During the epidemic, the forehead thermometer that “shoots” at you every day, do you know how it works?

“During the coronavirus epidemic, we all know that masks are out of stock and disinfectant alcohol is out of stock. In fact, medical forehead thermometers are also out of stock. It is hard to find a gun because of its rapid temperature measurement (1 second temperature measurement) and non-contact temperature measurement. , It is widely used in hospitals, airports, railway stations, communities, customs and other places, and the forehead thermometer has become a veritable anti-epidemic material.

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Today’s protagonist is the forehead gun.

During the coronavirus epidemic, we all know that masks are out of stock and disinfectant alcohol is out of stock. In fact, medical forehead thermometers are also out of stock. It is hard to find a gun because of its rapid temperature measurement (1 second temperature measurement) and non-contact temperature measurement. , It is widely used in hospitals, airports, railway stations, communities, customs and other places, and the forehead thermometer has become a veritable anti-epidemic material.

Any object above absolute zero (-273°C) will emit infrared rays, and the forehead thermometer receives infrared rays through the infrared temperature sensor to calculate the temperature data.

This article briefly talks about the hardware structure of the forehead thermometer.

High-speed rail station forehead thermometer to measure body temperature

The following is a comparison of common temperature measuring instruments. During the severe epidemic period, non-contact and fast temperature measurement are particularly important.

Appearance drawing

A complete forehead temperature gun product is composed of three parts: the shell, the internal circuit board and the battery. We will not talk about the shell, which belongs to the design category of structural engineers. The following focuses on the internal circuit composition.

From the structure diagram of the forehead thermometer below, it can be seen that the circuit devices include: LED Display, infrared temperature sensor, battery, button, buzzer, etc.

Appearance drawing of forehead thermometer

Hardware block diagram

According to the above composition, we can roughly draw the hardware structure block diagram of the forehead thermometer, and the functions of each part are introduced as follows.

Forehead thermometer hardware structure block diagram

Infrared temperature sensor: It is used to collect the temperature of the human body/object. The infrared forehead temperature gun is generally measured at a distance of 1~3cm from the forehead. The collected data is amplified by the signal amplification circuit. The signal amplification circuit is optional. Integrated signal amplification.

MCU microprocessor: accepts the data of the infrared temperature sensor for processing. For the analog sensor, it is necessary to add an A/D conversion circuit to convert the analog signal into a digital signal, and then process it to the MCU; for the digital temperature sensor, it can be directly to be processed by the MCU.

Button module: It is used for mode switching, measuring objects, measuring forehead, measuring neck, etc. The buttons can be customized, and can be used as a power button, a system reset button, etc.

LCD display: MCU control, used to display temperature data.

Buzzer module: Alarm module, MCU sets a normal temperature range, such as 35~37.3℃, if it exceeds the upper limit or is lower than the lower limit, it will give a buzzer alarm, and it is not normal for the temperature to be too low or too high.

Crystal oscillator circuit: It is used for the clock of MCU microprocessor to provide oscillation source.

Battery: The battery is the main power supply of the whole system. Because of the portability of the forehead thermometer, it must be battery-powered, battery-powered products, and the most important thing is low power consumption. When not working, the system needs to enter sleep mode to achieve better performance. Low power consumption and extended battery life.

Power management module: The power supply voltage of infrared temperature sensor, display screen, buzzer and other modules is not necessarily the same. The battery needs to be converted into an appropriate voltage value to supply power to each module. Of course, rechargeable batteries can also be used to add charging circuits.

Infrared temperature sensor

The forehead temperature gun is actually a single-chip processing system. The most important part is the infrared temperature sensor block. The sensor also determines the accuracy of the measurement. The following describes a commonly used infrared temperature sensor MLX90615. MLX90615 is a digital sensor. A/D conversion circuit, the highest precision can reach 0.2 ℃, because of its small size and convenient development, it is still used more.

Small size: reduce the space occupied by the internal circuit, which can make the product more portable.

Warm appearance

Working voltage 3V: Lower voltage means lower power consumption. The power-down mode current is only 1.1uA, that is, the thermometer consumes very little battery power when it is not working. This parameter is very important for battery-powered products.

Electrical parameters

As can be seen from the internal block diagram of the temperature sensor, the OPA amplifier circuit, A/D analog-to-digital conversion, filter circuit, and DSP are used for data processing.

Internal block diagram

Supports both PWM and SMBus modes

Pin Definition

Reference circuit:

In SMBus mode, two lines SDA and SCL need to be connected, and SDA and SCL need to be connected to pull-up resistors.

SMBus mode circuit

In PWM mode, connect SDA to the pin with PWM function of MCU, pull up a 10K resistor to the power supply of MCU, and connect SCL to the temperature sensor power supply VDD.

PWM mode circuit