Turkish srtudents built a DRAWBRIDGE by using ultrasonic sensors.
The ultrasonic sensor emits ultrasonic waves and listens to their reflection on objects from the environment. It compute the distance from the time spent between emission and reception.The ultrasonic sensor is very useful for robots that need to detect obstacle to navigate between them or for robots that have to detect the presence of objects without touching them.
The ultrasonic sensor can also work as a simple emitter to signal its presence to other robots for example or as simple receiver to react to the presence of an ultrasonic wave.
The ultrasonic sensor for Lego Mindstorms EV3 robots can compute distances between 1 and 250 cm with a precision around the centimeter. It has leds that are on when the sensor emits and blink when the sensor is listening.