Humans discover and learn about the world through their senses. We use these senses to guide us in what we do. If we see that the traffic light is red, we do not cross the street. If we hear a fire alarm go off, we rush towards an exit. If we taste something not to our liking, we spit it out. If we feel cold, we wear a jacket. If we smell something burning in the kitchen, we get ready to call for take-out. We rely on our senses to keep us informed about what is going on around us so that we can react appropriately to any particular situation.
We recognize the value of the senses so much that our scientists have even created technological equivalents of what nature has freely gifted us with. Thus, we have mechanical sensors that can detect motion, pressure, temperature, proximity and other kinds of stimuli. These mechanical sensors, in turn, allow us to continue discovering and learning about the environment around us.
There are a number of different methods by which motion can be sensed or detected by a device. For example, an automatic door opener, a feature usually found in malls, uses a radar-based motion detector. A device above the door emits bursts of either microwave radio energy or ultrasonic sound waves. It then waits for the energy to reflect and bounce back. If there is no one about to enter the building, the energy will bounce back in a regular pattern.
However, once someone enters the area, there will be a disturbance in the reflection pattern and the door then opens. In the case of a photo-sensor motion detector, a beam of focused light or a type of laser beam is aimed at a light sensor. If someone walks between the light source and the sensor, the path of the beam is disrupted. The sensor then registers the drop in the light level and sends a signal to its control box which emits an alarm.
Pressure sensors typically measure the pressure of either gases or liquids. These sensors are used for either control or monitoring purposes in numerous everyday applications. There are five types of pressure measurements, namely: absolute pressure sensors, gauge pressure sensors, vacuum pressure sensors, differential pressure sensors, and sealed pressure sensors.
Car alarm systems, for example, are some of the applications that utilize these pressure sensors. When the alarm system is turned on, the sensor measures the air pressure inside the car. If someone opens a door or a window, this pushes or pulls on the air inside the car, creating a change in the pressure, thereby triggering the alarm.
Perhaps the most common example of a temperature sensor is a thermometer. However, many other methods and devices have been developed for measuring temperature. Most of these devices rely on measuring a certain physical property of a material that can vary with changes in temperature. For example, a typical glass thermometer is filled with a fluid like mercury which expands as the temperature increases.
The resulting temperature then is determined by measuring the volume of the fluid. Since glass thermometers are usually calibrated, the temperature can be determined by simply observing the level of fluid within the tube. Other devices used to measure temperature are: thermocouples, thermistors, Resistance Temperature Detectors, pyrometers, Langmuir probes, and infrared.
A proximity sensor is able to detect the presence of nearby objects without the requirement of actual physical contact. These sensors usually emit an electromagnetic or electrostatic field, however, some types of proximity sensors emit a single beam of electromagnetic radiation such as infrared.
The sensors then look for changes in the field or the return signal and the object that these sensors detect is referred to as the target. Different sensors are needed to detect different targets. If the target is made of plastic, a capacitive or photoelectric sensor is better suited for detecting it. An inductive sensor, meanwhile, works best at detecting metallic targets.