This tutorial teaches you how to build a self resetting player sensor which will send a red stone signal when the player is within 16 blocks of its mob spawner.
My intended purpose for this device is for triggering traps based on the player's position, but there are indeed many other possible uses for it. An especially useful feature of it is a built in Peaceful Mode detector, which allows you to freely edit your survival maps in creative mode without fear of setting off your own traps during construction.
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