Presumably, you are not familiar with the word sensor principle, and you don't know what its general meaning is. Now let's find out. What is the principle of the sensor? We should pay attention to the following points: when we were in college, teachers often talked about the principle of sensors, and now they are just used. Share it with everyone! I didn't know what it meant before, but after listening to the last content and training, I probably understood something: Now let's understand: What is a sensor? In a broad sense, a sensor is a device that can sense external information and convert it into usable signals according to certain rules. Simply put, a sensor is a device that converts external signals into electrical signals. Therefore, it consists of two parts: the sensitive element (sensing element) and the conversion device. Some semiconductor sensors can directly output electrical signals and form their own sensors. There are many kinds of sensitive elements, which can be divided into ① physical types according to physical effects such as force, heat, light, electricity, magnetism and sound. ② Chemistry, based on the principle of chemical reaction. (3) Biology, based on the molecular recognition functions of enzymes, antibodies and hormones. Generally, according to its basic sensing function, it can be divided into ten categories (some people once divided sensors into 46 categories), such as heat sensitive elements, light sensitive elements, gas sensitive elements, force sensitive elements, magnetic sensitive elements, humidity sensitive elements, sound sensitive elements, radiation sensitive elements, color sensitive elements and taste sensitive elements. The commonly used heat-sensitive, light-sensitive, gas-sensitive, force-sensitive and magnetic-sensitive sensors and their sensitive components are introduced as follows. Temperature sensor and heat-sensitive element The temperature sensor is mainly composed of heat-sensitive elements. There are many kinds of thermistors, including bimetal, copper thermistor, platinum thermistor, thermocouple and semiconductor thermistor. Temperature sensors with semiconductor thermistors as detection elements are widely used, because semiconductor thermistors have the characteristics of small size, high sensitivity, high precision, simple manufacturing process and low price under the working conditions allowed by the elements. 1. The working principle of semiconductor thermistor can be divided into two types according to the temperature characteristics. With the increase of temperature, the resistance of PTC thermistor and negative temperature coefficient increases. (1) The working principle of positive temperature coefficient thermistor This thermistor is made of BaTio3 as the basic material, doped with appropriate rare earth elements, and sintered at high temperature by ceramic process. Pure barium titanate is an insulating material, but it becomes a semiconductor material after being doped with a proper amount of rare earth elements such as La and Nb, which is called semiconductor barium titanate. It is a polycrystalline material with grain boundaries between grains, which is equivalent to the barrier of conductive electrons. At low temperature, because of the electric field in BaTiO _ 3 semiconductor, conductive electrons can easily cross the barrier, so the resistance value is small. When the temperature rises to the Curie point temperature (that is, the critical temperature, the Curie point of barium titanate is generally 120℃ as the' temperature control point' of this element), the internal electric field is destroyed, which can't help the conductive electrons to cross the barrier, so it shows a sharp increase in resistance. Because the resistance of this element changes very slowly with temperature before reaching Curie point, it has the functions of constant temperature, temperature adjustment and automatic temperature control. It only produces heat, does not turn red, has no open flame and is not easy to burn. The voltage can reach 3 ~ 440 volts AC and DC, and the service life is long. It is very suitable for overheating detection of electrical equipment such as motors. (2) Working principle of negative temperature coefficient The negative temperature coefficient is made of metal oxides such as manganese oxide, cobalt oxide, nickel oxide, copper oxide and aluminum oxide through ceramic technology. These metal oxide materials have semiconductor characteristics, which are completely similar to germanium and silicon crystal materials. The number of carriers (electrons and holes) in the body is small and the resistance is high. With the increase of temperature, the number of carriers in the body increases and the natural resistance decreases. There are many types of negative temperature coefficient, which is used to distinguish low temperature (-60 ~ 300℃), medium temperature (300 ~ 600℃) and high temperature (>: 600℃). It has the advantages of high sensitivity, good stability, fast response, long service life and low price, and is widely used in automatic temperature control circuits that need fixed-point temperature measurement, such as refrigerators, air conditioners, greenhouses and other temperature control systems. When the thermistor is combined with a simple amplifier circuit, it can detect the temperature change of one thousandth of a degree, so it can complete high-precision temperature measurement with electronic instruments. The working temperature of general thermistor is -55℃ ~+3 15℃, and the working temperature of special low-temperature thermistor is lower than -55℃ and can reach -273℃. 2. Model of Thermistor Our domestic thermistor is designed according to the ministerial standard SJ 1 155-82, which consists of four parts. The first part: the main name, with the letter "m" for sensitive elements. Part II: Category, in which the letter "Z" is used to represent a positive temperature coefficient thermistor, or the letter "F" is used to represent a negative temperature coefficient. Part III: Use or characteristics, expressed by one digit (0-9). The general number' 1' stands for general use,' 2' stands for voltage stabilization (negative temperature coefficient),' 3' stands for microwave measurement (negative temperature coefficient),' 4' stands for indirect heating (negative temperature coefficient),' 5' stands for temperature measurement,' 6' stands for temperature control and' 7' stands for degaussing (positive temperature coefficient thermistor 9'. Part IV: Serial number, also expressed by numbers, represents specifications and performance. For the special needs of distinguishing this series of products, manufacturers often add' derivative serial number' after the serial number, which is a combination of letters, numbers and'-'. 3. Main parameters of thermistors The working conditions of various thermistors must be within the allowable range of their factory parameters. There are more than ten main parameters of thermistor: nominal resistance, service environment temperature (maximum working temperature), measured power, rated power, nominal voltage (maximum working voltage), working current, temperature coefficient, material constant, time constant, etc. Among them, when the nominal resistance is 25℃ and zero power, there is always a certain error, which should be within 10%. The working temperature range of common thermistor is wide, and it can be selected from -55℃ to +3 15℃ as required. It is worth noting that the maximum working temperature of different types of thermistors varies greatly, such as the negative temperature coefficient of the chip is+1 1, and the MF53-65438 is+1. General negative temperature coefficient is the first choice for experimental thermistors, because it is generally easier to observe when it changes with temperature than positive temperature coefficient thermistors, and the resistance value decreases continuously and obviously. If the thermistor with positive temperature coefficient is selected, the experimental temperature should be near the Curie point temperature of the component. Example MF 1 1 common negative temperature coefficient parameter main technical parameter name parameter value MF1/thermistor symbol shape icon scale resistance (kω) 10 ~ 15 sheet symbol rated power (W) 0.25 material constant b range (k Degree coefficient (10-2/℃)-(2.23 ~ 4.09), dissipation coefficient (mW/℃) ≥5, time constant (s) ≤30, and the maximum working temperature (℃) 125 thermistor value should be roughly measured by a multimeter with moderate measuring range and small thermistor measuring current. If the thermistor is about10kΩ, you can choose MF 10 multimeter, turn its gear switch to ohm gear R× 100, and clamp the two pins of the thermistor with a crocodile clip instead of a pen. When the ambient temperature is obviously lower than the body temperature, the reading is10.2k. When you hold the thermistor with your hand, you can see that the resistance indicated by the hands gradually decreases. After releasing the hand, the resistance increases and gradually recovers. You can choose such a thermistor (the highest working temperature is about 100℃). Several Practical Temperature Sensors Special temperature sensors in air conditioners: Heat sensitive elements are encapsulated in copper metal. B air temperature measurement sensor 2 optical sensor and photosensitive element optical sensor is mainly composed of photosensitive element. At present, photosensitive elements have developed rapidly, with various types and wide applications. Photosensitive resistors, photodiodes, phototransistors, photocouplers and photovoltaic cells are sold in the market. 1. Photosensitive resistor Photosensitive resistor consists of semiconductor photoelectric crystals that can transmit light. Due to the different components of semiconductor photoelectric crystals, they can be divided into visible light photoresistors (cadmium sulfide crystals), infrared light photoresistors (gallium arsenide crystals) and ultraviolet light photoresistors (zinc sulfide crystals). When the surface of semiconductor photoelectric crystal is irradiated by sensitive wavelength, the carriers in the crystal increase, which makes its conductivity increase (that is, its resistance decrease). Main parameters of the photoresistor: ◆ photocurrent and bright resistance: Under a certain applied voltage and illumination (100lx illumination), the current flowing through the photoresistor is called photocurrent; The ratio of applied voltage to current is bright resistance, which is generally several kω to several tens of kω. ◆ Dark current and dark resistance: Under a certain applied voltage, when there is no illumination (0 lx illumination), the current flowing through the photoresistor is called dark current; The ratio of applied voltage to current is dark resistance, which is generally above several hundred kω to several thousand kω. ◆ maximum working voltage: Generally, tens of volts to hundreds of volts. ◆ Ambient temperature: -25℃ to +55℃ in general, and -40℃ to +70℃ in some models. ◆ Rated power (power consumption): the product of bright current of photosensitive resistor and external voltage; 5mW to 300mW specifications are available. ◆ The main parameters of photoresistor include response time, sensitivity, spectral response, illumination characteristics, temperature coefficient, volt-ampere characteristics, etc. It is worth noting that the illumination characteristics (characteristics changing with the illumination intensity), temperature coefficient (characteristics changing with the temperature) and volt-ampere characteristics are not linear. For example, photoresistors containing CdS (cadmium sulfide) sometimes increase with the increase of temperature and sometimes decrease with the increase of temperature. Photosensitive resistance parameters of cadmium sulfide: model specification: mg 41-22mg42-16mg44-02mg45-52 ambient temperature (℃)-40 ~+60-25 ~+55-40 ~+70 rated power (MW) 20. 100 LX(kω)≤2≤50≤2≤50≤2 dark resistance, 0lx (mω) ≥ 1 ≥/kloc-0 ≥ 0.2 ≥1response time (ms) ≤ 20 ≤ 20 ≤ The area of the second PN junction is relatively large, but the electrode area is very small, which is beneficial to the photosensitive surface to collect more light; The third photodiode has a "window" sealed with plexiglass lens in appearance, which can collect light on the photosensitive surface; Therefore, the sensitivity and response time of photodiode are far better than that of photoresistor. Professional problems like this should be understood by now. It's difficult to type on it. Please read it carefully. I hope everyone can learn. It is very useful. I also learned how to operate the sensor through the above information. You can try! The above is some sharing about the sensor principle, I hope it will help you! Dear recognition is my greatest motivation! If you think it is not bad, you can share it with your friends!