Wireless temperature sensors , with their unique advantages and wide application scenarios, are gradually becoming an indispensable part of modern life. These sensors have the characteristics of miniaturization, digitization, and intelligence. Through wireless data transmission technologies such as WIFI , 433, Zigbee , etc., remote data collection and monitoring are achieved. Compared to traditional wired sensors, wireless temperature sensors have advantages such as easy installation, high flexibility, and low maintenance costs, and are widely used in multiple fields. In the field of smart home technology, wireless temperature sensors can monitor temperature changes in the home environment in real time and link with devices such as air conditioning and heating to achieve intelligent temperature control. By combining with security systems, it is possible to promptly issue alarms in case of fire or other abnormal situations, ensuring family safety.

In industrial automation, wireless temperature sensors are widely used for temperature monitoring of various devices. It can not only monitor the operation status of equipment in real time to prevent faults caused by overheating, but also combine with automation control systems to achieve precise control of the production process, improve production efficiency and product quality.

In terms of environmental monitoring, wireless temperature sensors can be deployed in complex and ever-changing natural environments to conduct long-term and continuous monitoring of temperature, soil temperature, etc. These data are not only of great significance for climate change research, but also provide valuable information support for environmental protection and agricultural production.

Specific application scenarios, such as solar wireless temperature sensors, adopt bidirectional communication, integrate transmission and reception, and can achieve point-to-point communication, point-to-point multi-point communication, flexible networking up to 60 points, and transmission distance up to 20KM. Suitable for wireless monitoring, wireless data acquisition, and wireless alarm systems with requirements for short distance, miniaturization, and low cost.

With the continuous development of Internet of Things technology, wireless temperature sensors are moving towards higher accuracy, lower energy consumption, and stronger anti-interference ability. With the popularization of new wireless communication technologies such as 5G and LoRa, the transmission efficiency and stability of wireless temperature sensors will also be further improved.

Summary and Application Analysis of Temperature Sensor Types

The main types of temperature sensors include thermocouple sensors, thermistor sensors, resistance temperature detectors (RTDs), and IC temperature sensors. IC temperature sensors include two types: analog output sensors and digital output sensors.

Thermocouple sensor

A thermocouple is a temperature sensing element and an instrument. It directly measures temperature and converts the temperature signal into a thermoelectric electromotive force signal, which is then converted into the temperature of the measured medium through electrical instruments (secondary instruments). The basic principle of thermocouple temperature measurement is to form a closed circuit with two different materials of conductors. When there is a temperature gradient at both ends, there will be current passing through the circuit, and there will be an electromotive force between the two ends - thermoelectric electromotive force, which is called the Seebeck effect. Two homogeneous conductors with different compositions are thermoelectric electrodes, with the working end at the higher temperature and the free end at the lower temperature. The free end is usually at a constant temperature. According to the functional relationship between thermoelectric electromotive force and temperature, create a thermocouple calibration table; The graduation table is obtained under the condition of a free end temperature of 0 ℃, and different thermocouples have different graduation tables.

When a third metal material is connected to the thermocouple circuit, as long as the temperature of the two contacts of the material is the same, the thermoelectric potential generated by the thermocouple will remain unchanged, that is, not affected by the connection of the third metal to the circuit. Therefore, when using thermocouples for temperature measurement, a measuring instrument can be connected, and after measuring the thermoelectric electromotive force, the temperature of the measured medium can be determined.

When selecting a thermocouple, the following factors should be considered:

1. Temperature range to be measured;
2. Required response time;
3. Connection point type;
4. The chemical corrosion resistance of thermocouple or sheath materials;
5. Resistance to wear or vibration;
6. Installation and restriction requirements, etc.

Thermistor sensor

The main component of a thermistor sensor is a thermistor. When there is thermal radiation around the thermistor material, it will absorb the radiation heat, generate a temperature rise, and cause a change in the material's resistance value.

The application of thermistor sensors

1. Temperature measurement using thermistor sensors

As a thermistor sensor for measuring temperature, its structure is generally simple and its price is relatively low. Thermistors without an external protective layer can only be applied in dry places; Sealed thermistors are not afraid of moisture erosion and can be used in harsh environments. Due to the high resistance value of thermistor sensors, the resistance of their connecting wires and contact resistance can be ignored. Therefore, thermistor sensors can be applied in temperature measurement over long distances of several thousand meters, and the measurement circuit often uses a bridge circuit. Its principle can also be used as other temperature measurement and control circuits.

2. Thermistor sensors for temperature compensation

Thermistor sensors can compensate for humidity in certain components within a certain temperature range. For example, the moving coil in the head of a moving coil instrument is made of copper wire wound. As the temperature rises, the resistance increases, causing temperature errors. Therefore, the negative temperature coefficient thermistor can be connected in parallel with the manganese copper wire resistor in the dynamic loop, and then connected in series with the compensated component to offset the errors caused by internal temperature changes. In transistor circuits and logarithmic amplifiers, thermistors are also commonly used to form compensation circuits. Compensate for drift errors caused by temperature.

3. Overheat protection of thermistor sensors

Overheat protection is divided into direct protection and indirect protection. For low current situations, the thermistor sensor can be directly connected to the load to prevent overheating damage and protect the device. For high current situations, it can be used for protection of relays, transistor circuits, etc. In either case, the thermistor is tightly integrated with the protected device, allowing for sufficient heat exchange between the two. Once overheated, the thermistor plays a protective role. For example, embedding a mutation type thermistor sensor in the stator winding of an electric motor and connecting it in series with a relay. When the motor is overloaded, the stator current increases, causing heating. When the temperature exceeds the sudden change point, the current in the circuit can suddenly change from a few tenths of milliamperes to several tens of milliamperes, thus the relay operates and achieves overheating protection.

4. Thermistor sensors for measuring liquid levels

Applying a certain heating current to the NTC thermistor sensor will result in its surface temperature being higher than the surrounding air temperature, resulting in a lower resistance value. When the liquid exceeds its installation height, it will take away its heat, causing its temperature to decrease and its resistance to increase. By judging its resistance change, one can determine whether the liquid level is below the set value. The oil level alarm sensor in the car fuel tank is made using the above principles. Thermistors are also used in cars to measure oil temperature, coolant mixing, etc.

Resistance temperature detector (RTD)

RTD is typically made of platinum, copper, or nickel. The resistance temperature relationship of these metals is shown in the figure. They have a large temperature coefficient, respond quickly with temperature changes, can resist thermal fatigue, and are easy to manufacture into precision coils.

RTD is currently the most accurate and stable temperature sensor. Its linearity is superior to thermocouples and thermistors. But RTD is also a temperature sensor with slow response speed and relatively expensive price. Therefore, RTD is most suitable for applications that have strict requirements for accuracy, but speed and price are not very critical.

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Application of Resistance Temperature Detectors (RTDs)

Resistance temperature detectors are widely used in nuclear power plants to measure temperature. The temperature measurement principle of RTD is that the resistance of pure metals or certain alloys increases with increasing temperature and decreases with decreasing temperature. Therefore, RTD is somewhat like a thermoelectric converter, converting temperature changes into voltage changes. The most suitable metal for RTD use is pure metal that remains stable within a given temperature range. The relationship between resistance and temperature change is best linear, with a larger temperature coefficient (defined as the change in resistance per unit temperature) being better, and it should be able to resist thermal fatigue and respond sensitively to temperature changes. Only a few metals can meet this requirement.

IC temperature sensor

(1) Simulated integrated temperature sensor

Integrated sensors are made using silicon semiconductor integration technology, hence also known as silicon sensors or single-chip integrated temperature sensors. The analog integrated temperature sensor was introduced in the 1980s. It is a specialized IC that integrates temperature sensors on a single chip and can perform temperature measurement and analog signal output functions. The main characteristics of analog integrated temperature sensors are single function (only measuring temperature), small temperature measurement error, low price, fast response speed, long transmission distance, small volume, micro power consumption, etc. They are suitable for long-distance temperature measurement and control without the need for nonlinear calibration, and the peripheral circuit is simple. An integrated sensor that is still widely used both domestically and internationally, AN6701, an IC temperature sensor with high sensitivity and precision, is introduced below.

The schematic diagram of AN6701 is shown in the following figure, which consists of three parts: temperature detection circuit, temperature compensation circuit, and buffer amplifier.

The detection circuit of the IC temperature sensor works by utilizing the principle of the voltage difference (VbC) between the base and emitter caused by the current density difference between the two emitters in a transistor. The following diagram shows the temperature detection and temperature compensation circuit diagram. In the above figure, T1-T5 is the detection circuit, and the circuit composed of T8-T11 and RC generates a current proportional to its absolute temperature. This current is injected into T7 through T12 and T13 to obtain the compensation temperature corresponding to the injected current. RC is an external resistor, making the calibration of the sensor more convenient.

(2) Digital output sensor

Digital temperature sensors were introduced in the mid-1990s. It is the crystallization of microelectronics technology, computer technology, and automatic testing technology (ATE). At present, various intelligent temperature sensor series products have been developed internationally. Intelligent temperature sensors all contain temperature sensors, A/D converters, signal processors, memory (or registers), and interface circuits. Some products also come with a multiplexer, central controller (CPU), random access memory (RAM), and read-only memory (ROM). The characteristic of intelligent temperature sensors is that they can output temperature data and related temperature control variables, and are compatible with various microcontrollers (MCUs); And it achieves testing functions through software on the basis of hardware, and its intelligence and harmony also depend on the level of software development.