Over the years, I have made several Allstar, Echolink, and APRS nodes1. A few months ago, my friend JC (W6IPA) told me he wanted to build an Allstar node. I gave him this piece of advice: if I had to make a new Allstar node, I would create a Raspberry-Pi hat with a radio module like the SA818. This module can output 1 Watt of power with 5 volts, or ½ a Watt on the low power setting. It only costs $10 to $14, depending on your provider.
My previous nodes used a Baofeng BF-888S or UV-82HP and a RIMLite or a USB dongle as an audio card. This set up works fine but is challenging to integrate into a nice box because of all the cables, making the setup fragile.
For good results, the Baofeng radio has to be modified. It works for Allstar and Echolink, but this solution is lousy for digital. For digital, you need to get the audio before the de-emphasis and pre-emphasis stage, which is not possible on a Baofeng. Finally, the Baofeng radios add distortion, making it hard to decode packets, especially on a weak signal.
After brainstorming with my friend, we decided to build a polyvalent Raspberry-Hat that we could use for voice and digital, with an embedded or an external radio. You can even use the board with any other computer by installing some connectors.
The Raspberry-Hat
The two pictures show the hat we have created. It uses a CM108 audio chipset.
You can install an SA818 UHF or VHF radio, or the header J3 to connect an external radio.
Power the board with the Raspberry-Pi 5-Volt, or the Raspberry-Pi can be powered through the J4 connector.
Finally, the SA818 can be configured using the Raspberry-Pi GPIO connector, or you can use any computer with an FTDI dongle and the J5 connector.
We have some of the PCBs in stock with all the SMD components already installed. If you are interested, you can visit my friend's site on Tindie.com
Using an external radio
This blog post will show you how to connect this board to a TYT-9000D radio data port for APRS. I have also run some tests with Allstar. It makes an excellent high power Allstar or Echolink node.
The TYT-9000D is a clone of the Alinco DR-138. The Alinco comes standard with a DB9 data port installed on the back of the radio. On the TYT, the DB9 is missing, but it is easy to install. The hole for the DB9 is hidden under a cover, and an AMP Mini-CT connector is present on the mainboard. This operation is straightforward, but you can always get an Alinco DR-138 if installing a DB9 is not for you.
TYT-9000D connector installation
To install the DB9 connector on the TYT, you will need to order a cable assembly Mini-CT 6 positions from Digikey. The part number is A100196-ND.
As you can see in the following picture, installing a DB9 connector is easy. I have also put a dab of hot glue to ensure the Mini-CT connector doesn't come out with vibrations.
For safety reasons, I haven't connected the red wire (+5v). I have not tested it, but I think this 5-volt wire can be used to power the Raspberry-Pi.
You can connect the wires on the DB9 anyway you want, but I would recommend using the same pinout as the Alinco. It would make it more straightforward in the future to plug an accessory made for the Alinco.
TYT and Pi-Hat pinout
| TYT Pin | Color | Function | DB9 Pin | Pi-Hat |
|---|---|---|---|---|
| 1 | White | Ground | 5 | 3 and 5 |
| 2 | Yellow | TX Audio | 9 | 4 |
| 3 | Orange | COS | 1 | 6 |
| 4 | Blue | RX Audio | 4 | 2 |
| 5 | Black | PTT | 7 | 1 |
| 6 | Red | 5 Volt | Not connected | Not connected |
Word of caution
The Alinco radios have an excellent reputation, and I know people who use them to build simplex or full-duplex repeaters. When they operate a low or mid-power, they can handle a high duty cycle. The TYT is a clone of the Alinco, and I am not sure it can take the high duty cycles required for a repeater. For an APRS digipeater, the TYT-9000D should be ok, especially if you set the power to medium or low2. But for a high power Allstar node, make sure you are not running the radio at the full 60 Watt. It would be a good idea to consider installing a FAN to help dissipate the heat.