A simple prox switch is just an on-and-off switch with a lever attached. For example, when a box moving down a conveyor belt hits the lever of the prox switch, the switch turns off. This switch controls the motor of the conveyor, shutting off the conveyor so the box stops. Other machinery fills the stationary box. When the box is full, it triggers a weight prox switch at its bottom, and the conveyor starts up again. Simple prox switches figure extensively in automation, and design engineers specify where they wants the switches placed.
A light prox switch, also an on-off switch, works with light. The relatively simple operation of this switch consists of a beam of light hitting a strategically placed photocell. The photocell's output then feeds into a controller. When an object crosses in front of the beam, it blocks the beam so light no longer shines on the photocell. The controller "reads" the photocell as off, then activates other circuitry. Burglar alarms often use this type of prox switch. For example, a thin and invisible beam of light shines across a painting. Visitors to the museum cannot see the beam of light, so it doesn't interfere with the painting. If a burglar lays his hands on the painting, however, he breaks the beam.The controller senses no light at the photocell, which sets off an alarm.
In 2010, computers control robotics and automation systems. Simply telling the computer whether the object is there via a simple on-off switch is not enough; the system must also find out the position of the object, which occurs via a proximity sensor. The closer an object moves to a sensor, the higher its output. For example, a car body is moving down a conveyor belt. If the conveyor used a switch, it would stop instantly, which could potentially throw the heavy car body off the belt. A sensor tells the computer the car body is approaching its destination, so the conveyor decelerates gradually. Engineers specify the conditions for the sensor, specifying its exact location in the automation machinery. Many of these sensors are extremely complex, using radar-like sound or infrared light, while others use magnetism of objects. The closer the object, the stronger its magnetic field. Inventors Andrew Ziegler and associates invented a cleaning robot that uses proximity sensors to detect walls and obstacles. When the prox sensor detects an obstacle, the on-board computer steers the robot away from the object.