- Performs simple, rapid and accurate temperature measurement
- Equipped with temperature monitoring MCU, DC-DC converter, LCD display, thermopile and buzzer
Health monitoring at home and in public places has become a critical requirement these days. Due to the impact of the pandemic, the world has drastically changed in its operations in public places by checking health conditions through temperature monitoring. In such conditions, rapid measurement and accuracy play an important role.
Compared to traditional mercury thermometers and electronic thermometers, forehead thermometers have the advantages of performing simple, rapid and accurate temperature measurement. At the same time, forehead thermometers perform contactless measurement.
With this in mind, Holtek has developed the BH67F2742 which is a dedicated MCU for contactless infrared thermometer applications. The MCU Flash architecture facilitates production parameter adjustment, product calibration and data storage. Compared with traditional solutions, the integrated LDO, EEPROM, LED and LCD drivers, low noise Operational Amplifier and 24-bit A-D Converter reduce the requirements for large numbers of external components.
BH67F2742 has built-in LCD driver used to drive an LCD to display temperature. Buzzer and motor are used for vibration and sound output alert when the measurement is complete.
Forehead Thermometer circuit description
DC-DC Converter
The HT7733SA is a high-efficiency PFM synÂchronous step-up DC-DC converter. The device has the advantages of extremely low start-up voltage as well as high output voltage accuracy. It has a higher operating frequency and also offers the advantages of much reduced audio frequency noise. The devices require only three external components to provide a fixed output voltage of 2.7V, 3.0V, 3.3V, 3.7V or 5.0V. The HT7733SA devices include an internal oscillator, PFM control circuit, driver transistor, reference voltÂage unit and a high-speed comparator. These devices are available in space-saving 3-pin SOT89, 3-pin SOT23 and 5-pin SOT23 packagÂes. For 5-pin SOT23 package types.
LCD
This device contains an LCD Driver function, with internal LCD signal generating circuitry and various options, will automatically generate these time and amplitude varying signals to provide a means of direct driving and easy interfacing to a range of custom LCDs. For large volume applications, which incorporate an LCD in their design, the use of a custom display rather than a more expensive character-based display reduces costs significantly.
Thermopile
Thermopiles are designed to measure temperature from a distance by detecting an object’s infrared (IR) energy. The higher the temperature, the more IR energy is emitted. The thermopile sensing element, composed of small thermocouples on a silicon chip, absorb the energy and produce an output signal.
This device contains a high accuracy multi-channel 24-bit Delta Sigma analogue to digital converter which can directly interface to external analogue signals. PGA gain control, A-D converter gain control and A-D converter reference gain control determines the amplification gain for A-D converter input signal.
| Feature Category | Specifications | Unit | Remarks |
| Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Thermistor (NTC) | |||
| R25 | 100 ± 0.2 | KΩ | 25°C |
| B Value | 3950 ± 0.5% | K | |
| Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Thermopile | |||
| Output Voltage | 360 ± 30% | uV | Blackbody Temperature: 37°C
Ambient Temperature: 25°C |
| Thermopile temperature range | 75 ± 20 | KΩ | 25°C |
| Working Temperature range | -20~100 | °C | |
| Storage Temperature range | -40~100 | °C | |
Switch, Buzzer and Vibration
Switch is used for ON/OFF operation as well switch is used for multipurpose operations to control thermometer while taking readings. To indicate completion of reading, vibration and buzzer indication are provided to intimate user the completion of the reading process.
Advantages of Forehead Thermometer
A Forehead thermometer is a mercury-free, user-friendly, touch-free infrared type thermometer that can be often used at home. It can accurately measure a person’s temperature within a few seconds via the forehead by scanning the temporal artery blood flow. For this reason, it is a great tool to measure body temperature.
Additional features such as the sound/silent button or an LCD blink alarm are attached for a high-temperature alert. Memory and advanced measurement functions are available.
