MMDVM hotspot – hardware network switch

posted in: DMR, Ham radio 30

Several months ago, John VK4JWT asked me if I knew of a way to switch his JumbSpot between Brandmeister and DMR Marc using some sort of physical switch.

John was interested in this feature, because he does a lot of mobile operation and it would be much easier to be able to flick a switch on the hotspot, when driving along in he car, than it would be to mess around with different channels and reflector codes.


Unfortunately at the time I had a lot of other projects on the go, and only had time to investigate the theory of how this could be done; however the recent changes to the Australian DMR MARC network , to use IPSC2 and direct talk group access to the Australian MARC network, got me thinking about this problem again, and I have finally come up with a solution, where I have an external 2 position toggle switch connected to my duplex hotspot, where one position tells PiStar to connect to DMR MARC and the other position tells PiStar to connect to Brandmeister.



Before I go into the details.. Although I have made this modification to my duplex hotspot, I have checked the schematic for the simplex hotspot aka JumboSpot and as far as I can see, it should work fine on the simplex hotspot, because it uses the same pins to connect from the Raspberry Pi to the hotspot – specifically Pin 7 seems to be available on the top of the hotspot, and can be soldered onto.


The first part of this system is the hardware switch itself.

Looking in my box of spares, and other junk, I found a toggle switch which had a central wiper and 2 other pins. In one position the wiper is connected to one pin and in the other position its connected to the other pin. The 2 “other” pins are not connected to each other at any time (This is important)


To connect the switch to the Raspberry Pi, I needed to find a solder pad on the duplex hotspot that was connected to an unused GPIO pin on the RPi board.

Most duplex hotspots, do not have connections to the entire Raspberry Pi pins, they only have two 10 pin headers, one at each end of the bank of pins on the Raspberry Pi, and of course some of these pins are used for the communication etc between the hotspot and the RPi

Looking on github, I found the original open source design for the duplex hotpot which was used as the basis for the duplex hotspots that are available on eBay and AliExpress from China.

This showed which pins on the RPi were actually used.

Because I also need a 3.3V supply to connect to the switch, I chose Pin 7 (aka BCM4_GPCLK0), as it was close to pin 1 which has 3.3V on it (and also right next to a ground , which I also needed)


In order for the RPi to sense the position of the switch, the voltage on Pin 7, needs to be 3.3V when the switch is in one position and 0V when its in the other position. Although it you could connect Pin 7 directly to the wiper, and 3.3V and 0V to the other 2 pins respectively, its normally good practice to pull up or pull down GPIO pins via a resistor.

In my case I had a 4.7k resistor on my workbench, left over from a previous project, so I decided to use that, but I think the optimum value is normally 10k (but don’t quote me), anyway 4.7k works fine 😉

So I soldered the 4.7k to Pin 7, then connected the switch via  flying lead, where I used red for 3.3V, black for 0V and brown for the switched input Pin 7



After powering up the hotspot, and making sure I’d not done any harm to the board, e.g. like shorting the power rail, I checked that Pin 7 was indeed being switched to have either 0V or 3.3V depending on the position of the switch.


The next step in the process is to make separate profiles for Bransmiester and DMR MARC on  PiStar and add a script to switch between them

To do this you need to login to the PiStar console , by using the “SSH Access” option in the Expert settings.

and create a folder to store the configuration files

using the commands


mkdir configs



The command rpi-rw makes the SD card writable, and you may need to use this command again if you find that the SD card becomes non writable – because the RPi operating system seems to run a daemon which keeps changing the SD to read only





The next step is to use the PiStar configuration screen to change to DMR MARC only, e.g. on my case..



Then press the Apply Changes button


Then go back open the SSH Access screen again and copy the active MMDVMHost file to the configs folder


sudo cp /etc/mmdvmhost configs/marc


Then reconfigure PiStar for Brandmeister using the configuration screen (again), then once the Brandmeister configuration has been installed by PiStar, make a backup of this profile as well


sudo cp /etc/mmdvmhost configs/bm



Now that you have the 2 configurations backed up, you need to make a script file to check the status of  GPIO Pin 7 and copy the relevant mmdvmhost profile file to the master location and then restart MMDVMHost.


This is the script I created to do this, which can be downloaded from this link


But to get it onto the RPi you can use this command from the SSH window



curl >


Now that you have the scripts (in /home/pi-star) you need to make it executable using this command

chmod +x


The next step is to test whether the script actually works. by running it in the foreground, by typing the command to run it


sudo ./


Depending on the position of the switch , you should see the message “DMR MARC mode” or “Brandmeister mode”


Looking on the PiStar dashboard, you should see the “DMR Master” change (in the lower left corner of that screen).

Note. Although MMDVMHost is restarted and connects fairly quickly, I’ve noticed on IPSC2 and Brandmeister that it takes around 10 secs for the connection to the server to be fully operational.


Now to make this script run when the PiStar starts up.




I used the method described on this page


Specifically “method 1” which adds the script to /etc/rc.local


To do this you need to edit that file e.g.


sudo nano /etc/rc.local


and add the following line to the second to last line in the file. (just above the line that reads “exit 0”

The text to add is


/home/pi-star/ &


Once you have added this line and saved the file, all you need to do is reboot (or power cycle) your PiStar hotspot, and after its finished rebooting, you can confirm that the switch is still working by looking at the dashboard


(Note. There are other ways to run a script at boot time, but this method works for me)



For anyone who wants to read the script, here it is



# Script to allow switching of DMR network using a hardware switch
# connected to a gpio pin
# by Roger Clark VK3KYY / G4KYF

# Before using this script you need to make backup copies of your DMR Marc
# and Brandmeister mmdvmhost setting files into /home/pi-star/configs
# the script assumes the files are called marc and bm
# You need to make the configs folder
# e.g.
# rpi-rw
# mkdir /home/pistar/configs
# To create these files. Use the Pi-Star configuration screen to switch to
# DMR Marc, then copy /etc/mmdvmhost to /home/pi
# cp /etc/mmdvmhost /home/pistar/configs/marc
# Then reconfigure to Brandmeister and save that config
# cp /etc/mmdvmhost /home/pistar/configs/bm

#--------- Script code starts here ----------------------------------------

gpio mode 7 in
# Set the lastVal to a non binary value to force the script to copy the
# config set by the switch when the script starts, in case the switch
# was changed when the script was not running, e.g. if the Raspberry Pi was
# not powered up etc

#Loop indefinitely
while :
b=$(gpio read 7)
if [ $b != $lastVal ]
#need to mount the file system as read write
#as it defaults to read only

mount -o remount,rw /
if [ $b -eq 1 ]
echo "Brandmeister mode"
cp /home/pi-star/configs/bm /etc/mmdvmhost
echo "DMR MARC mode"
cp /home/pi-star/configs/marc /etc/mmdvmhost
mount -o remount,ro /
systemctl restart mmdvmhost.service
sleep 1

30 Responses

  1. Robert

    Thanks for your work Roger, I am looking at applying this to a jumbo spot, but I am not able to do so, what is the chip number. I am assuming its the same on a duplex board as is on a simplex board

  2. Roger Clark


    It should work on a JumboSpot. I checked the schematic and its connections to the RPi are the same as far as I can tell.

    Pin 7 seems to be free and available on the top of the hotspot

  3. Robert

    Thanks, but what is the chip number ? is it the RF chip ?

  4. Roger Clark

    It doesnt matter what RF chip they use, but AFIK they all use the ADF7021

  5. Anonymous

    Thanks Roger, it was not clear to me that it was the RF chip.

  6. Roger Clark

    Looking under the microscope they are marked as

    ADF7021 BCPZ #1733 3949267.1

    I’m not sure what chip you are looking at, but mine are 48 pin,

  7. ken


  8. Roger Clark

    LOL… I’m not sure it deserves a “wow” 😉 but the technique may come in handy …

  9. Robert

    I worked it out, its the header pins not an IC on your diagram… Thanks

  10. Roger Clark


  11. Robert

    Roger, I am getting :Could not resolve host errors
    when running the netswitcher script

  12. Roger Clark


    That command worked for me the other day

    Try this instead

    curl >


    curl -O

    and then rename the file to remove

    mv net-switcher.sh_.tx

    make sure the SD card is writeable first by doing rpi-rw

  13. Robert

    Thanks, that worked and the script is also working

  14. Anonymous

    All done and dusted, works good Roger
    Thank you

  15. Robert

    All working, Thank you Roger

  16. Roger Clark


  17. Roger Clark


    I’ll update the post with the ammendment to the curl command


  18. Robert

    VK7HSE will send you all the changes to your instruction that made things work.

  19. Roger Clark

    That would be great.


  20. ALex VK2PSF

    You may need to move dmrgateway as well for some configs

  21. Roger Clark


    I totally agree.

    I’m now working on a revised script which does this.


    I just tried this, but it looks like dmrgateway is only running if its needed, so attempts to restart it, if its not running give a warning / error message

    I’m not sure if its better / quicker to use ps to find if the service is running, and then only restart it if it is running, or simply try to restart it regardless of whether its running or not

    I suppose the latter would result in the log files getting messy 🙁

  22. Alex VK2PSF

    I suspect mmdvmhost will look after it by inspecting the enable flags in each section and starting as needed will investigate.

  23. Alex VK2PSF

    OK, so the configure scripts action is to stop all services (including cron) make changes then start all services using sudo systemctl start dmrgateway.service > /dev/null 2>/dev/null & try and ignore any return values, the start script for dmrgateway looks for to decide if it should execute. most have been installed to systemctl as disabled so there is no automated restarting so just start the services anyway.

  24. Roger Clark

    I think PiStar dashboard has code to handle when DMR Gateway is selected as DMR Master; but I’d need to look in the source code to confirm

  25. Roger Clark


    For mmdvmhost I copied the restart code from the dashboard code, but it looks like things are different for DMR Gateway

    BTW. I just tried running

    sudo systemctl restart dmrgateway.service


    I think you already got past this point, and decided it best to try to start it, regardless and then hide the errors

    Did you check if the errors get logged ?

    and this doesn’t return any errors if the service is already running, but I don’t know if the command actually restarted it or not.


    sudo systemctl start dmrgateway.service

    even if the service is already running, doesn’t seem to show any errors

    However, if I change the configuration in the dashboard and remove DMR Gateway as DMR Master and then try to start the service, it fails.

    I’m not sure why this is, because the dmrgateway config file is present in /etc

  26. Bevan VK5BD

    Good work Roger, have you considered adding some scripting to the Pi-Star remote commands so that the swapping on modes could be done remotely by private calling 9999991 or 9999992 (examples only) that way there are no hardware mods, I do understand it will be harder for the user to know what mode there hotspot is in and have to rely on memory to know the mode they have left it in if they cant see the dashboard

  27. Roger Clark

    Several people have suggested this, and I’ll probably have a go at implementing it when I get chance.

  28. Robert Alford

    Did you ever manage a Open GD77 Implementation so I can use both my Open GD77 Radios to switch from 1 to the Other ?

  29. Roger Clark

    I only wrote a system to copy the mmdvmhost config files for us with hotspots connected via GPIO to the RPi.

    It may work to switch between 2 USB based hotspots (like the GD-77) but I’ve never tried that, as I can’t see a use case for that.