In anticipation of Trump’s terrible tariffs, I made a few strategic purchases last month to upgrade my very old electronics. My biggest move was buying an M4 Mac Mini to replace my late-2013 27 inch iMac. The Mac Mini wasn’t as expensive as I feared, but a new monitor as good as my iMac would have killed me. So I did some research and learned about Luna Display, a little red plug-in device with associated software that lets you use an iMac as a monitor for under $100.

The short version:

I’ve got Luna Display working and it’s good enough for me to put off buying a new monitor for a while!

Small but key hack: turning wifi off on my iMac makes sure the system only connects via Ethernet, which is key.

Very happy both about saving money and keeping good electronics out of landfills.

The longer version:

To make Luna Display work with my setup, I had to stick the little red plugin into my Mac Mini and install Luna Display software on both the main computer (my new Mac Mini) and the display computer (my old iMac). I also connected the Mac Mini to the iMac via Ethernet (absolutely critical in my case!). (One quirk — I’m pretty sure you already need an external monitor in order to set everything up, because you can’t manage your Mac Mini without a monitor and your iMac won’t work as a monitor until you’ve set everything up, natch. Fortunately, I have a small, cheap external monitor that worked fine for that purpose.)

After running through the settings and enabling some permissions, the whole thing just worked — the iMac became a monitor for the Mac Mini! But there was terrible latency — meaning there was a lag between moving the mouse and seeing it actually move on screen — because the system automatically connected via internet instead of Ethernet. When I went through the settings and told the system to only connect via Ethernet, the connection improved fantastically. There’s still a tiny lag that I can see while typing, for example. But it’s miniscule and doesn’t bother me.

The main problem is that when the computers go to sleep, the connection is usually lost. So waking the computers up from sleep usually requires restarting the Luna Display program on both machines. And then the system would usually default to wifi again — even though I had it set to only connect via Ethernet. This was really frustrating — it added extra minutes every time I sat down at my desk, which isn’t fun or good.

Eventually, I figured out that turning off wifi on my iMac would disable the wifi connection and now the system only connects via Ethernet! I still typically have to restart Luna Display on the computers when waking the computers up from sleep, but they connect again immediately via Ethernet, so it’s vastly less annoying.

UPDATE!

A few more details/quirks worth sharing:

1. The iMac’s webcam doesn’t work when you’re using it as a display with Luna Display. So I guess I have to figure out how to connect my iPhone to my computer to use as a webcam.
2. The keyboard command for increasing the brightness of the screen doesn’t work from the Mac Mini on the iMac when it’s connected to Luna Display. You have to take the iMac out of Luna Display and use a keyboard connected to the iMac to change its brightness (and presumably to calibrate it).
3. Connecting the iMac’s screen to a Mac Mini via Luna Display does not connect its speakers, sadly. I’ve connected a Tivoli bluetooth radio to my Mac Mini for sound, which is fine.
4. I can’t get video from AppleTV to play on my iMac screen when it’s connected via Luna Display. The sound from videos will play, but not the video. This is similar to the way videos would not play on my non-HD monitor. I assume something about the way the iMac screen is connected to the Mac Mini is triggering the same kind of problem.