Speedometer
I built a speedometer for my EE109 class that measured speed using an ultrasonic range finder with a start and stop button. It had a rotary encoder where you could change the "speed limit" and if it detected that an object had exceeded the speed limit a buzzer would run and an RGB light would blink red. A servo motor read down the countdown limit of 10 seconds which was also displayed on an LCD. There was a tri-state buffer to send and receive information from the other similar systems. This taught me how to build circuits, work in C and work with arduino.