New Year’s eve building marathon: TNC-Pi APRS gate

I used the New Year’s eve long weekend to finish another project – TNC-Pi kit from Coastal Chipworks. I already experimented with APRS TinyTrak beacon before and noticed bad APRS coverage throughout Toronto, especially, east of DVP so I decided to put my own RX gateway to improve reception in the area.

The ordered kit arrived fairly quickly. Building the kit was easy – thanks to great instructions on the website.

For the radio part, I used a spare Baofeng UV-3r found in a drawer. Its output power is 2W only but with a good antenna it should be enough. Either way, the height above average terrain (HAAT) will be 20-30 feet at best. To interface it, I soldered a 4-wire ipod wire to a spare serial male connector – worked the first time. All parts, including Baofeng radio, ordered from Chinese vendors on eBay. The radio is powered using USB charging cable from a 5V supply – same as the Raspberry Pi. I added a few ferrite cores on all power supply and interface wires.

For the software part, I downloaded and compiled APRX gateway software. For some reason, the package is no longer available from repositories so I downloaded and compiled it from as per the instructions from Coastal Chipworks.

Everything seemed to work except the APRS-IS part – the received packets weren’t routed to it. Apparently, a passcode is now required even for RX-only gateways. To get the passcode, I used callpass cmd-line utility from Xastir package.

For the antenna, I used a spare 1/4 wave vertical mobile whip. It is not tuned for best SWR yet so the gateway is currently running in test mode – RX-only. Eventually, once I put a better antenna, it will be switched to a full digipeater mode.

For the enclosure, a tin cookie jar of a decent size was used.


The first packets received were from quite a far given the low HAAT and low gain antenna:


The APRS coverage east of DVP has now improved – the following packets would go unnoticed if not for the new gateway:


Future planned improvements are:

Also coming soon – finished build of a Nescaf QRP switched capacitive filter kit


As per request from a reader, the pin-out for Baofeng UV-3r is as follows:

pinout tncpi

Serial male connector Audio plug On photo
Pin 5 (right-most pin upper row) Tip (spkr out) red wire
Pin 1 (left-most pin upper row) Ring (mic in) white wire
Pin 3 Ring (ptt in) brown wire
Pin 7 (bottom row) Sleeve (gnd) black wire

The order of pins is when looking at the actual connector pins side (not the soldering side) – from top row left-to-right. As in here:

About va3paw
Ham radio enthusiast, software developer

One Response to New Year’s eve building marathon: TNC-Pi APRS gate

  1. Jwahar Bammi says:

    I landed on your blog post looking for the wiring diagram between the TNC-pi’s DB-9 connector and the Baofeng mic/spkr dual jack. Could you kindly describe the connections, will be very helpful.
    73 de kc1ccr

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