The PL Tone Decoder does what its name implies - it decodes PL tones. But wait, there's more! It also handles DCS codes as well. The form factor is small - it is the same size as a Raspberry Pi, and in fact can be plugged into a Pi 3 or 4 using the 40-pin GPIO connector. When connected to a suitable audio stream from a ham radio, this unit will decode and display the PL tone (or DCS code) on the built-in 7-segment display that's large enough to be seen across a room. Yes, you can do the same thing with a scanner, HT or mobile unit, but those solutions are cumbersome to use. The PL Tone detector is a custom-built PC card that uses a Raspberry Pi Pico to do the hard lifting. The rest of the card handles power, display and communications to connected computer (try that with your scanner...).
The best way to use the card is to connect it to a radio that has a 9-pin DIN data port that is capable of 9600 baud. The 9600 baud configuration gives unfiltered access to the audio output of the radio. You can try it at 2400 baud but we found that this connection tends to filter out the higher PL tones which for our use was unacceptable. Several radios have been tested and found to work including radios from Kenwood and Icom.
The PL Tone card requires two connections: the data connection discussed above and a 5 volt/USB connection for powering the unit. The 5 volt/USB power can be supplied in one of three ways:
1. Stand-alone USB power: use a USB cable with a mini-USB connector and plug it into any USB-A adapter. In this mode, the unit will display the tones/codes on the integrated 7-segment display.
2. USB power from a laptop/computer: This connection is similar to the stand-alone version with the exception that the PL tone card will output the tone information over the serial connection of the USB cable. This way an application can be used to read this information and process it as you wish.
3. Power from a Raspberry Pi: Here the PL Tone card is connected to the 40-pin GPIO connector of the Pi. Power is taken from the Pi and the data stream is written to the Pi over the I/O pins. This way an application can be used to read this information and process it as you wish.
The PDF document below describes the general operation of the PL Tone decoder card and includes instructions on how to update the software on the Pico to the latest release of software.
The software is delivered as compressed/zip file. The zip file contains a single file with the .uf2 extension which can be downloaded to the PL Tone decoder card. The version number is in the file name - Tone15.zip is version 1.5 of the PL Tone decoder software. The PDF manual explains how to update the card to the latest release.