One of my projects uses IR break beam break sensors to trigger an action on a microcontroller.  The project called for an object to pass through the beam in one direction then again in the other direction, but I only wanted to act on the first pass.

This could be done in software by only acting on the odd numbered passes but I choose to use a CD4013 flip-flop IC (Datasheet). This chip has two output pins, Q and Q-bar, that have opposite values: one 0, the other 1. There is also a data pin and clock pin When the clock pin is pulsed the value at the data pin is delivered to output Q and Q-bar is set to the opposite.  If Q-bar is connected to the data pin then each time the clock pin is pulsed the inputs will swap values.

In the video below the legs of a red/blue bi-color LED are connected to the two output pins of a flip-flop chip.  The output from an IR photo interrupter is connected to the clock pin of the flip-flop, each time the pencil tip breaks the IR beam the outputs are reversed, causing the LED to change color. This example demonstrates the simplicity of controlling electronics using a special purpose chip without the complexity of programming a microcontroller.