Step 1: The Input

To have an input you will need a sensor to have as an input. Your sensor could be a switch, ultrasonic sensor, potentiometer, etc,.

Step 2: The Processor

This is the brains of the operation. You can use an arduino or another simple circuit that will control your output. This is just simply a circuit that will take the input and turn it into an output.

Step 3: The Output

This uses the output from the processor and uses it to do something. This could be a servo or an led.

Step 4: Example

This is an example you can do if you have the Board Of Education Shield kit by parallax. I used one of their projects as an example. In the project the input is an inferred detector and uses an arduino to process the signal that comes in when ever a light is shined in and the stops the servo output. You press reset and the servos go until a light is shined at the infrared detector and that is processed and the servos are stopped.