Sensor is a device which responds to an input physical quantity by converting it into an electrical signal. Sensitivity of a sensor shows how much its output changes with change in its input. Sensor measuring very small changes must have very high sensitivity. Sensors are designed in such a way that they cause only a small effect on what is measured. That is why most of the sensors are small in size. Analog Sensors produce a continuous output signal or voltage which is generally proportional to the quantity being measured. Digital Sensors produce a discrete output signal or voltage that is a digital representation of the quantity being measured.
Voltage sensor converts potential difference measured between two points of an electrical circuit into proportional electrical signal. This signal can be stored for further analysis or can be utilized for control purpose.
Current sensor can be used to measure current of an electrical circuit. It converts current of an electrical circuit into proportional electrical signal. This signal can be utilized for further analysis or control purpose.
Temperature sensors are electronic devices used to measure temperature of a medium. It provides a signal proportional to the temperature of the medium. Analog and digital temperature sensors are available. Both types are suitable for use in a wide range of applications. Mainly there are two types of temperature sensors contact type sensors and non-contact type. Contact type sensors measure the temperature of an object or liquid by physical contact. These types of sensors are mainly used for low temperature measurements. Non-contact type sensors use the thermal radiation of the heat source to monitor change in temperature. Non-contact sensors are used when the object is moving or when contact with the object changes its temperature. For very high temperature measurement this type of sensors are used. Thermistor and thermocouple are two commonly using temperature sensors.
Thermistor is a resistor whose resistance change proportionally with change in temperature. In most of the thermistors, resistance decreases with increase in temperature. Reverse type is also available. In normal cold condition, resistance is high due to limited number of free electrons. They are in bond state in atoms and they cannot move. As temperature increases, these bonds will break and more free electrons are created. This increase conduction and produce electrical signal proportional to increase in temperature. Thermistors have very high speed of response to any changes in temperature. They are passive resistive devices i.e. we need to pass a current through it to produce a measurable voltage output. Generally thermistors are constructed using ceramic or polymer material.
This figure shows the circuit for measuring temperature using thermistor. Resister R1 pulls thermistor to reference voltage Vref. Resister and thermistor combination forms a voltage divider. According to temperature, thermistor resistance will change. This results in varying voltage at the junction. This voltage is amplified by an operational amplifier.
Thermocouples are temperature sensors that consist of two junctions of dissimilar metals, such as copper and constantan that are welded or crimped together. One junction (J1) is connected to the object whose temperature is to be measured. This junction is called measuring junction. Other is connected to a known temperature. This junction is known as reference junction. Due to the temperature difference in two junctions, current will flow through thermocouple. We can measure the potential difference and by processing this voltage we can calculate the temperature of the body. Thermocouple is the most commonly used temperature sensor. It has widest temperature range from -200oC to 2000oC.
Light sensor means a passive electronics device which can be used for measuring light. It produces a signal proportional to the intensity of light falls on it. Light sensors are commonly known as "Photoelectric Devices" or "Photo Sensors" because they convert light energy into electricity.
Light Dependent Resistor (LDR):
LDR normally have very high resistance about 1000000 ohms. As light fall on it, its resistance decreases to a great extent. When this material is exposed to light by creating hole-electron pairs its resistance will drop from several thousands of ohms to only a few hundred ohms. It is mainly made up of semiconductor materials such as cadmium sulphide.
Photo diode is a device which converts light into current or voltage, according to the mode of operation. It is similar to conventional PN-junction diode, because it is a regular semiconductor diodes whose outer casing is either transparent or has a lens to focus the light onto the PN junction for increased sensitivity. Photo-diode varies the current conducted through it according to the intensity of light falls on it. As light fall in it more charge carriers are created. Photodiodes have a more linear response than LDRs. Photo diodes are very accurate and stable than LDR. Commonly used materials to produce photo diodes are silicon, germanium, lead sulfide etc.
Colour sensor is used to detect colours. Colour sensors mainly consist of an array of photo detectors. Each of photodiodes will have red, blue or green filter or no filter. Photo diodes with each filter are present in equal numbers. They are also distributed evenly throughout the array. Colour filter only allow the particular Colour to pass through it and block the rest. So using this array intensity of light can be easily calculated. An oscillator present in the sensor will produce a square wave with frequency proportional to the intensity of the chosen colour.
Electronics pressure sensors are used to measure pressure, mainly of liquids and gases. They record the pressure and convert it into proportional output electrical signal. Capacitive pressure sensor consists of two thin metal plates as capacitor. One side of this capacitor is exposed to measuring pressure and other to a reference pressure. Change in pressure changes the gap between the plates. So capacitance varies with difference in pressure. By processing this capacitance we can measure pressure. Pressure sensors have wide variety of application in different fields such as automobile, manufacturing, aviation, bio medical measurements, air conditioning, hydraulic measurements etc.
Smoke sensors are used to detect smoke. It is widely used for many industrial applications as well as commercial application. Smoke sensors work either by optical detection or by ionization process. Some sensors use both the detection methods for higher sensitivity to smoke. In optical detection method sensor have a light source producing a light beam and also a photodiode. When smoke enters in to the sensor some light is scattered by smoke particle. Scattered light is detected by the photodiode. In ionization process sensor uses a radioisotope, placed in an ionization chamber. Chamber consists of two plates. When voltage is applied to these plates, ionization occurs and electrons are released from the atom due to the alpha particles released by the radioisotope. By the movement of electrons to positive plate and ions to negative plate small continuous electric current is created between the plates. When smoke particles enter the chamber it combines with ion and neutralizes it and drop the current. By monitoring this current we can detect smoke.
It is an electronic sensor which measures atmospheric humidity by changing its electrical characteristics such as resistance or capacitance according to the amount of atmospheric humidity. Output voltage or current is produced proportional to the fluctuations in humidity.Most common humidity sensors use capacitive measurements. They consist of two metal plates with a non-conductive humidity-sensing film between them. This film is to collect moisture from the air. Moisture cause minute changes in voltage between two plates. This change in voltage is proportional to the amount of humidity.