Infrared Thermometer

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Infrared Thermometer

Introduction of the scheme

1.  Overview

      In recent years, non-contact infrared temperaturemeasurement has developed rapidly. The performance and functions have greatlyimproved. The variety of applications continue to increase and expand. Incomparison with contact temperature measurement, non-contact infraredtemperature measurement is superior. This is due to faster response time,safety, and a longer service life.

      The infrared thermometer used tomeasure the temperature of the human body is called the human infraredthermometer. Because both manufacturing principles are the same, no specific medical or industrial infraredthermometer has been developed. Currently, there are only two kinds of infraredthermometers; high precision, high performance infrared thermometers, with ahigh distance coefficient ratio and infrared thermometers with low precisionand performance, and low distance coefficient ratios.As long as theemissivity of the infrared thermometer is set to 0.95 human skin is generally in this value), it matches the requirements for human bodytemperature measurement.This article introduces the human infrared temperaturemeasurement solution based on SD8000 series IC of Hangzhou SDIC Microelectronics.

Figure 1. Infrared wavelengthspectrum

Application Characteristic

2.  Principles

2.1  Infrared wavelength

      The infrared light in the infrared temperature sensor is alight that is invisible to the human eye, but in fact, it is an objectivesubstance like any other light.The physical nature of infrared radiation isthermal radiation.The greater the temperature of the object, themore infraredradiation emitted, and the energy of the infrared radiation is stronger.Any object with a temperature higher than zerothermodynamics, will have infrared radiation emitted to the surroundingenvironment.Thefunction of the infrared thermometer is collecting infrared rays emitted by objects. Itdoes not emit any harmful radiation, so it is harmless to humans. There isa misunderstandingthat infrared thermometers emit radiation to the human body to produce readings, butthis concept is wrong.Infrared light has a wavelength range ofapproximately 0.75 to 100 μm. Refer to figure 1. 

2.2  Introduction of thermopileinfrared sensor

      Thermopile in a thermopile infrared temperature sensor allows it to become a temperature measuring element. It consists of two interface originals: Thermopile and Thermistor. The components and structure diagrams are shown below:

The thermopileinfrared temperature sensor senses heat radiation directly and is used to measure small temperature differences oraverage temperatures.With SD8005B or SD8000W ICs developed by SDIC, it provides a perfect solutionfor non-contact temperature measurement. This article is specific to SD8005B.


Figure 2.Thermopile component        


Figure 3. Schematic diagram of thermopile structure

2.3  Description

      The SD8005B is a CMOS SoC with built-in 24-bit ADC and abundantperipheral resources: RTC, selectable voltage sources, flexible PGIA,voltage step-up module, UART, I2C, SPI, TIMER, PWM/PDM, PFD,CAPTURE, and LCD driver.Functional block is shown below:

Figure 4. Functional block diagram


High precision ADC, ENOB=18.8bits@8sps, 2 differential or 4single-ended inputs

Low noise,high input impedance preamplifier with selectable gain: 1, 12.5, 50, 100, or200

8 bits RISC ultra-low power MCU. The MCU current consumption is 300uA typically at 3V and 2MHz operating clock rate. Standby current is 1.5uAat 32kHz clock, and less than 1uA at sleep

16K Bytes OTP, 512 Bytes SRAM

Low OTP programming voltage, can replace external EEPROM

Multiple clock sources, flexible clock selection, external oscillator malfunctiondetection

ADC output rate: 8SPS–2kSPS

24SEG X 4COM LCD drive, ultra-low powerconsumption and high driving capability, programmableboost module to maintain luminance at low supply voltage

Built-in temperature sensor, supportssingle point calibration

1.2V low temperature drift voltage reference output

Selectable voltage source for external transducer excitation: 2.4V/2.6V/2.9V/3.3V

Flexible battery voltage detection range 2.0V~ 3.3V

◆External or internal voltage reference for ADC, multiple internal voltage references

◆RTC module provides second output

◆Abundant peripheral resources: UART, I2C, SPI, PWM/PDM, PFD, TIMER, CAPTURE

◆Low voltage detection and power on reset circuit

◆Operating voltagerange: 2.4V~ 3.6V

◆Operating temperaturerange: -40 ˚C~85 ˚C

3.  HardwareDesign

      The infrared temperature measurementsystem consists of thermopile infrared sensor, control IC (SD8005B or SD8000W),display, buttons, power supply, buzzer and indicator circuit. The circuit issimple with fewer peripheral devices, and low power consumption. The solutionprovides a large number of stable shipments to many large manufacturers. The circuitschematic is shown below.

4.  Introduction of infrared temperature measurement process

      The thermopileinfrared sensor receives the infrared radiation of the target and generates avoltage signal (at both ends of the Thermopile).Therelationship between the voltage signal and the target temperatureTobjand the ambienttemperatureTambis as follow:


      Where K is the correctionconstant; F is a function, related to the sensor

Thevoltage signal is measured after the SD8005B built-in PGIA amplification and high-precision ADdigital-to-analog conversion. SD8005B collects the NTC resistance value (thesignal at both ends of the thermistor), and determines the ambient temperatureTamb by looking up thetemperature-resistance table. The temperature of the target Tobj is obtained by calculation or by thetable. After getting the target temperature, drive the LCD to displaythe actual temperature through SD8005B, and complete the process from infraredtemperature measurement to display temperature.Otherfunctions such as setting temperature thresholds and alarms can be added basedon the needed requirements.


Application block diagram

SD8000 series infrared temperature measurement solution .pdf

Figure 5. SD8005B Infrared Temperature Measurement Circuit Schematic

Figure 6. SD8000W Infrared Temperature Measurement Circuit Schematic


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