A simple but capable AM transmitter for the Raspberry Pi pico!
Note: Use the https://github.com/earlephilhower/arduino-pico core and install
the RP2040_PWM library in your Arduino IDE.
The FCC link for unlicensed educational transmissions is here:
https://www.fcc.gov/media/radio/low-power-radio-general-information
This simplest summation is:
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Never interfere with anything.
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Move your frequency if you are being interfered with, DO NOT COMPLAIN.
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Do not transmit more that 200 feet.
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Use the broadcast AM and FM bands (COMMON SENSE RULES!)
Unlicensed operation on the AM (535 to 1705 kHz) and FM radio broadcast bands (88 to 108 MHz) is permitted for some extremely low powered devices covered under Part 15 of the FCC's rules.
Unlicensed operation on the AM and FM radio broadcast bands is permitted for some extremely low powered devices covered under Part 15 of the FCC's rules. On FM frequencies, these devices are limited to an effective service range of approximately 200 feet (61 meters). See 47 CFR (Code of Federal Regulations) Section 15.239, and the July 24, 1991 Public Notice (still in effect). On the AM broadcast band, these devices are limited to an effective service range of approximately 200 feet (61 meters). See 47 CFR Sections 15.207, 15.209, 15.219, and 15.221. These devices must accept any interference caused by any other operation, which may further limit the effective service range.
https://www.chathammarconi.org/
picoAMSine is being taught in an after school STEM program at the Museum in Chatham Mass mid June 2024 by the Buzzards Bay Radio Club WB2TEV with instructor Bradshaw Lupton, with the gracious continued assistance of VU3CER Dhiru Kholia, KL7TF, Tom Farrington, N2EMU Mark Dionne, W1HD William Way, Eben Franks and Scott Haigh, David Weikel each of whom cheerfully tested WSPR and/or FT* communication from the beginning of the COVID pandemic. The program will be detailed here for other STEM teachers, hams and STEM students on Cape Cod and throughout the State.
The Pi generates a 1557 kHz PWM signal, which will be the carrier wave. The
frequency is not changed, only the pulse width. The pico does pulse width
changes according to the signal, and outputs the AM signal on pin 15.
A sine wave signal is available on the GP12 pin! This GP12 pin can be
connected to the ADC pin in order to send a RF sine signal out.
Update: The Für Elise melody is available on the GP13 pin now - enjoy
responsibly! Connect GP13 pin directly to the ADC pin using a jumper cable.
Update 2: Morse message tune is available on GP16 pin!
export PATH=$PATH:$HOME/.local/bin
sudo apt install make python3-pip -y
make install_arduino_cli
make install_platform
make install_deps
make default
Once these steps are executed, the resulting firmware can be found at the
build/rp2040.rp2040.rpipico/picoAMSine.ino.uf2 path.
- Flash the .ino file using the Arduino IDE
- Tune in to 1557 kHz AM on the radio
Do you want to transmit an actual voice signal instead of the (possibly annoying) sine wave?
Just connect the MAX4466 Electret Microphone Amplifier with Adjustable Gain Module to the Pico, and there you go!
pip3 install -r requirements.txt
From https://github.com/rgrosset/pico-pwm-audio?tab=readme-ov-file#usage-guide,
Launch the notebook user interface by issuing the command below in the same folder as this project. This will open the Jupyter user interface. Open the notebook file in the user interface.
jupyter notebook
The notebook itself is fairly self explanatory. Run each cell in order using the run buttons in the UI. The final cell will create a data array that you can copy and paste into your project. The notebook is configured to convert just about any WAV file to a mono 11 kHz data which you can then use in your projects!