Basic Sensors

There are hundreds of sensors available, and some of the most commonly used sensors are described below.

• Temperature sensors measure the ambient temperature. For example, such sensors are used in home fire alarms or cars to warn against engine overheating. They are also used to control appliances; for example, to turn off an oven when it reaches the required temperature or to turn on an air conditioner if the room temperature crosses a threshold.

There are different types of temperature sensors categorized by the way they detect temperature. Each type of sensor has a range, sensitivity, and accuracy and is therefore suited for a particular application. The temperature can be recorded in Fahrenheit, Kelvin, or Celsius.

The common types of temperature sensors include:

• Thermocouple: This sensor works on the principles of the Thermoelectric effect. When two dissimilar electrical conductors are joined together, they form what is known as an electrical junction. When the temperature applied at the junction changes, the voltage across the junction also changes. By measuring the voltage, the temperature can be calculated and calibrated based on the properties of the two materials.

• Resistor Temperature Detectors / Thermistors: These sensors work on the principle where the resistance of a conductor is proportional to the temperature. As a general rule, the resistance of a conductor increases with an increase in temperature. But in some materials, it is the opposite, where the resistance decreases with an increase in temperature. These sensors require a current to be passed through, and as the temperature changes, so does the resistance, and therefore, the current. By measuring the current, the temperature can be calibrated based on the conductor material and its variance in resistance due to temperature.

• Semiconductors: These sensors use the working principles of semiconductors and the fact that voltage across the p-n junction of a semiconductor changes with temperature. They are manufactured as integrated circuits and are therefore small in size and easy to use in electric circuits. They also have several handy features, such as high accuracy, high sensitivity, and excellent linearity across a good range of temperatures (about -50 to +150 degrees Celsius). LM35 is one of the more popular IC temperature sensors, and we will be using that in our project.

• Gas sensors detect the presence of and measure the concentration of some gases around them, including combustible, flammable, and toxic gases. They are particularly useful in safety systems where they can alert humans to the presence of harmful gases so that the source of the gases can be contained or the area be evacuated.

The common types of gas sensors include:

• Electrochemical: This type of sensor works by allowing gases to diffuse through a porous membrane to an electrode where it is either chemically oxidized or reduced. The amount of current produced is determined by how much gas is oxidized at the electrode, indicating its concentration.

• Infrared point: Infrared (IR) point sensors use radiation by passing it through a known volume of gas; energy from the sensor beam is absorbed in certain wavelengths, depending on the properties of the specific gas. The energy in this wavelength is compared with the range of outside absorption wavelength; the difference in energy between these two wavelengths is proportional to the concentration of gas present. This type of sensor is advantageous because it does not have to be placed in the gas to detect it and can be used for remote sensing.

• Semiconductor: Semiconductor sensors detect gases by a chemical reaction that takes place when the gas comes in direct contact with the sensor. Tin dioxide is the most common material used in semiconductor sensors, and the electrical resistance in the sensor is decreased when it comes in contact with the monitored gas. This change in resistance is used to calculate the gas concentration. Semiconductor sensors are commonly used to detect hydrogen, oxygen, alcohol vapor, and harmful gases, such as carbon monoxide. In our projects, we will be using the MQ5 sensor.

• Light sensors detect the current ambient light level, that is, how bright or dark it is. These sensors can be used in automated lighting control, particularly in street lighting systems where lights can be turned on or off automatically depending on the ambient light level in the area.

• Water/Humidity sensors detect the presence of water or moisture. They can be used to switch off the water supply when a water storage container is filled or in irrigation systems can be controlled based on moisture in the soil in the agricultural industry.

• Motion sensors detect moving objects and are also used in automated lighting control based on the presence of people in a room or in security systems that alert if unexpected motion is detected.

• Proximity sensors can detect the presence of nearby objects without any physical contact. It works by emitting a beam of electromagnetic radiation (such as infrared) and looks for changes in the return signal. A variation of this type of sensor is the ultrasonic distance sensor which can determine the distance to an object. A typical ultrasonic distance sensor consists of two membranes. One membrane emits sound, and the other catches the reflected echo if it encounters an object in its path. The object distance can be determined by measuring the time taken by the sound to reflect.

The object being sensed is often referred to as the proximity sensor's target. Different proximity sensor targets demand various sensors depending on the target's material and the sensor design. For example, a capacitive proximity sensor or photoelectric sensor might suit a plastic target; an inductive proximity sensor always requires a metal target.

Proximity sensors have quite a few uses in everyday life. They are used to replace measuring tapes in measuring devices at construction sites. Cars are equipped with ultrasonic sensors to alert them if they get too close to another object, especially when parking. Without needing to measure distances, they can only register the presence of an object, for example, machines working in danger zones. A proximity sensor adjusted to a short-range is often used as a touch-free switch.

• Tactile sensors measure inputs from humans, generally finger touch and pressure. A common application of tactile sensors is in touchscreen devices, including mobile phones. Sound sensors can detect and measure ambient sound levels. Microphones could be considered specific and accurate sound sensors in which human speech is detected and can be played back or analyzed to create voice-controlled solutions.

• Speed sensors are most commonly seen as speedometers in vehicles and are used to measure the speed of the vehicle. An accelerometer is designed to measure non-gravitational acceleration. When the object is integrated with it and goes from a standstill to any velocity, the accelerometer is designed to respond to the vibrations associated with such movement. It uses microscopic crystals that go under stress when vibrations occur, and from that stress, a voltage is generated to create reading on any acceleration. Accelerometers are also used in devices that track human movement, such as for tracking fitness activities.

• A Gyroscope is not a sensor in a true sense, but it provides information about the environment just like sensors and has extremely important applications. It is a device that uses Earth's gravity to help determine orientation. Its design consists of a freely-rotating disk called a rotor, mounted onto a spinning axis in the center of a larger and more stable wheel. As the axis turns, the rotor remains stationary to indicate the central gravitational pull, and thus which way is down. Gyroscopes are used in an aircraft to indicate the rate of rotation around the aircraft roll axis with its turning angle.