Air-Gapped Audio Recorder on a Raspberry Pi
I needed long-run audio recordings that were defensible as well as audible -- the kind where "when was this recorded, and has anyone touched it since?" has an actual answer. Phones and handheld recorders are fine for capturing sound, but they're terrible at proving anything about it. So I built a Raspberry Pi that does one job: power it on, and it records. No app, no network, no buttons.
The whole thing is air-gapped on purpose. Wi-Fi and Bluetooth are disabled at the device tree and kernel level and the firmware blobs are deleted outright, and there's a boot check that flat-out refuses to start recording if it detects a live radio. Time is the other hard problem -- a Pi has no clock of its own -- so the build stacks a DS3231 RTC module with optional wired NTP or GPS, plus a time-seed floor written at flash time, and the LED tells you whether the clock is actually trusted before you walk away from it.
Audio lands in 10-minute segments so a yanked power cable costs you a second or two, not the session. On ingest every file gets SHA-256 hashed into an append-only, hash-chained manifest, with optional S3 mirroring where you can verify bucket contents against the seals using server-side checksums without paying to pull the audio back down. The recorder process itself runs unprivileged with network denial and syscall filtering, so it can't do much beyond writing wav files even if something goes sideways.
It's microphone-agnostic -- I ran a miniDSP UMIK-1 because I already had one on the shelf, but that's a measurement mic and honestly the wrong tool here. Anything USB and reasonably sensitive will do better, so long as the quality holds up.
Build time is about 15 minutes from a Mac: verify and flash the image, add an exFAT data partition, inject the provisioning payload, boot it once and let it self-configure. Collection is just as dumb by design -- pull power, pull the USB stick, run umik download to seal and clear it. Two units have been running in the field so far, 362 sealed files, no data loss. It's MIT licensed if you want to build one.