The power widget that comes with Android can easily switch between brightness levels with one screen tap. It's one of the features of the power bar. The Power bar also allows one tap control of 4 other commonly used features that you might want to toggle on/off. The others are Wifi on/off, Bluetooth on/off, GPS on/off, Auto-Sync on/off.
To access the power widget press and hold the screen and when the menu pops up, choose widgets. Scroll thru and find Power Control and tap it.
You can also use an app like widgetsoid to make your own custom power bar.
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