65 lines
3.9 KiB
Markdown
65 lines
3.9 KiB
Markdown
# picoAM
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A simple but capable AM transmitter for the Raspbery Pi pico!
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(image deviates slightly from current version of the circuit, apologies!)
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## Disclaimer!!
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The Pi is, to my knowledge, not able to output enough power to disrupt anything significantly as long as you're not near an airport (if you
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are, then it ABSOLUTELY IS), but with a long enough antenna, will get you in trouble and annoy others. Please use this only for experimental
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and educational purposes and do NOT use this in a way which might cause any disruption!
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My antenna for this is barely large enough for half of my room to have reception. Please don't go much further than that!
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**Also**, this project will overclock your pi. However, 200MHz is fine for 99.9% of picos and is SUPER unlikely to damage it unless you
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leave it running for a very long time (days or something in a closed environment).
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### Most important disclaimer
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Legal restrictions likely aren't enough for some people to not do this. Including me actually (tho minisenders with low power like
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this are actually allowed where i live, but they have to be certified). However, there is one **EXTREMELY** importatnt thing to add:
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This uses square waves, so there are GOING to be harmonics. One of those is near 99.6MHz, in the FM band. You will hear the sender there
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as well in some cases as FM receivers *can usually* receive AM, its just gonna be really noisy. Your neighbors, sadly, might also hear it
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there, as 99MHz FM is is in the FM radio bands. MAKE SURE YOUR ANTENNA IS SMALL!!!
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There is also a MUCH STRONGER harmonic at ~155.726MHz FM, in the 2m amateur radio band. PLEASE, PLEASE, PLEASE don't use a large
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enough antenna for it to be able to send that very far. And this signal will actually reach FURTHER than the AM one because it
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is a shorter wavelength and therefore sends better using a short antenna. ***__PLEASE add the low pass filter I described.__***
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## How it works
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The Pi generates a 1557kHz PWM signal, which will be the carrier wave. The frequency is not changed, only the pulse width.
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(Sadly, I was not able to make an FM version because FM would require more precise frequency shifts, which the pico just can't do.)
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Then, an audio input is taken in on the right side of the breadboard (blue and green wire), which is clamped and slightly loaded
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so that we don't get nasty interference.
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Then, this clean signal is given to the pico on pin 26. The pico does pulse width changes according to the signal, and outputs the
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AM signal on pin 15.
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## Circuit Diagram and explanation
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The part which prepares the audio for the pico is not strictly needed, but if you use line inputs, you reeeally should have it.
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It has the following jobs:
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- D1 and D2 clamp the signal to acceptable voltages
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- **An extra resistor of about 50-400 ohms may be added from the AI+ point to the AI- point to load the signal.** This is optional, but
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*can* clean up a little bit of noise if you have long cables.
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**ERRATA :warning:**<br>
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If you aren't an asshole, ***PLEASE*** add the low pass filter as mentioned in the Disclaimer:
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- Add the 100ohm resistor (R3 in new schematic) between pico and antenna
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- Wire a 2nF (2000pF) capacitor (C1 in new schematic) between the new start of the antenna and ground
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## How to use it
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- Flash the ino file using the Arduino IDE
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- Cut open an audio cable and take one of the channels and the ground out of the cable's mantle
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- Connect those to jumper wires
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- Take a breadboard and put the pico on there, wire the circuit
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- Add the audio inputs
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- Add a **sufficiently long but not too long** antenna. Mine is about 2m-ish long.
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- Connect the Pi to your PC using the USB port for power
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- Connect the audio input to your PC
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- Tune in to 1557kHz AM
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