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Overview
Body temperature checks are a common requirement at home, workplaces, public entrances, and during travel. Infrared body temperature detectors use non-contact measurement to help reduce transmission risk. This article describes the system and key design approaches for such detectors.
The block diagram below shows a system based on the MSP430 microcontroller family together with power management, amplifiers, and temperature-sensing components from TI.
Figure 1 System block diagram of an infrared body temperature detector
Main controller: MSP430 microcontroller family
The MSP430 microcontroller family from TI is a 16-bit ultra-low-power RISC mixed-signal processor first introduced in 1996. It is widely used in sensing and measurement end products because it integrates high-performance ADCs, LCD drivers, serial communications, PWM outputs, and other modules on-chip. Together with TI's online hardware and software resources, these devices can simplify development and accelerate prototype creation while saving PCB space.
Peripherals and features useful for thermometer design
- SAR ADCs or high-resolution sigma-delta ADCs on MSP430 devices, combined with TI amplifiers such as the TLV333, can sample the high-precision analog output of infrared temperature sensors and convert it to digital temperature values. The ADC can also monitor battery voltage in real time.
- On-chip LCD driver modules help implement LCD displays quickly. Devices such as the MSP430FR4133 include up to a 4×36 or 8×32 segment LCD driver with flexible segment and COM pin configuration to simplify PCB layout.
- I2C serial interfaces support high-precision digital temperature sensors, digital infrared sensors, and digital proximity sensors for auxiliary sensing and data input.
- On-chip timers can output multiple PWM channels to drive indicator LEDs, buzzers, and other indicators.
- GPIO interrupts enabled in ultra-low-power modes provide fast button response while maintaining standby power savings for battery-powered thermometers.
Low-power operation
The MSP430 family has emphasized ultra-low-power operation since its introduction. Because infrared thermometers are typically battery powered and may be used frequently, low-power operation is a key design requirement. MSP430 devices provide low-power peripherals and modes to address this challenge.
Memory and scalability
The MSP430 family offers on-chip memory starting at 16 KB to meet the needs of most thermometer products, and the product family scales up to devices with up to 512 KB of memory. That range allows designers to migrate designs between devices with minimal effort. Recommended MCU models are shown below.
Table 1 Recommended MCU models for infrared thermometers
Power management
The schematic in Figure 1 also includes a selection of power management, signal-chain, and sensor products.
The TPS61099 series of boost converters targets ultra-low-power applications. It offers a quiescent current of 800 nA and supports input voltages down to 0.7 V, enabling single-cell alkaline battery operation. For an input of 1.5 V and an output of 3.3 V at 10 μA load, the device can reach about 80% efficiency. TPS61099 series devices are available in adjustable and fixed-output versions; the fixed versions cover common output voltages from 1.8 V to 5.0 V.
The TPS62170 buck converter provides low IQ to extend battery life, especially when the system is idle. It supports switching frequencies above 2 MHz for high efficiency and to reduce the required inductor size, enabling a more compact solution.
Amplifiers and signal chain
The TLV333 operational amplifier is a zero-drift amplifier family with high precision and low power consumption. It features ultra-low input offset voltage (15 μV max) and low drift (0.02 μV/°C), which help minimize temperature measurement error. Its rail-to-rail input/output maximizes dynamic range. The device also offers low quiescent current (28 μA max), wide supply range (1.8 V to 5.5 V), and small packages (SC70 option), with an operating temperature range of ?40°C to +125°C. Dual and quad channel variants are available.
For systems requiring faster settling time and lower noise to speed up measurements, the OPA388 can be used as an alternative. It provides lower input offset voltage (5 μV max), lower noise (7 nV/rtHz), and faster settling time (2 μs), which help reduce settling time and the number of samples required to reach a specified temperature resolution.
TI offers multiple operational amplifiers for the interface between analog sensors and ADCs. The table below lists other amplifiers suitable for this design; all listed amplifiers are available in dual-package options.
Table 2 Recommended amplifiers for the signal interface
Temperature sensors
TI provides several temperature sensors. The high-precision digital sensor TMP117x offers ±0.1°C accuracy over ?20°C to +50°C and integrates a 16-bit ADC with I2C/SMBus communication. It targets battery-powered systems with low shutdown Iq (150 nA) and a conversion current of 3.5 μA at 1 Hz. For systems that use the MCU ADC, TI also offers analog temperature sensors and thermistors: LMT70 provides a voltage output corresponding to temperature with up to ±0.13°C accuracy between 20°C and 42°C. The TMP61 linear thermistor provides 1% temperature tolerance and simplifies calibration compared to traditional NTC parts. For cost-sensitive digital applications, the TMP1075 offers ±1°C accuracy from ?25°C to +100°C. The TMP23x analog sensors offer design flexibility with selectable accuracies and gains from ±0.5°C to ±6°C.
Low-noise analog rails
Sensitive analog and sensor circuits generally require a low-noise voltage rail. Low-dropout regulators (LDOs) are commonly used to provide clean, low-noise power for ADCs and sensors. For this purpose, the TPS7A20 offers ultra-low output noise (6 μVRMS), high ripple rejection (85 dB at 1 kHz), and low quiescent current (typical 6 μA, 150 nA in shutdown), making it suitable for low-noise analog rails while maintaining low standby current for battery-powered designs. The TPS7A02 is another option that provides nA-level IQ (25 nA, 3 nA in shutdown) with high PSRR, suitable for post DC/DC regulation, and it also delivers good transient response for pulsed loads.
Wireless connectivity and power gating
Some higher-end products include low-power Bluetooth Low Energy (BLE) connectivity. CC2640R2F ICs or CC2650MODA modules are commonly used for BLE integration. TI's SimpleLink software suite can assist in developing wireless features.
To reduce current draw, load switches with integrated fault protection, such as the TPS2051x family, or switches with ultra-low leakage like TPS22916xx, can be used to selectively disconnect a BLE module or other subsystems from the battery. This approach extends battery life while enabling additional functionality.
Conclusion
The devices and components described above can be used to expedite the design of infrared body temperature detectors, offering options for low-power boost/buck regulation, low-noise analog rails, precision amplifiers, and temperature sensors suitable for battery-powered handheld devices.
