The camera knows when someone is watching

A Raspberry Pi clipped to the 3D printer streams the nozzle with one line: ffmpeg to MPEG-TS, piped into curl -T - against a pipe URL. Anyone with the link opens it with ?video and gets a player.

The part nobody expects is what the Pi sees. The response body of its own upload is a status stream: [INFO] Waiting for 1 receiver(s) to connect..., then [INFO] A receiver connected. A shell script reads those lines and drives a GPIO pin. A small red LED on the printer lights when someone is watching and goes dark on A receiver disconnected.

No viewer-count API, no webhook, no analytics tab. The upload told the uploader. Same trick for a workshop stream, a lab bench, a kid's telescope.

The camera knows it's being watched because the upload said so.