Projector Fun with Raspberry Pi, DEFCON Badge & Debian aarch64

Mario Duhanic, 2025-08-31

Sometimes the best projects are the ones that are dead simple and just work.
Here’s a little build I put together:

Brain: Raspberry Pi (running Debian aarch64, headless, no X, no window manager)
HMI (Human-Machine Interface): the DEFCON 29 badge
Display: Acer beamer
OS: Debian aarch64 Linux
Goal: loop a video + audio, shift the picture around so the beamer doesn’t burn in, and power down on a single keystroke.

The Setup

The Raspberry Pi boots straight into two things:

Audio loop

mpv --loop /home/video/video/audio.wav &

Video loop (dual-pan, left/right)

pan=0.7
left="--video-pan-x=-$pan"
right="--video-pan-x=$pan"
loop="--loop-playlist"
dir="/home/video/video"
file="$dir/movie.mov"

mpv --ao=null $loop --\{ $right $file --\} --\{ $left $file --\} &

No desktop, no fluff. Just mpv doing its thing.

Programming the DEFCON 29 Badge

Before the badge can act as a shutdown button, it needs to be programmed so one of its keys actually sends the letter “S” over USB and glows to make it obvious.

⚠️ Important: Make sure you connect the badge with a proper USB-C data cable, not a charge-only cable. Otherwise, the serial connection and keystrokes will never show up.

Connect the badge via USB serial.
Use a terminal program like screen, picocom, or minicom. Example:
```
screen /dev/ttyUSB0 115200
```
Flash or configure the badge firmware so that one button glows red and, when pressed, transmits the S keystroke.
Test it on a regular computer: pressing the glowing red button should type S.

Now the badge is not just decorative—it’s a shutdown button with built-in backlight feedback.

Control via Badge

With the badge programmed, pressing the glowing red button triggers a clean shutdown thanks to this script:

#!/usr/bin/env bash
# shutdown_on_S.sh
DEV="${1:-/dev/input/event0}"
LOG="/home/video/shutdown_on_S.log"

# wait for device
while [ ! -e "$DEV" ]; do sleep 1; done

(evtest --grab "$DEV" 2>&1 || evtest "$DEV" 2>&1) | awk '
/EV_KEY/ && (/KEY_S/ || /\(S\)/) && /value 1/ {
  system("sudo systemctl poweroff || sudo shutdown -h now");
  exit 0
}'

And in crontab:

@reboot /home/video/shutdown_on_S.sh /dev/input/by-id/usb-MK_Factor_DEF_CON_29_Badge_12345-if02-event-kbd
@reboot /home/video/video.sh

Why This Rocks

Lightweight: no X, no window manager. Just raw Linux + mpv.
Beamer-safe: video pans left and right, avoiding static burn-in.
Fun HMI: glowing DEFCON badge doubles as the shutdown switch.
Visual cue: the shutdown button glows red, so you can’t miss it.
Autonomous: power on → boot → video + audio loop, hands-free.

Next Steps

Add randomized pan positions so the projector image moves around more.
Maybe use the badge LEDs as status indicators (playing / stopping).
Expand the video loop into a mini playlist for variety.
Mute the audio before power-off.

Final Thoughts

This is one of those projects that makes you smile:

plug in power,
projector starts looping audio + video,
press the glowing red badge button → sends S → clean shutdown.

Simple. Hackerish. Effective.

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