Infrared Thermometer Technical Principles
All objects with temperatures above zero degrees (-273.15°C) continuously emit infrared energy into the surrounding space. Their radiation characteristics, the magnitude of radiation energy, wavelength distribution, etc., are closely related to the object’s surface temperature. Conversely, by measuring the infrared energy emitted by the object itself, we can accurately determine its surface temperature, which is the mechanism behind infrared radiation temperature measurement.
Like other living organisms, the human body also radiates infrared energy in all directions, with a wavelength generally between 9-13μm, which falls within the near-infrared spectrum of 0.76-100μm. Since the light in this wavelength range is not absorbed by air, the amount of infrared radiation emitted by the human body is independent of environmental influences and only related to the amount of energy stored and released by the body. Therefore, by measuring the infrared energy emitted by the human body, we can accurately determine its surface temperature. Infrared temperature sensors for the human body are designed and manufactured based on this principle.
Infrared Temperature Sensors
Currently, the infrared temperature sensors available on the market can be categorized into several types based on the materials used for energy conversion:
(1) Pyroelectric type: Trisulfate, Lithium Tantalate, etc.
(2) Thermopile type: N-type and P-type polycrystalline silicon
(3) Diode type: Monocrystalline or polycrystalline PN junction
(4) Thermocapacitor type: Bimetallic thin film
(5) Thermistor type: Vanadium oxide, amorphous silicon, etc. In fact, these types differ only in the conversion methods and material efficiency after receiving infrared energy.
Currently, both ear and forehead thermometers use thermopile infrared sensors, with the basic physical principle being the Seebeck effect.
Thermopile temperature sensor parameters:
Measurement range: -50℃-100℃;
Measurement accuracy: within 0.1℃, ±0.1%;
Operating temperature: -20℃-+85℃;
Maximum measurement distance: ≦0-50mm (with lens 300-500mm)
Output voltage: 0-10mV;
The maximum range of thermopile temperature sensors can reach from -60℃ to +1200℃.
The main packaging forms of infrared thermopile sensor products are TO46(TO18)/TO39(TO5);
Infrared Thermometer Chip
The core chip of the infrared thermometer mainly consists of an ADC chip and a control chip, enabling functions such as button control, LCD display, and battery detection.
The working process of the infrared thermometer: It consists of an optical system, photoelectric detector, signal amplifier, signal processing, and display output. The optical system collects the infrared radiation energy of the target within its field of view, which is determined by the optical components and position of the thermometer. The infrared radiation emitted by the measured object first enters the optical system of the thermometer, which focuses the incoming infrared rays, concentrating the energy; the concentrated infrared rays are then directed to the photoelectric detector, whose key component is the infrared sensor, tasked with converting light signals into electrical signals; the electrical signals output from the photoelectric detector are amplified and processed according to the instrument’s internal algorithm and corrected for the target emissivity, transforming them into the temperature value of the measured target.
Calibration of Human Forehead Thermometers
Environmental temperature: Place the sensor in a water tank at 25.00 +/-0.02 degrees, wait for stabilization, and press a button to confirm, usually taking about 30 seconds.
Black body temperature: First, set the black body furnace to 37.00 +/-0.02 degrees, then insert the sensor into the black body furnace, wait for stabilization, and press a button to confirm. This usually takes about 20 seconds.
What to Prepare for Scheme Design and Product Production
1. How many function keys are on the shell, and what are their functions?
2. How many pins does the LCD screen need?
3. Is the sensor digital or analog?
4. Product specifications: What kind of battery is used, home use or medical use, does it require calibration?
Production Testing Requirements
2. Does the factory have a constant temperature room? Can it adjust to high, medium, and low temperatures?
3. Do you know how to handle heat sinks? If one of the structure, optical components, or sensors has a problem leading to continuous testing and correction, it is also an unclear factor, and electronic design engineers may not be able to solve these issues.
4. Do you have clinical trial resources? (Look for hospitals; they have patients with and without fever to collect testing data over a period.)
5. If doing medical regulations, do you have domestic CFDA medical qualifications? Abroad, it’s called FDA and CE Medical (CFDA approval can take up to a year and a half).
Infrared Thermometer Design Scheme
Scheme Overview
Non-contact infrared thermometers are a new type of instrument that uses modern sensor measurement technology, microelectronics technology, and other technical means to measure the temperature of the object being tested. When the infrared thermometer is aimed at and brought close to the object being measured within the effective distance detected by the digital proximity sensor, pressing the power/measurement button and holding it for several seconds will allow the internal infrared sensor module of the thermometer to collect temperature and convert it into an electrical signal. This signal is then converted into a digital signal and transmitted to the microcontroller through a communication interface. The microcontroller uses the digital temperature sensor to collect the current environmental temperature and performs corresponding temperature compensation on the transmitted temperature digital signal, storing the corrected temperature as the current record number, while determining the LCD backlight range of the temperature: (35.0-37.4℃) normal body temperature green backlight, (37.5-37.9℃) elevated body temperature yellow backlight, (38-42.9℃) high body temperature red backlight. If the body temperature is abnormal, it will also sound an alarm and finally announce the current temperature. If not used for more than 30 seconds, it will automatically enter low-power mode to save energy.
Scheme Hardware Diagram
This non-contact infrared thermometer scheme consists of an ultra-low power, high-security MCU: RJM8L151, power module, button module, infrared sensor module, temperature sensor (module), digital proximity sensor module, environmental temperature sensor, voice module, speaker, and LCD display module. The hardware block diagram is as follows:
1. Microcontroller MCU
Utilizes the ultra-low power, high-security RJM8L151 series MCU launched by Wuhan Ruina Technology Co., Ltd. This chip has excellent operating and standby power performance, supports 4-channel IO wake-up of the chip, and integrates a 12-bit high-precision successive approximation ADC, a 2-channel multifunction comparator, timer, real-time clock (RTC), watchdog timer (WDT), programmable voltage detection (PVD), interrupt controller, hardware true random number generator, AES/DES/SM4 hardware encryption engine, and communication interfaces: UART, SPI, IIC, GPIO, ISO7816, JTAG, etc. It features high cost performance, large storage capacity, high security, low power consumption, and rich interfaces. The RJM8L151 series secure MCU hardware block diagram is shown below:
2. Power Module
Powered by 2 AAA batteries, using a voltage conversion chip to maintain the output voltage of the power module to ensure that the MCU, infrared sensor, digital proximity sensor, voice chip, LCD screen, and other modules work normally and stably even when battery voltage drops.
3. Sensor Module
A) Uses an integrated infrared sensing module, including an infrared thermopile sensor and a dedicated processing chip. The advantages are stable circuits, simple interfaces, and ease of debugging, while the downside is the higher price. Sensor signals can use MLX90614, HMS K1C1 F5.5, and 10TP583T, among others, as examples:
The infrared sensor MLX90614’s infrared sensing part is an 81101 thermoelectric unit, which receives the infrared radiation signals from the target object and then processes the signals through the internal DSP signal processing dedicated integrated chip MLX90302 with a 17-bit ADC for signal amplification and processing, outputting via PWM or SMBus (compatible with standard speed IIC) with a measurement accuracy of up to 0.2%, fast measurement speed, and low power consumption. MLX90614 supports sleep mode. The digital temperature sensor is mainly used to measure the ambient temperature to enhance accuracy by compensating the infrared sensor measurement, while the digital proximity sensor is used to detect the effective distance during temperature measurement.
B) Uses a thermopile sensor + peripheral circuit. The advantage is lower cost, while the downside is that small signal amplification and processing circuits, and temperature compensation circuits need careful design and debugging, resulting in a longer product design cycle. Sensor signals can utilize MTP10-B7F55, RTP678, etc., among which Sunnyqi’s MTP10-B7F55 can achieve fast, high-precision temperature measurement. It has an NTC for ambient temperature measurement to enhance accuracy, with flexible application methods, based on the following principles:
(a) Software temperature compensation (b) Hardware temperature compensation
(c) Analog interface (d) Digital interface
3.2 Digital Proximity Sensor
Used to determine the distance between the measured object and the PIR sensor, measurement is only activated under optimal distance conditions to ensure the accuracy of measurement data, such as TMD26721.
3.3 Environmental Temperature Sensor
Its role is to obtain the environmental temperature during measurement, allowing the infrared temperature sensor’s measurement value to exclude environmental temperature, thus deriving the measured human body temperature, compensating the infrared temperature sensor. Some environmental temperature sensors are integrated into thermopile sensors, such as MTP10-B7F55.
4. Button Module
The design includes 5 independent buttons: power/measurement button, SET button, UP button, DOWN button, and MODE button, with the first four buttons having sleep wake-up functions.
Power/Measurement button: Press the button to wake up from sleep, after which the MCU executes a temperature measurement, displaying the current temperature upon completion.
SET button: Press the button to wake up from sleep, entering function setting mode, each press corresponds to the following functions:
• F1 Display unit switch: Default is °C, pressing UP switches to °F, pressing DOWN switches back to °C
• F2 Voice broadcast switch ON/OFF: Default is ON, pressing UP keeps it ON, pressing DOWN switches it OFF
• F3 Delete temperature records
UP button: Scrolls up from the current temperature record until temperature record 32, scrolling again returns to temperature record 1
DOWN button: Scrolls down from the current temperature record until temperature record 1, scrolling again returns to temperature record 32
MODE button: Switch between human/body surface modes
5. Voice Module
Utilizes voice chip WTN1010/VC505 to store voice segments, such as ‘ten o’clock’, ‘℃’, ‘℉’, ‘one’, ‘two’, …, ‘nine’, ‘beep beep beep beep beep’, etc., playing selected segments to form complete speech, driving the speaker to produce sound.
6. LCD Display Module
Utilizes RFD-TA20001ZT-11 to display current temperature, current memory temperature, measurement mode, battery status, voice switch status, and other information.
Scheme Functional Specifications
2) Measurement distance: 3~8CM
3) Measurement modes: • Human mode: 32.0-42.9℃ (89.6-109.3°F)
• Body surface mode: 0.0-99.9℃ (32.0-211.9°F)
4) Measurement time: <6s
5) Measurement accuracy: ±0.2℃ (35.0°-42.0℃); ±0.3℃ (other ranges)
6) Display resolution: 0.1°C or 0.1°F
7) Display units: °C (Celsius) and °F (Fahrenheit)
8) Supports 32 groups of memory storage
9) Supports three-color backlight display
• Normal body temperature green backlight (35.0-37.4℃)
• Elevated body temperature yellow backlight (37.5-37.9℃)
• High body temperature red backlight (38-42.9℃)
10) Supports low battery voltage display
11) Supports voice broadcast
12) Supports automatic shutdown to save power
Advantages of RJM8L151:
1) Ultra-low power consumption
Up to 6 low-power management modes are available, with deep sleep power consumption at 5nA, external wake-up taking less than 5us, and supporting 4-channel IO wake-up. In Halt mode, power consumption can be as low as 0.4uA while maintaining RAM data, significantly extending battery life.
2) Built-in PVD programmable voltage detection
Can quickly stage detect and manage input voltage after battery boost.
3) Built-in 12-bit high-precision successive approximation ADC
Can perform real-time, accurate voltage acquisition of the battery for digital display and monitoring of battery usage status.
4) Rich communication interfaces
The chip supports UART, SPI, IIC, GPIO, ISO7816, etc., providing more communication options for peripheral devices for functional expansion.
5) 15-bit watchdog timer
Includes a 1-channel 15-bit watchdog timer that can reset the system in case of abnormal operation or system freeze, restoring normal operation and further enhancing system stability.
6) Memory security
High-strength physical protection circuits are designed for internal Flash, SRAM, and other storage units, effectively preventing malicious code theft and reverse analysis.
Copyright Statement:This scheme is an original article by CSDN blogger “1028_YangYang”, following the CC 4.0 BY-SA copyright agreement. Please include the original source link and this statement when reprinting.
Original link:https://blog.csdn.net/weixin_44246277/article/details/104365045
Digital Circuit Technology of Ear Thermometers
The principle of digital processing is to convert the signals from thermistors and thermopiles into electrical signals, amplify them, and collect data through A/D conversion by the microcontroller to obtain corresponding temperature values.
Complete circuit system
2. Clock Circuit
3. Reset Circuit
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