build our kits in a particular order. This is usually mechanical, resistors, capacitors then finally semiconductors. We are NOT going to do that. We are going to build a stable platform for which to install all the parts. The platform will be made out of the electrolytic capacitors and other mechanical parts. This will help prevent the board from rocking whilst you are soldering on the other parts.

Install the header strip into the Arduino Due. This will greatly aid in aligning the headers when you mate the INADVM board. Make sure to install the long part of the header into the Arduino. The short part will go through the board and get soldered.

Place the board onto the Arduino and header pins. Solder only the pins that stick though the board.

You should end up with this when you complete this step.

Install all the electrolytic capacitors. Take care to orientate them correctly. The long leg goes to the "+" on the legend.

Now turn the board over and check for stability. Better, right?

Now it's time to take a well earned swig of your Brownian Motion Producer. Go ahead and treat yourself to one of those chocolate cookies too.

Install the rest of the mechanical parts. These are the IC sockets, DB9 connector, headers and jumpers. I also installed the variable resistors too. Pay attention to my orientation. The legend on the board is WRONG!! If you installed your part per the legend you'll be forced to adjust your modem in the reverse direction. Not a big deal but catch it now whilst you can. Install the jumper into position "A" on the yellow header.

Install the resistors, capacitors and the diodes. DO NOT install any of the transistor or IC parts! Install the board onto the Arduino and power it up. I connected my shack supply to the Arduino which can take up to 36V via the barrel connector.

Observe the board for blue smoke. If you see any disconnect the power IMMEDIATELY!  Using your meter check for 3.3V on pin 8 of both IC sockets. The "power" LED should also light up.

Unplug the board. Install the remaining parts paying attention to install the IC's the right way around.

That's it. You are now ready to install the firmware onto the Arduino and enjoy your modem. Go ahead and eat your last cookie. Heck! Get yourself a beer. You earned it!