Crash‑proof dashcam
The whole 2592×1944 sensor, scaled (never cropped) to 1440×1080 H.264 at 15 fps. Ten‑minute clips that survive a yanked power cable with at most two seconds lost.
- MPEG‑TS
- fsync 1 s
- loop recording
- protect button
Screen
MAPCAM
Raspberry Pi CM4 · composite video · 5″ CRT
fallout.nav
A 2001 black‑and‑white reversing monitor, a Raspberry Pi CM4 and a wasteland navigator.
A dashcam, a moving satellite map and a Pip‑Boy‑style HUD, fed into a WAECO Mobitronic tube straight from the CM4's own composite DAC. The screen in the photo is the real display code driving the Las Vegas Strip in your browser.
2592×1944full‑sensor dashcam
10 HzGPS + compass fusion
480i/60composite from pin 111
≤ 2 slost when power is cut
Live, not a video. The same renderer, HUD and speed logic that run on the Pi, in demo mode: simulated GPS on a loop around the Strip, Esri satellite tiles, a still frame for the camera view.
Take the wheel
Every switch below talks to the live screen above. demo link: waiting
Speed
0km/h
Heading
---°
Zoom
--
Map
HDGN↑
Position
--.----- --.-----
Las Vegas Strip loop
Drive
Map orientation
Compass style
Screen mode
Map layer
01 What it does
One small box, six jobs
A Raspberry Pi Compute Module 4 behind the dashboard records the road, knows where it is and which way it points, and draws all of it on a tube older than most smartphones.
Moving satellite map
A hand‑written Canvas renderer for Esri World Imagery and streets. In heading‑up mode the car icon turns first and the map catches up on a spring. Zoom follows speed; when you stop, the screen switches to the camera.
- Esri tiles
- 1 GB cache
- zoom 15–19
- heading/north‑up
Wasteland HUD
Three compass designs (Riveted dial, Heading tape, Brass compass), tall Oswald digits, stencil warnings and a spiked wasteland muscle car, all sized for a 92 × 69 mm black‑and‑white picture.
- v1 · v2 · v3
- SVG muscle car
- OFL fonts
Sensors that tune themselves
A Holybro M8N configured over UBX for 10 Hz fixes and static hold, plus an IST8310 compass that calibrates itself against the GPS course while you drive. GPS time keeps the clock right with no RTC.
- UBX
- 10 Hz
- auto‑calibration
- gpsd + chrony
Web control room
A password‑protected web UI on the laptop over Wi‑Fi: dashboard, live WebRTC video, recordings, map and compass settings, skyplot, and a 1:1 preview of the CRT in real millimetres, calibrated against a bank card.
- FastAPI
- WebSocket 10 Hz
- WebRTC
- 1:1 preview
Composite, straight to the tube
No HDMI converter. One wire soldered to pin 111 (VDAC_COMP) of the CM4 goes into the monitor's DIN socket. NTSC 480i/60, with margins and mirroring tuned to the RV‑50.
- pin 111
- NTSC 480i
- mirror_x
- KMS margins
02 In the van
Bolted to the dashboard
The RV‑50 sits on top of a Peugeot dashboard, where a reversing monitor belongs. The Pi, camera and GPS live behind it on a single 12 V feed.
- 92×69Millimetres of pictureThe visible area of the 5″ tube. Every font, the compass and the car were sized for it in the 1:1 preview.
- ~6 hOf looped videoAbout 17 GB of the 32 GB eMMC is set aside for clips; the oldest go first, protected ones never.
- 1 GBOf map tiles cachedTiles come through the phone hotspot. Roads you have already driven work offline.
- 0Converters in the video pathComposite leaves the CM4 on a single wire and goes straight into the monitor's channel‑2 input.
03 Hardware
Parts bin & wiring
Off‑the‑shelf modules and one very old monitor. The interesting parts are a 10‑pin GPS harness and a single wire on a 0.4 mm pitch connector.
| Compute | Raspberry Pi Compute Module 44 GB RAM, 32 GB eMMC, Wi‑Fi. Raspberry Pi OS (Debian 13 trixie, 64‑bit) |
|---|---|
| Carrier | Waveshare CM4‑NANO‑BUSB‑C power, 40‑pin GPIO, CSI camera port |
| Camera | OV5647, 5 MPCSI0 ribbon; full sensor 2592×1944 at 15.6 fps |
| GPS + compass | Holybro M8Nu‑blox M8N GNSS + IST8310 magnetometer, safety switch with LED, buzzer, on one JST‑GH 10‑pin plug |
| Display | WAECO Mobitronic RV‑50 (2001)5″ black‑and‑white CRT reversing monitor, two camera inputs on DIN plugs |
| Power | DC‑DC 12 → 5 V, 3 AOne shared ground for the Pi and the monitor |
| Software | Python · FastAPI · Picamera2 · MediaMTXcage + Chromium kiosk, gpsd + chrony, vanilla JS on Canvas |
Video: pin 111 → DIN camera input
The CM4's composite DAC is on the module connector but the carrier board doesn't route it out, so it gets a wire of its own.
GPS: Holybro 10‑pin → Pi 40‑pin header
RX/TX are labelled on the module side, so they cross over. Everything is 3.3 V logic, so it connects directly; only the buzzer needs a MOSFET.
IST8310 answers on I²C address 0x0E. The safety switch protects the current clip from loop deletion, and its LED shows REC.
04 Architecture
Four services, no single point of failure
Recording runs in its own process: if the web server falls over, the dashcam keeps rolling. systemd restarts anything that dies, and the display is just a web page in a kiosk browser.
sensor busesvideodata / HTTP
05 The hard parts
Lessons from the wasteland
Most of it went to plan. These parts didn't, and they're the reason the device works.
A recorder that survives the ignition key
Car power dies without warning. An MP4 keeps its index at the end of the file, so if the power goes mid‑clip the whole file is unreadable. fallout.nav writes MPEG‑TS instead: independent 188‑byte packets with no index, playable up to the last byte written.
The recorder calls fsync() every second, places a keyframe every second, and on boot trims the last file to a whole number of packets. Pull the plug and you lose at most two seconds, not ten minutes.
LessonPick formats that fail gracefully, then make the OS flush on your schedule.
Pin 111, not pin 109
The CM4 has a composite video DAC, but the Waveshare carrier doesn't route it to a connector. So a wire goes straight onto pin 111 (VDAC_COMP) of the module's J2 connector. Counting pins at that pitch goes wrong easily; the grounds make a better landmark. On the inner row a ground comes after every two signal pins:
101103105107 ⏚109111113 ⏚115
The right pin is the left one of the pair between the first two grounds. One pin to the left is 109, PCIe_nRST: a flat 0 V and an hour of head‑scratching.
LessonNavigate by landmarks, not by counting.
PAL jumped, NTSC held
The plan said PAL 576i/50. The 2001 tube couldn't hold vertical sync: the frame jumped with 288p, mono, firmware or KMS output, dimming and DC‑level shifts alike. At NTSC 480i/60 it locks solid. At 60 Hz the monitor only shows the central ~80 % × 90 %, so kernel margins squeeze the picture into the visible area.
LessonOld hardware has opinions. Try the boring alternative early.
The monitor mirrors everything
The RV‑50 is a reversing monitor: it flips the picture so the rear view reads like a mirror. The first map came out backwards. Now the display renders mirrored (mirror_x), and even the 12 px centring shift goes the "wrong" way: right on the screen is left in the signal.
LessonRead the device's purpose, not just its spec sheet.
Two inputs, one ghost
The monitor isolates its camera inputs poorly. The van's own reversing camera on the neighbouring channel bleeds into ours, and ours into it. You can't fix that with a soldering iron inside a 25‑year‑old monitor, so the fix is in software: a control that blanks fallout.nav's output, so the reversing camera gets a clean picture when it matters.
Ground matters too: Pi, DC‑DC and monitor share one ground, otherwise the DC‑DC converter's noise shows up as rolling bars.
LessonWhen the hardware leaks, give the user an off switch.
A compass that calibrates itself
Inside a steel van, a magnetometer mostly measures the van. Instead of a figure‑of‑eight dance in a car park, fnav‑core collects samples while you drive straight above 5 m/s, compares them with the GPS course, and fits hard‑iron offsets and a deviation curve. Drive in different directions and the coverage ring fills up. Above 3 m/s the heading comes from GPS anyway; the compass covers slow speeds and stops.
LessonUse the sensor you trust to teach the one you don't.
06 Screenshots
From the tube and the laptop
The HUD designs, real frames from the Pi's composite output, and the web UI running against the mock backend.
07 Built with AI agents
One lead, seven agents, about a day
fallout.nav was written by a team of Claude agents working in parallel. A lead agent wrote the plan and the contracts, gave each agent its own part of the repository, and then did the integration and all the work on the real device.
7agents in parallel, each in its own file zone
~1 dayfrom plan to a picture on the tube
~250automated tests (195 Python + 53 JS)
~20klines of Python, JS and shell
How it stayed sane
- Contracts first. Telemetry, REST API, settings schema and display components were fixed in writing before anyone started coding.
- One zone per agent. No two agents ever edit the same file; only the lead changes contracts.
- Mock everything. A simulated GPS track, compass and camera let all of it be built and tested on a PC while the Pi was still being flashed.
- One device, one driver. Deployment and on‑device tests are serial: the lead, or one agent at a time.