Jul 192013

Think about the following rather unusual scenario: You own a current generation MacBook Air and a new iMac that you don’t use. This blog post will show you how you can use the iMac as a pure monitor without any manual intervention.


The current generation iMacs (21″ and 27″) can be used as a Thunderbolt display using Target Display Mode. To enabled this mode you have to boot both Macs and connect them using a Thunderbolt cable. After that you have to press [Command]+[F2] on the keyboard of the “display” iMac. Although this feature is great for rare usage – it’s not very comfortable if you want to use the iMac as a permanent secondary display.


thunderbolt-logoTo solve this problem I created iMacMonitor. It’s a small application running on the “display” iMac that monitors the Thunderbolt bus and automatically switches to Target Display Mode if necessary. Thereby one only has to connect the main computer to the Thunderbolt cable and a few seconds later the iMac can be used as a secondary monitor. This works because Mac’s wake from standby if a Thunderbolt device is connected. As soon as the main computer is disconnected the display will sleep again. Be aware that you should not shutdown the “display” computer as it won’t switch to Target Display Mode anymore.


  1. First you have to download iMacMonitor.app on the “display” iMac and set it as Login Item.
  2. Then make sure that a user is automatically logged in (see this link).
  3. Restart
  4. Voilà

Now you can connect a Thunderbolt enabled Mac and the “display” iMac will switch to Target Display Mode automatically.


As I have written this application for fun you can also download the Xcode project. It’s released under the MIT licence.

Update: Here’s the Xcode 5 Project for 10.9

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