Second Screen Hardware and Wiring

Second screen: hardware and wiring

Part of Second Screen (Raspberry Pi) ‐ experimental. For what has actually been tried, see Tested Hardware.

Shopping list

Part	Notes
Raspberry Pi 3 Model B or B+	The 3B's Wi-Fi is 2.4 GHz only; the 3B+ also does 5 GHz, which leaves more room for video. Its VideoCore decodes H.264 in hardware. Newer Pis should work but haven't been tried.
microSD card, A1 / U3, 32–64 GB, from a known brand	Avoid no-name "1 TB" cards: they are fakes and fail. A USB SSD in an enclosure also works (a 3B needs program_usb_boot_mode=1 set once to boot from USB).
12 V → 5 V, 3 A synchronous buck converter (8–36 V input), e.g. Pololu D24V30F5 / D36V28F5	Set to 5.1–5.2 V. A Pi 3 draws up to 2.5 A. Bare LM2596 boards sag under load: the Pi then reports undervoltage, crashes and can corrupt its card.
Short, thick micro-USB cable	Thin cables drop voltage; vcgencmd get_throttled should read 0x0.
Heatsink, and some air around the Pi	A Pi 3 shut in a warm dash slows itself down. The head unit's second-screen settings say when the Pi is under-powered or hot.
Monitor: HDMI, or RCA (composite)	Any 7" 1024×600 HDMI panel, a headrest monitor, a small TV.
Fuses and fuse holders	One per branch: 3 A for the Pi's converter, 2 A for the monitor.
Optional: 12 V delay-off relay module + PC817 optocoupler module	Clean shutdown when the ignition goes off (see below).
Optional: ignition-sense power HAT (e.g. Mausberry car switch)	Does the relay + sense job in one board.
Optional: USB Ethernet adapter + a short Ethernet cable	A cable from the head unit to the Pi instead of Wi-Fi, see below.

Power: from the head unit's wiring, not its USB

A head unit's USB port gives 0.5–1 A: not enough for a Pi 3, and the monitor needs 12 V anyway. Tap the head unit's ISO / Quadlock harness on the car side and give each branch its own inline fuse:

Harness wire	Goes to	Fuse
Yellow, constant 12 V	The 12 V → 5 V 3 A buck converter, then the Pi's micro-USB	3 A
Red, ACC 12 V	The monitor's 12 V input (7" and headrest monitors draw 0.5–1 A)	2 A
Black, ground	Shared ground for the converter and the monitor	–

 Yellow 12 V ──[3 A]──► delay-off relay ──► buck 5.1 V ──► Pi (micro-USB)
 Red ACC ──────[2 A]──► monitor 12 V
      └──────────────► relay trigger
      └──► PC817 opto ──► GPIO3 (pin 5) / GND (pin 6)      (--acc-sense)
 Black ground ────────► converter, monitor, relay

An under-powered Pi slows down and stutters whatever the Wi-Fi does. The head unit now says when the Pi is under-powered, hot or slowed down by its firmware, in Settings → Connections → Second screen: use a 5 V, 2.5 A supply over a short, thick cable, and give the Pi air or a heatsink. A test in the car on 2026-10-07 found a hot, under-powered Pi to be the real cause of a stuttering picture.

Clean shutdown

Cutting a Pi's power while it writes can corrupt its SD card. Pick one:

• Relay and signal. Power the converter through a 12 V delay-off relay, fed from yellow and triggered by red (ACC); it keeps power on for about 60 s after the ignition goes off. Also feed ACC through a PC817 optocoupler to GPIO3 (pin 5) and ground (pin 6), wired so the opto pulls GPIO3 low while ACC is on. Install by hand with --acc-sense, since the ready-made card doesn't sense ACC: when ACC goes off, GPIO3 rises and the Pi shuts down before the relay cuts power.
• Ready-made board. An ignition-sense power HAT does the same in one module.
• Signal alternative. The blue/white REMOTE (amplifier turn-on) wire follows the head unit's own power, sleep included. It can replace ACC as the sense signal, but it carries only 100–300 mA: never power anything from it.
• Read-only card. Once paired, the Pi turns its card read-only by itself, so a sudden power cut can't damage it.

Cable from the head unit

New on 2026-10-09, not car-tested yet. Two Wi-Fi hops through a phone are the usual cause of a late or stuttering picture, and at home the head unit and the Pi easily end up on different networks. A cable avoids all of it:

• a USB Ethernet adapter on one of the head unit's USB ports (the common AX88179 or RTL8153 ones are the ones to try), and
• an Ethernet cable from it to the Pi's own Ethernet port.

Nothing to set up: the Pi hands the head unit an address on that cable, with no gateway, so the head unit keeps using the phone's Wi-Fi for the internet, and finds the Pi over the cable within a second. Wi-Fi stays as the fallback. Do not plug the Pi's Ethernet port into a home network: it hands out addresses there too.

Video output

• HDMI: plug in the monitor. The installer keeps HDMI on even if the monitor powers up after the Pi.
• RCA (composite): use the Pi's 3.5 mm jack with a TRRS-to-RCA cable and install by hand with --composite; the ready-made card is for HDMI. The Pi puts video on the sleeve and ground on the second ring; many camcorder cables swap these, so if there's no picture try the red or white plug. overscan=5 is then set in display.conf; adjust it to what the monitor crops off. Composite reports no size, so set width/height there if needed (720×576 PAL, 720×480 NTSC).

Example build: Citroën C4 Picasso (2011)

The project car has a factory 7" screen at the top of the dash (the RT5 navigation screen). With an aftermarket head unit fitted, that screen is dead. The plan is to swap its panel for a 7" HDMI panel and keep the original bezel.

• Opening measured in the car: window about 155 × 87.5 mm, cavity about 185 × 115–120 mm, depth 45–50 mm.
• Fits any standard 7" 1024×600 panel (viewing area about 154.2 × 85.9 mm, driver board about 164.9 × 100 mm).
• Panel ordered (2026-10-03, not yet received): a 7" 1024×600 high-brightness panel (070JI42-HX-0, sold as 1000 nits) with a driver board offering full-size HDMI, micro-USB 5 V, audio jack and a keypad (OSD buttons) connector. Brightness matters: a dash-top screen faces the sun.
• Fallback if it's too dim: an industrial sunlight-readable 7" HDMI module (e.g. Midas MVT-H070001).
• Rejected: reusing the original RT5 screen (Ampire, CVBS → LVDS through the RT5): without the RT5 it is likely to stay dark, and it would need a composite chain.

Dimming (built 2026-10-05): the Pi "presses" the panel board's brightness keys through its keypad connector, two GPIOs through a divider (the keys sit at about 4.6 V: 220 Ω from the key, 14.7 kΩ to ground). The head unit sets a day and a night level. See Setup and Pairing.

Later ideas: a BH1750 light sensor for automatic levels, PWM on the backlight boost converter.