A keyboard and mouse can simply be plugged into the full-sized USB ports. You could use wireless keyboards and mice as well, as long as they use standard USB HID (human interface device) drivers. Multimedia keys and other custom keys on fancier keyboards won't work.
Once the physical connections are made, ensure the display is configured to use the correct input and simply slide the phone onto the dock, inserting the dock's male connecter into the phone's MHL/micro-USB port. When the smartphone is fully connected to the dock, its screen will go dark and output will be sent to the secondary display. Boom! Your old smartphone is now essentially an Android PC. It really is that simple!
On the Samsung smartphones, everything we tried automatically rotated into landscape mode, which made it ideal for multi-media playback or working on office documents.
Navigating through the various menus on Android can be somewhat clunky when using a keyboard and mouse, but it's fairly intuitive once you start poking around. Right-clicks act as a back button, while left-clicking behaves like a tap of the touchscreen. Many in-application icons and menus--even the mouse cursor itself--appear oversized on a big screen, but they function as expected.
We wouldn't trade an actual PC for a smartphone and USB OTG / MHL dock, but for a relatively small investment (especially if you've already got the monitor and input devices on hand) you can easily get some work done on one of these setups. Wondering which apps to stock on your new phone-slash-PC? Check out PCWorld's guide to highly productive Android apps and Greenbot's list of killer free apps for students to find some starting suggestions.
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