Floureon PMR446 connector pinout for APRS tracker
As part of looking at typical PMR446 coverage, I want to use APRS on PMR446. I want to get a Freetrak APRS tracker to automatically key the Tx and then walk/bike around to see what sort of coverage I get. These Floureon PMR446 rigs are sometimes on offer at Amazon for ~ 14 a pair.
There’s no information in the manual about how this damn thing is connected. It is a 2.5mm TRS jack plug, and a DVM showed me that sleeve is sensibly connected to ground. I started off assuming it was connected like Motorola T series but no.
| pin | function |
|---|---|
| T | PTT/mic (rests at 1.8V on Rx) |
| R | Spkr |
| S | Ground |
A nasty little wrinkle is that the rig will only switch to PTT with a resistance to ground of <= 2.2kΩ, 4.7kΩ is too much and won’t put the rig into Tx. This means only some electret mics will work, if you are trying to make a headset with this. Although the first electret capsule I tried didn’t work because of this, it is not a problem for me because I am simply going to pull this down via 2.2kΩ through a transistor, and the rig goes into TX even with the about 0.3V VCEsat in series. I inject the signal into the non-transistor end of the 2.2kΩ via 150kΩ to pad down the signal to microphone level.
Wrangling Freetrak
This turned out to be a heck of a lot more involved than I had expected. Some of this was due to the limitations of PMR446 rigs, and some of it was due to changes in typical GPS NMEA outputs over the last 10 years wit the increasing popularity of drones, favouring a higher update rate. I have moved this to patching Freetrak