Turn an iPad or iPhone stuck on iOS 12 into a second display for your Mac (macOS 26), over Lightning or Wi-Fi — free, self-hosted

Built and running for real on an iPad Air (Model A1475, iOS 12.5.8), both over a Lightning cable and over Wi-Fi. H.264 video + touch + two-finger scroll + a real mouse cursor all work reliably.

No cable required: the Mac app auto-discovers the iPad over Bonjour on the same Wi-Fi network and connects directly. It prefers USB when a cable is plugged in (lower, steadier latency) and falls back to Wi-Fi automatically otherwise — you don't have to choose a mode, it just works either way.

Keywords / What this is

Use an old iPad on iOS 12 / 12.5.x as a free second display for Mac (Apple Silicon or Intel, modern macOS). Open-source alternative to Sidecar, Duet Display, and Luna Display for devices too old to run OpenDisplay's own client (which needs iPadOS 17+). Works over USB/Lightning and Wi-Fi.

The idea

Skip the paid apps (Duet Display, Luna Display...). Instead:

1. Mac side: build and run, unmodified, the open-source OpenDisplay app (GPL-3.0, free). It already handles the hard part: creating a virtual display on macOS (CGVirtualDisplay), capturing it (ScreenCaptureKit), hardware-encoding H.264 (VideoToolbox), and transporting it either over Wi-Fi (discovered via Bonjour) or by talking directly to macOS's usbmuxd over Lightning/USB-C — no extra tool like iproxy needed.
2. iPad side: OpenDisplay requires iPadOS 17+, so it can't be installed on an iOS 12 iPad. That's why the iOS/ folder in this repo is a purpose-built iOS 12 client: a clean-room reimplementation of OpenDisplay's network protocol (read from their public source), so it can talk to the unmodified Mac app above without touching it.

Since the Mac side only needs an unmodified build, the only thing you're actually "writing" yourself is the iPad app — much lighter than reimplementing the whole pipeline from scratch.

Repo layout

ipad12-second-screen/
  README.md
  LICENSE                 <- MIT, covers iOS/ (code written for this project)
  iOS/                    <- iOS 12 client, original code, MIT
    LegacyPadDisplay.xcodeproj  <- committed and ready to open, no build step
    project.yml           <- xcodegen config (source of truth if you edit the project;
                              re-run `xcodegen generate` after changing it)
    App/
      AppDelegate.swift
      ReceiverViewController.swift  <- fullscreen video view + touch/scroll capture + cursor sprite
      VideoReceiver.swift    <- core: TCP listener, frame deframing, H.264 decode, control message send/recv
      LaunchScreen.storyboard
      Assets.xcassets/
  Mac/                    <- git submodule, upstream OpenDisplay, UNMODIFIED, GPL-3.0

Mac/ is a git submodule pointing straight at the upstream peetzweg/opendisplay repo — nothing copied or edited, so it's clear this is someone else's code (their GPL-3.0 license stays intact), and it's easy to git submodule update --remote when they cut a new release.

Minimum requirement: iOS 12.0 (iOS/project.yml → deploymentTarget), tested for real on iOS 12.5.8.

Does this work on iPhone too?

Yes, no code changes needed. TARGETED_DEVICE_FAMILY is already set to "1,2" (iPhone + iPad), and VideoReceiver.swift already reports "device": "iPhone" vs "iPad" based on UIDevice.current.userInterfaceIdiom in its hello message — this mirrors upstream OpenDisplay itself, which also targets both (Mac/project.yml's local-network usage description literally says "connects to your iPad or iPhone"). The Mac side sizes the virtual display from whatever pixel dimensions the device reports in hello, so it isn't hardcoded to iPad proportions either.

Build/install is the same flow as the iPad steps below — just pick the iPhone as the destination device in Xcode. If it's also stuck on an old iOS version, the same DeviceSupport caveat further down applies. The one real downside: an iPhone screen is a lot smaller, so it's a much less useful second display in practice than an iPad.

Step 0 — Clone with submodules

git clone --recurse-submodules https://github.com/cuongpham1/ipad-iphone-second-monitor-ios12-free.git
cd ipad-iphone-second-monitor-ios12-free

If you already cloned without --recurse-submodules:

git submodule update --init --recursive

Step 1 — Get the Mac app (OpenDisplay, unmodified)

Two options:

• Prebuilt (recommended, no Xcode needed): grab OpenDisplay.dmg from the latest release — signed and notarized by the author. Open it and drag OpenDisplay.app into /Applications first — don't run it straight out of the mounted .dmg. Running it from the disk image works for a first test, but the volume unmounts on reboot and takes the app with it, and macOS ties the Screen Recording/Accessibility grants to that specific app path — moving the app afterward means re-granting both permissions again.
• Build from source: open Mac/ in Xcode and run it — see OpenDisplay's own build instructions for details.

Either way, on first launch macOS will ask for Screen Recording and Accessibility permissions (System Settings → Privacy & Security) — grant both, fully quit the app (Cmd+Q) and relaunch it (flipping the toggle isn't enough; the app needs a restart to actually pick up the new permissions), then run it again.

Step 2 — Build the iPad app (iOS/ in this repo)

open iOS/LegacyPadDisplay.xcodeproj

In Xcode:

1. Select target LegacyPadDisplay → Signing & Capabilities tab → set Team to your personal Apple ID.
2. Plug the iPad into the Mac with a Lightning cable, select it as the destination device in Xcode's toolbar.
3. Hit Run. On first install, go to Settings → General → VPN & Device Management on the iPad to "Trust" your developer certificate.

The app should show a fullscreen black screen with "Listening on :9000" at the bottom — meaning it's waiting for the Mac to connect.

Xcode says "Failed to prepare the device for development"

Recent Xcode versions don't ship iOS DeviceSupport for old iOS 12 builds anymore. You need to add the matching support folder to ~/Library/Developer/Xcode/iOS DeviceSupport/. This repo was actually brought up using the 12.5 support bundle from apptim/iPhoneOSDeviceSupport:

curl -L -o /tmp/12.5.zip https://raw.githubusercontent.com/apptim/iPhoneOSDeviceSupport/master/12.5.zip
unzip -o /tmp/12.5.zip -d /tmp/ds125
cp -R "/tmp/ds125/12.5" ~/Library/Developer/Xcode/iOS\ DeviceSupport/

It doesn't need to match the exact build number (e.g. 12.5.8/16H88 worked fine with a plain "12.5" folder) — a close minor version is enough. Other community repos like filsv/iOSDeviceSupport work the same way if you need a different version. Copy the folder into place, fully quit Xcode, reconnect the iPad, and reopen.

Free Apple ID — the app expires after 7 days

This is an Apple limitation, not something this project can fix. After 7 days, plug the cable back in, open Xcode, and hit Run again to reinstall. To avoid repeating this, you'd need a paid Apple Developer Program membership ($99/year) — signs for a full year.

Step 3 — Connect

1. Make sure both apps are running (Mac app has Screen Recording + Accessibility permissions; iPad app is open).
2. Two ways to connect, no mode switch needed on the Mac side — it picks automatically:
   • Cable: plug the iPad into the Mac over Lightning/USB-C. The Mac app recognizes it through usbmuxd just like a normal OpenDisplay device (same id/port 9000 as the original client).
   • Wi-Fi: put both devices on the same Wi-Fi network, no cable. The Mac app browses for _opensidecar._tcp over Bonjour and connects directly to the iPad's port 9000. If the iPad asks for Local Network permission the first time, allow it — Wi-Fi discovery needs it.
   • If both are available at once, the Mac app prefers USB (lower, steadier latency) and falls back to Wi-Fi automatically if the cable is pulled.
3. Once connected, the iPad should switch from a black screen to showing the macOS desktop (with a real mouse cursor), and a new display should appear under System Settings → Displays on the Mac — drag a window over to it and you're set.

If it won't connect — check these first

• No new virtual display shows up under System Settings → Displays: almost always missing Screen Recording/Accessibility permission for the Mac app, or granted but the app wasn't fully quit and relaunched afterward (see Step 1).
• Check the Mac app's console log (Xcode → View → Debug Area → Console while running the OpenSidecarMac scheme) to see whether it's seeing the device over usbmuxd at all.
• If the picture stays black/frozen after switching back from another app, give it a second or two — ensureListening() reconnects automatically on foreground return, which briefly flashes black while the Mac redials.
• If the Mac app sends an updateRequired/incompatible-version message — a newer OpenDisplay Mac build may have changed the protocol. VideoReceiver.swift intentionally omits the pv (protocol version) field, so it's always treated as "protocol 1", which every current OpenDisplay Mac release documents as backward-compatible.

What works / what's missing

Works: video, touch to click/drag, two-finger scroll, a real mouse cursor (position + shape, synced over its own control message, smooth), reconnect on foreground return (the listener/connection can die silently while backgrounded since no background networking mode is declared — ensureListening() forces a clean reconnect every time the app comes back, at the cost of a brief flicker).

Missing: an external keyboard, automatic rotation to match the virtual display, latency measurement.

License

• iOS/ (iOS 12 client): MIT, see LICENSE.
• Mac/: submodule pointing at peetzweg/opendisplay, GPL-3.0, copyright held by its original authors — unmodified, not vendored into this repo.

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Keywords: iPad iOS 12 second monitor Mac, old iPad external display, Sidecar alternative iOS 12, Duet Display free alternative, Luna Display free alternative, OpenDisplay iOS 12 client, Lightning USB second screen, legacy iPad second monitor, LegacyPadDisplay.