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Building a Digital Mobile Radio Repeater Part 3: Cabling


Cabling power switching and distribution, plus interfacing and Raspberry Pi control.

This series of posts covers the building of a Digital Mobile Radio (DMR) repeater for amateur radio use on the 70cm band (UHF). The first post in the series introduced DMR basics, before then moving on to the construction and programming of a modem platform based around an STM32F446 NUCLEO board plus an interface shield.

The second post covered the major components and those which would be used for power distribution, and housing this all in a rack mount enclosure. In this post we take a look at power cabling and switching, together with interfacing and its cabling.

Power distribution


We fitted a length of DIN rail running almost the entire width of the enclosure, to accommodate terminals for power distribution, along with fusing and a DC-DC converter. The first components to be secured to this were a fuse holder for the 12VDC input and then three “always-on” terminals.


One of the nice things about DIN rail terminals being that you can neatly gang them together for power distribution, using cross connector (039-9105) cut to the length required.


Just to the side of the main fuse we added an earth terminal, which clamps to the DIN rail to providing an earthing point. You can see above that the 12V always-on terminals are capped at the open end with an end plate (872-4726) . A white wire sends 12V from the terminals to the key switch fitted to the front panel. When its contacts are closed this is returned via the 2nd white wire which is connected to the coil of the plug-in relay.

Another wire runs from the always-on terminals to a relay contact. In this case allowing us to easily switch a load of up to ~120W, without having to use a heavy duty switch and run thicker cabling to this, while also giving us the benefit of a spare set of contacts for future use.


To the right of the relay we have the switched 12V terminals, with thin wiring bottom-left that connects to the red front panel indicator and ebm-papst fan secured to the rear of the enclosure. This is actually a PWM-capable 4-wire fan and since want to pin it on full-speed, we’ve connected the purple wire (control) to the same terminal as the red (power). As you might imagine, grounding the purple wire would stop the fan altogether.


A fourth white wire is for the tacho, which in our case will not be used and so this wire was simply secured out of the way — to the side of the fan.

Next to the switched 12V terminals we have two more fuse holders, one for each radio. Next to these we then have the 5V output DC-DC converter, which will be used to power the Raspberry Pi.


The 5V output is fed via another fuse holder to a set of three terminals. To the right of which we have a block of black terminals for negative — common to both 12V and 5V rails.


It should be noted that with all these connections we terminated the cable ends with crimp ferrules, as this gives a much more satisfying result, where you are not left wondering if multi-core cable has splayed in some odd/messy fashion as pressure from the terminal is applied.


Note that if using ferrules you will also need a suitable crimp tool.


We fitted second-hand PMR radios and these came with power leads which had been cropped very short. However, this was not a problem as we used butt splices which are first crimped and then heat shrunk (458-0724) to safely and securely join sections of new cabling.

MMDVM Interface


A small adapter PCB is used together with a PS2 style Mini-DIN cable to connect each PMR radio up to the MMDVM interface shield.


Prior to installation some configuration will be required with audio levels being set.


The front panel LCD, which provides repeater status, was also cabled up to the interface shield.

Raspberry Pi


A 4-way header was connected to the Raspberry Pi P1 header pins 30, 32, 34 and 36, which gives access to two each ground and GPIO.


One GPIO pin is connected via a resistor to a front panel LED for Pi status, while the other can be shorted to ground via a momentary push button to trigger the Pi to shut down. Since it would be quite useful to have persistent logging enabled the Raspberry Pi filesystem will be mounted R/W, which means we would benefit from a simple mechanism whereby anyone could easily shut it down and check it is safe to power off, without having a keyboard and monitor attached.


Power is supplied to the Pi also via P1, using a Molex KK connector.

Other cabling


Other cables were also fitted, e.g. the Ethernet and HDMI cables, and two USB cables. It should also be noted that one of the nice features of this rack mount enclosure being that the adjustable internal bars provide handy points for neatly securing cables.

Next steps

Now that we have everything secured into place and cabled-up, in Part 4 we will move on to setting up software on the Raspberry Pi.

Andrew Back


Open source (hardware and software!) advocate, Treasurer and Director of the Free and Open Source Silicon Foundation, organiser of Wuthering Bytes technology festival and founder of the Open Source Hardware User Group.
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