I wanted to build the most inexpensive SvxLink “node” or “station” I could using off-the-shelf parts. I bought an inexpensive BaoFeng “S9Plus” (it says “UV-S9X3” on the side) radio on Amazon for $28. For the computer system, I used a Raspberry Pi 4 that I already had, however, I also found a Raspberry Pi B+ board on Digi-Key for $25. For the interface between the HT and RPi, I bought a Masters Communication RL-20 board for $25 as an unassembled kit. All prices exclude sales tax and shipping charges (free shipping from Amazon). So for about $78 + tax + shipping, you can have your own SvxLink Node!
Why would you want one? Well, if you like to use EchoLink to talk to folks all over the world from your HT or want to monitor the repeater where you used to live, this might allow you to do that. You could also set up an “intercom” with friends and family using the SVX reflector software that’s included as part of the SvxLink build. An SvxLink node provides an internet connected radio node that you can use with an HT or mobile radio and ultra low power.
I disassembled the “UV-S9X3” to find that it was actually the same as a “UV-5R” but with the addition of the 220 band. I had previously purchased an identical looking “UV-9S” radio. Testing of that radio revealed that there was no filtering added for this band, so transmitting on 220 also means you’re putting out a pretty decent signal at the 2nd harmonic on the 440 band as well. Best not to use the 220 band, or at least not with any decent antenna! I plan to use this on the UHF band where SCRRBA has allocated frequencies to use with EchoLink (E.g. 440.000 MHz & 445.000 MHz)
The first thing I tackled was how to obtain a “COR” or Carrier Operated Relay (or for “COS”, “switch”). Naturally someone had already figured this out. I found the TDA2822 Dual Low Voltage Amplifier chip and verified there was a switched voltage (V-supply to the amplifier chip) on pin #2 which corresponded to the presence of a signal with qualifying PL.
I lazily buzzed out the TX & RX Audio along with PTT from the 2.5mm & 3.5mm Speaker/Mic jacks instead of digging deeper to find discriminator audio and a maybe a better place to inject transmit audio. This means the receive audio is “deemphasized” or “cooked” with the level being adjusted by the volume pot. Transmit audio will be Pre-emphasized. SvxLink can handle that just fine.
Initial testing was promising. I used SvxLink’s Parrot feature to test the audio quality and it sounded good.
After testing, I removed the on-board electret mic element. Doing that required the somewhat complicated surgery of removing the display, separating the board from the chassis and desoldering the mic element. Another option would be to plug a dummy plug into the Mic jack.
The RL-20 board is designed for “Radioless” applications where you’re using a speaker and microphone instead of attaching it to a radio. Using an RA-35 or RA-42 board would give you more hardware audio level adjustability. I believe the RA-35 kit is $35.
I designed a case for the hacked BaoFeng HT. To speed up printing, I used only 2mm thick walls which turned out okay. The press-on lid did not fit snuggly so I redesigned that to have a rubber band o-ring slot to give it more friction. I included some ventilation holes as well.
The DB9 connector is wired using the Masters Communication RA-35/RL-20 standard. This is also compatible with the Repeater Builder STM32-DVM digital MMDVM board.
The completed setup.
I’m working on a more efficient and manageable way to interconnect everything. Perhaps designing a single case for all three major components would be better? A Raspberry Pi Zero W (or Zero 2W) would work for this just as well and it is much smaller.
The HT is powered by 7.2-9Volts DC. I used a buck converter / switching DC regulator to step down 13.8 VDC to 7.3VDC. Instead of installing this in the case with the radio, I designed a small case for the regulator. Powerpoles provide the input and output connections.
After my initial design was done, I decided that there were too many separate components. So, I set out to consolidate the computer and audio interface board into one assembly. To do this, I used a smaller Raspberry Pi Zero W single board computer. It’s much smaller than the Pi3 & Pi4 boards and has sufficient memory and CPU performance for the job. Instead of using the large “USB B” connector, I soldered a right-angle USB micro pigtail onto the RL-20 board directly.
So that I could run the combined system on conventional “shack” supply, 13.8V, I included a small buck converter/switching regulator board. Instead of trying to fit another USB Micro connector into the cramped chassis, I connected the voltage regulator directly to the 5V bus on the Raspberry Pi’s GPIO connector.
The new RL20+RPi configuration can be used with higher power transceivers such as the Motorola CDM-750.
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