PicoFox – A Hackable 2m Fox Transmitter

PicoFox is an open source fox transmitter for the 2-meter amateur band, built around the RP2040 microcontroller (the same chip used in the Raspberry Pi Pico). It’s designed to be simple, functional, and easy to modify.

PicoFox works well as a standalone fox for portable or walking hunts, but it’s also a great platform for experimentation. There's ample CPU, flash, and GPIO available if you want to add features like sensors or interactivity. The transmitter can generate a frequency or phase modulated signal with up to 2.5kHz (data) bandwidth.

Key Features

• Transmits FM in the 144–148 MHz band (ITU Zone 1: 144–146 MHz).
• Configured by editing a text file — no software or terminal needed.
• Transmits any 5kHz 16-bit PCM WAV file.
• Automatic Morse code ID with configurable tone, speed, and spacing.
• Scheduled start and stop times with low-power sleep before and after a hunt.
• Percentage-based or fixed-time transmit cycles.
• 15 MB user volume for custom audio and configuration files.
• RF output > 18dBm at full power. Down to ~ -20dBm with full attenuation.
• Charges over USB.
• Expansion header provides:
  • 12 GPIOs (including I2C, SPI, UART, ADC).
  • Regulated 3.3V and raw battery voltage.
  • Two additional SI5351 clock outputs

Getting One

Fully assembled PicoFox units are available from AI6YM.radio. An enclosure, battery, and antenna are included. I ship nearly anywhere in the world.

You can also build your own - both the hardware and software are open source for non-commercial use. For commercial inquiries, contact [email protected].

Configuration

To configure your PicoFox, connect it to any computer using a USB-C cable. It will appear as a USB mass storage device (like a flash drive). Open settings.txt with a text editor and adjust the configuration values as needed. Eject the drive, disconnect USB, and power cycle the PicoFox to apply the new settings.

To reset to factory settings, delete settings.txt, eject the drive, disconnect USB, and power cycle the PicoFox. A default settings file will be regenerated.

Note: if the PicoFox is powered on but does not appear as a USB storage device when connected over USB there is a 99% chance you have a bad cable. The current record is three bad cables in a row before a good cable was found. The world is full of bad cables and cables built for power only with no data wires.

This document applies to PicoFox r3 - all PicoFox orders from January 2026 on are r3 hardware. The r2 PicoFox had a lower output power (15mW / 12dBm) and did not include the in-series attenuator (so the ATTENUATION configuration field does nothing). It used a micro USB connector instead of USB-C and could not charge while off. Otherwise the information in this document applies.

Settings Overview

These settings reflect the current firmware version (v2.1). If your settings.txt file does not contain all of these settings that indicates you are running an older firmware version.

• CALLSIGN: Alphanumeric callsign (max 12 characters).
• ITU_ZONE: ITU zone where the transmitter operates (1, 2, or 3).
• FREQ_MHZ: Transmit frequency in MHz.
• ATTENUATION: Attenuation level 0–151, approximately 0.25dB steps.
• DUTY_CYCLE: Transmit duty cycle percentage (1–100), or 0 to use the fixed-time cycle settings.
• INITIAL_SLEEP_SECS: Seconds to remain in low-power sleep after startup and before the first transmission (0 starts immediately).
• RUNTIME_SECS: Seconds to run transmit cycles after the initial sleep (0 runs indefinitely).
• CYCLE_ON_SECS: Transmit time per cycle when DUTY_CYCLE=0.
• CYCLE_OFF_SECS: Sleep time between transmissions when DUTY_CYCLE=0.
• MORSE_WPM: Morse ID speed in words per minute.
• MORSE_FARNSWORTH_WPM: Slower Farnsworth spacing speed (ignored if lower than MORSE_WPM).
• MORSE_TONE: Morse tone frequency (100–2000 Hz).
• MORSE_TONE_VOL: Morse tone volume percentage (1–100).

Note: Invalid or missing values may disable the transmitter or revert to safe defaults.

Scheduling and Duty Cycle

All timing values are whole seconds and support at least a full day (86400). The default remains continuous transmission at 100% duty cycle, starting immediately and running indefinitely. The 60-second fixed-cycle values are present but do not take effect unless DUTY_CYCLE is changed to 0:

DUTY_CYCLE=100
INITIAL_SLEEP_SECS=0
RUNTIME_SECS=0
CYCLE_ON_SECS=60
CYCLE_OFF_SECS=60

INITIAL_SLEEP_SECS provides time to hide the fox before it starts transmitting. The radio output remains off and the processor uses low-power sleep during this delay. After the delay, RUNTIME_SECS starts counting. When a nonzero runtime expires, PicoFox stops the normal cycle, transmits its CW callsign ID one final time, switches the RF output off, and returns to indefinite low-power sleep. The final ID is always the last transmission, regardless of the other cycle settings.

For the original percentage-based behavior, set DUTY_CYCLE from 1 to 100. Each transmission contains audio.wav followed by the CW ID; the firmware calculates the off time needed to produce the selected percentage.

For exact on/off times, set DUTY_CYCLE=0. PicoFox repeats or trims audio.wav to fill CYCLE_ON_SECS, reserves the end of that interval for the CW ID, then sleeps for CYCLE_OFF_SECS. If the configured on time is too short to contain the complete ID, the firmware raises CYCLE_ON_SECS to the required minimum and writes the corrected value back to settings.txt.

For example, the following waits two hours, transmits in 60-second on/120-second off cycles for three hours, sends a final CW ID, and sleeps until the unit is collected:

DUTY_CYCLE=0
INITIAL_SLEEP_SECS=7200
RUNTIME_SECS=10800
CYCLE_ON_SECS=60
CYCLE_OFF_SECS=120

Using the Transmitter

After configuring settings as described above, the transmitter will turn on following the provided settings the next time it is turned on. The transmitter may be powered by an external USB power supply or the internal battery.

A full battery will last about ten hours at 100% duty cycle.

Updating the Firmware

Most PicoFox devices in the wild are running firmware v1, this is true for both r2 and r3 hardware. The firmware is the same for either hardware version but some features will not work on r2.

You can identify your firmware version by looking at the settings.txt file. If the file contains the INITIAL_SLEEP_SECS setting you are running v2 or later; if not, you are running v1.

v2 and Later Easy Update Method

Firmware v2 and later supports entering the bootloader without opening the enclosure or holding the physical BOOTSEL button:

1. Back up settings.txt and audio.wav before updating. The update normally preserves the user volume, but keeping a copy protects your custom configuration and audio.
2. Create an empty file named exactly BOOTSEL (with no extension) in the root of the PICOFOX USB drive.
3. Eject the PICOFOX drive safely, then reset or power cycle the PicoFox.
4. Wait for the RP2040 bootloader drive named RPI-RP2 to appear.
5. Copy releases/pfoxr3v2.1.uf2 to RPI-RP2. Do not copy the UF2 to the normal PICOFOX drive.
6. The board will reboot automatically. When PICOFOX reappears, restore or edit your settings and audio as needed, eject it safely, and power cycle once more to apply them.

The firmware removes the BOOTSEL marker before entering the bootloader, so later boots return to normal operation.

v1 Serial Touch Method

Older firmware does not recognize the BOOTSEL marker file, but it can still enter programming mode through its USB serial port. Nothing needs to be typed into the console: opening the port at 1200 baud and then closing it tells the PicoFox to start its bootloader.

Before starting, close any files on the PicoFox and wait for file copies to finish. Use a USB cable that supports data, not a charge-only cable. Use one of the methods below according to the operating system you are using.

Once the PicoFox is in programming mode a drive named RPI-RP2 should appear. Copy releases/pfoxr3v2.1.uf2 to that drive; the PicoFox will reboot automatically when the copy finishes.

If this method does not work, or you prefer not to mess with serial consoles, you may use the physical buttons on the board as described in the next section.

Linux

1. Connect the PicoFox and find its serial port:

   ls /dev/ttyACM*

2. Run the following command, replacing /dev/ttyACM0 if a different port was listed:

   stty -F /dev/ttyACM0 1200

   If the command reports "permission denied," run it again with sudo.

macOS

1. Connect the PicoFox and find its serial port:

   ls /dev/cu.usbmodem*

2. Run the following command using the complete port name reported above:

   stty -f /dev/cu.usbmodemXXXX 1200

Windows

1. Connect the PicoFox.

2. Open Device Manager, expand Ports (COM & LPT), and note the PicoFox port, such as COM5.

3. Open PowerShell and run the following commands, replacing COM5 with the correct port:

   $p = [System.IO.Ports.SerialPort]::new()
   $p.PortName = "COM5"
   $p.BaudRate = 1200
   $p.DtrEnable = $true
   $p.Open()
   Start-Sleep -Milliseconds 100
   $p.DtrEnable = $false
   $p.Dispose()

Note: I offer no support for this method. Windows is a terrible environment for any sort of embedded programming or development work of any kind.

Physical Button Method

Open the PicoFox enclosure and find the buttons on the board. One is labelled RESET and the other BOOTSEL. With the PicoFox connected to your computer and powered on hold the BOOTSEL button, press and release the RESET button, then release the BOOTSEL button. A drive labelled RPI-RP2 should appear. Copy releases/pfoxr3v2.1.uf2 to that drive; the PicoFox will reboot automatically when the copy finishes.

Replacing the Audio File

To change the audio transmission, replace the audio.wav file in the device’s flash storage. The audio must be sampled at 5kHz, mono (one channel), and encoded as signed 16-bit PCM WAV.

Using Audacity (free software available for all major OSs):

1. Set "Project Rate (Hz)" to 5000.
2. Import, record, or generate your audio.
3. Mix down to a single mono track.
4. Export as WAV using Signed 16-bit PCM encoding.
5. Name the file audio.wav and copy it to the device.
6. Eject the drive and disconnect USB.
7. Power cycle the PicoFox.

Notes:

• Improper format may prevent audio transmission or cause noise to be transmitted instead.
• The audio is transmitted, followed by the morse ID, and then the transmitter turns off if DUTY_CYCLE < 100. In this way the length of the audio determines the transmit cycle time and the frequency of morse code ID transmissions. YOU ARE RESPONSIBLE FOR COMPLYING WITH LOCAL LAWS FOR TRANSMITTING YOUR CALLSIGN!
• To restore the default audio, delete audio.wav, eject the drive, disconnect USB, and power cycle the PicoFox.

Design Brief

An SI5351 clock generator, locked to a TCXO, provides the RF signal. Modulation is applied in software by the RP2040. To achieve smooth FM output, modulation is handled on core 1 with I²C running at 1 MHz. Register writes are minimized and no other devices are on I²C0.

Core 0 manages flash operations. Most extensions or new features should run on core 0. The latest firmware uses a two core buffer approach to reduce the clock speed required and so reduce power consumption. You may need to raise the clock speed to support any features you add.

Note: Mounting the device as a USB mass storage device will interrupt clean modulation as both cores are blocked during flash operations.

The SPI flash device is split into program memory and a 15 MB FAT12 filesystem. The filesystem contains a text file for user-supplied settings, an audio file for transmission, a second audio file containing the generated morse code ID, and a hash file used to detect changes to the settings file. Missing user files are re-created from defaults in the firmware binary. A damaged filesystem is reformatted automatically as FAT12 before the default files are restored.

The design intentionally includes large amounts of free CPU cycles, RAM, and flash so that cool things can be built on the PicoFox.

Development Guide

Build Environment Setup

• Install Arduino IDE >= 2.3.
• Install libraries - Adafruit SPIFlash, Adafruit TinyUSB, Etherkit Si5351.
• Install board package from my fork of Arduino-Pico. This will be merged upstream eventually but for now you'll need the changes in my fork.
• In Tools -> Board select the Raspberry Pi RP2040/RP2350 Boards -> AI6YM PicoFox.
• Connect your PicoFox and select the appropriate TTY / COM port.

On Linux it may be necessary to add udev rules for this device. The relevant rules from my system are below. I have no idea how this works on other operating systems. Mac is probably similar, Windows should not be used for software developement of any kind (or really anything).

# Make an RP2040 in BOOTSEL mode writable by all users, so you can `picotool`
# without `sudo`. 
SUBSYSTEM=="usb", ATTRS{idVendor}=="2e8a", ATTRS{idProduct}=="0003", MODE="0666"

# Symlink an RP2040 running MicroPython from /dev/pico.
#
# Then you can `mpr connect $(realpath /dev/pico)`.
SUBSYSTEM=="tty", ATTRS{idVendor}=="2e8a", ATTRS{idProduct}=="0005", SYMLINK+="pico"

# Adafruit TinyUSB stack.
SUBSYSTEM=="tty", ATTRS{idVendor}=="239a", ATTRS{idProduct}=="cafe", MODE="0666"
SUBSYSTEM=="usb", ATTRS{idVendor}=="239a", ATTRS{idProduct}=="cafe", MODE="0666"

Compilation Settings

• Flash Size: 16MB (Sketch: 1MB, FS: 15MB)
• CPU Speed: 200 MHz
• USB Stack: Adafruit TinyUSB

Do not select a different partition layout for a release build. The firmware expects the 15 MB filesystem layout; another choice can hide or overwrite the user volume. If the expected filesystem is missing or damaged, the firmware formats a new FAT12 volume and writes defaults. The current firmware uses about 697 KB of the 1 MB sketch partition.

All other settings are fine at default values.

Recovering a Board

In general the UF2 upload process will work fine. Click the upload button, after compilation the IDE will push the board into UF2 update mode, write the new binary, and then reset the board to normal operation.

If that doesn't work try the following:

• Use the BOOTSEL marker-file method described in Updating the Firmware.
• For firmware that does not support the marker file, use the USB serial method.
• Reset the board and try again. Just tap the RESET button on the board to reset the RP2040.
• Force the device into BOOTSEL mode. Hold the BOOTSEL button, tap the RESET button, and then release the BOOTSEL button.
• Force the device into BOOTSEL mode AND change Tools -> Upload Method to PicoTool.
• When all else fails grab a PicoProbe (or a Pico / Pico2 flashed with the PicoProbe binary), connect that to the debug header of the PicoFox, and change Tools -> Upload Method to Picoprobe.
• If all of the above fails, well, he's dead Jim.