Due to the needs of medical development, in many situations, ordinary thermometers can no longer meet the requirements for rapid and accurate temperature measurement, such as in densely populated areas like train stations and airports. Although this temperature measurement technology is relatively mature abroad, it is still under development in China. Therefore, to meet the needs of medical development and effectively measure temperature in special environments, thereby effectively controlling and preventing the spread of special diseases such as SARS, there is an urgent need to design a thermometer with fast measurement speed and high accuracy. Addressing the insufficient accuracy of general industrial infrared thermometers, we have designed an infrared temperature measurement circuit based on the principle of infrared thermometry. This circuit improves the accuracy of infrared thermometers through the selection of key components, the design of the aiming system, and the automatic adjustment of temperature compensation. This circuit is used for rapid human body temperature measurement in densely populated and high-traffic areas. 1. Principle of Infrared Thermometry All objects in nature with a temperature above absolute zero (-273.15℃) continuously radiate electromagnetic waves, including the infrared band, into the surrounding space due to the thermal motion of their molecules. The relationship between the radiation energy density and the object's temperature conforms to the law of radiation. The principle of external radiation—the law of radiation: E = σ / (5.67 × 10⁻⁸ W / (m²·K⁴)) where: E is the radiant exitance, W/m³; σ is the Stefan-Boltzmann constant, 5.67 × 10⁻⁸ W/(m²·K⁴); ε is the emissivity of the object; T is the temperature of the object, in K; T₀ is the ambient temperature of the object, in K. By measuring the emitted E, the temperature can be determined. Temperature measuring instruments based on this principle are called infrared temperature instruments. This type of measurement does not require contact with the object being measured, therefore it is a non-contact measurement. Infrared temperature instruments have a wide temperature range, from -50℃ to above 3000℃. In different temperature ranges, the wavelength distribution of the electromagnetic wave energy emitted by the object is different. In the normal temperature range (0～100℃), the energy is mainly concentrated in the mid-infrared and far-infrared wavelengths. Instruments used for different temperature ranges and for different measuring objects have different specific designs. According to the principle of equation (1), the infrared radiation measured by the instrument is: Where: A is an optical constant, which is related to the specific design structure of the instrument; ε1 is the emissivity of the object being measured; ε2 is the emissivity of the infrared thermometer; T1 is the temperature (K) of the object being measured; T2 is the temperature (K) of the infrared thermometer; it is measured by a built-in temperature detection element. Emissivity ε is a coefficient used to express the ability of an object to emit electromagnetic waves, with a value ranging from 0 to 1.0. The most ideal radiating object is an object with an emissivity of 1.0, which is called a blackbody in physics. This is a theoretical concept, and in reality, no object has an emissivity that can reach 1.0. However, it is possible to manufacture an actual blackbody that is very close to ε=1.0 for the calibration of the thermometer. All real objects, including the surfaces of various parts of the human body, have an ε value that is lower than 1.0. Since the ε value is extremely difficult to measure and uncertain, T1 calculated according to equation (2) after the instrument measures E will have an error. In practical work, the instrument is calibrated at the factory on a blackbody with ε=1.0. Only when measuring an object with ε=1 does the reading represent the actual temperature of the object. If the object's ε is not equal to 1, the instrument reading does not represent the actual temperature of the object and must be corrected. The human body mainly radiates infrared rays with wavelengths between 9 and 10 μm. By measuring the infrared energy radiated by the human body itself, the surface temperature of the human body can be accurately determined. Since light in this wavelength range is not absorbed by the air, the infrared energy radiated by the human body can be used to accurately measure the surface temperature of the human body. The infrared radiation characteristics of the human body are closely related to its surface temperature. Therefore, by measuring the infrared energy radiated by the human body itself, the surface temperature of the human body can be accurately determined. The biggest advantage of infrared temperature measurement technology is its fast testing speed, which can be completed within 1 second. Since it only receives the infrared radiation emitted by the human body and no other physical or chemical factors act on the human body, it is harmless to the human body. 2 Structural Block Diagram and Circuit Design 2.1 Structural Block Diagram Design Figure 1 shows the overall structural block diagram of the designed system. 2.2 Temperature Sensor This temperature measuring device uses an infrared sensor, which receives infrared radiation emitted by objects and converts it into a voltage signal. I selected the PM611 unit pyroelectric sensor. Although this sensor is a single-element sensor, its structure of one receiver and two parallel compensation elements connected in series effectively compensates for ambient temperature fluctuations and the effects of vibration and other disturbances. Its operating temperature is -20 to +70℃, making it particularly suitable for measuring human body temperature. Furthermore, the PM611 has good performance indicators, hence its selection as the probe for the temperature meter. As shown in Figure 3: Connect the sensor's D, S, and E terminals to the D, S, and E terminals marked in the circuit diagram, respectively. 2.3 Measurement Circuit The measurement circuit is shown in Figure 4. 2.4 Integrating Circuit In converting the analog signal to a digital signal, the ICL7106 integrating A/D converter was selected. Its main advantages are: high conversion accuracy and low cost; its accuracy is independent of the accuracy of the integrating resistor and capacitor, thus reducing the requirements for component quality; strong anti-interference capability; and simple peripheral circuitry. However, the ICL7106 also has some drawbacks: changes in the reference power supply directly affect conversion accuracy. Since the internal reference voltage source is generally greatly affected by temperature, an external reference voltage source should be used when high accuracy is required; its operating speed is relatively low, typically 1-20 times/s. Therefore, considering the main characteristics of the ICL7106, especially to improve accuracy, an external reference voltage source was used in the design. Furthermore, since the fastest temperature measurement speed is only 1 time/s, the ICL7106's low operating speed does not affect our design, but its advantages are very suitable for the design needs. Therefore, the ICL7106 is the first choice for A/D conversion. 2.5 LCD Display Circuit The display section consists of a multi-bit liquid crystal display driver (ICL7106), a standard segment liquid crystal display (EDS801), and other components. 3. Debugging In the experiment, the magnitude of the A3 output signal was changed by adjusting the 10 kΩ variable resistor at the A1 output terminal. When the A3 output is less than 2.0 V, it can adapt to the 2.0 V operating characteristic of the ICL7106. Therefore, the two potentiometers of A3 are used to adjust the A3 output to ensure that it does not exceed 2.0 V at high temperatures. During debugging, place an object with a known temperature (20-70℃) in front of the probe and adjust R12 in the ICL7106 section until the LCD displays the correct temperature. Then, change to another object with a known temperature and continue adjusting the potentiometer until the displayed value is accurately equal to the known temperature value. Repeat this adjustment several times. Note that the temperature limits of the PM611 sensor and the ICL7106 A/D integrator should not be exceeded.