Proximity Sensors The most popular proximity sensors fall in to two categories:
OPTICAL and ULTRASONIC.

Ultrasonic Sensors: Ultrasonic proximity sensors for mobile robotics often depend on surplus sensors used in early Kodak cameras. The first generations of "autofocusing" cameras often used ultrasonic range finding that operated on a relatively simple principle. An impulse of sound is radiated by a small piezoelectric transducer. The sound travels until it reflects from an object, and the reflected sound bounces back and is detected by a second ultrasonic transducer. By measuring the time delay, the distance to the object can be measured as:
d = (t_rec - t_xmit)/(2*V) where V is the sound velocity. (Note: in some systems, the same transducer is used to both send and receive the acoustic impulse). Ready made ultrasonic rangers are available for ~$50, which can be interfaced to a microcontroller relatively easily. The following source supplies a Polaroid 7000 Series distance transducer for about $45.

Acroname


Optical Sensors: Optical sensors can offer a lower cost option to simple proximity sensing, particularly when the goal is simply sense e.g., wether the robot is nearing an obstacle and needs to take evasive action. Such sensing is readily accomplished by using systems such as the one pictured below.

More on Optical Proximity Sensing

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