How to Use an Old Laptop as a Second Monitor

Tech site Engadget has published a guide on using an unused laptop as a second monitor. The approach lets people extend their desktop or mirror their screen without buying new hardware. Mirroring is useful for presentations, while extending gives more room for open tabs and easier multitasking.

Before starting, users should check device compatibility and choose between a wired and a wireless connection. Simply plugging an HDMI cable between two computers is not enough.

Windows computers

If both machines run Windows 10 or 11, the built-in wireless technology Miracast can connect them over Wi-Fi. On the spare laptop, the user adds the optional feature "Wireless Display" under the "Projecting to this PC" settings, sets it to "Available everywhere" and opens the app. On the main computer, pressing the Windows key + K, selecting the spare laptop and entering the PIN completes the connection. Windows key + P switches between mirroring and extending. Both machines need Wi-Fi, and some lag may appear, so this is not the best option for gaming.

Mac and other systems

If both devices are Macs, AirPlay can be used, but only if the MacBook is from 2018 or later. Both must be signed into the same Apple ID. AirPlay also runs over Wi-Fi, so a slight delay is possible.

For different operating systems, or any setup involving Linux, a program called Deskreen is needed. It is a free, open-source tool that works over Wi-Fi. Both computers must be on the same network, and the spare laptop's browser opens the QR code or URL that Deskreen provides. The display is mirrored by default; to extend it, a cheap virtual display adapter, or "dummy plug," must be inserted into the main computer's HDMI port.

HDMI connection

Most laptop HDMI ports are video-out only. A user therefore needs an HDMI-to-USB video capture card: the main device's HDMI cable goes into the card, and the USB end goes into the spare laptop. The card acts as a webcam, and the feed can be viewed in the camera app or in OBS Studio. A PlayStation or Nintendo Switch console can be connected in the same way. Lag is usually lower than with Wi-Fi-based connections.