Minecraft – How to testfor blocks not people

Place a command block with the /testfor @p[whatever] command.
Then, a redstone comparator facing outwards from the command block.
From there, place your redstone or any redstone contraption that needs the input.

Note: If the comparator's signal isn't strong enough (1 block worth of power), place a Redstone Repeater after the comparator and any output should now have 15 strength.

Once it turns on the lamp trigger this command:

/blockdata (pos of the command_block with the clear command) {SuccesCount:0}

Then do a delay with redstone repeaters that triggers a command block with this command:

/setblock x y z minecraft:unpowered_comparator

Where the x, y, and z are the coordinates of your comparator. This will reset the comparator to an off state.