I used a photocell to activate the motor on a car whenever an object passed over it.  Putting your finger or any other object over the photocell results in the car moving forward. Input: light blocking, output: car movement.

alternative link:  https://drive.google.com/a/andrew.cmu.edu/file/d/0B1I19a0bJ9L7eENpLU1odWtYdEk/view?usp=sharing