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  • Human Skin Temperature Sensing Wrist Watch

  • Created: Sep 19, 2016

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This reference design introduces the use of LMT70 temperature sensor of Texas Instruments to measure human skin temperature in a wearable device. This wristwatch optimizes the sensor’s ability to accurately measure skin temperature. In addition, MSP430 family of ultra-low-power microcontrollers is also used in the circuit. It features a powerful 16-bit RISC CPU, 16-bit registers, and constant generators that contribute to maximum code efficiency. The digitally controlled oscillator (DCO) allows wake-up from low-power modes to active mode in less than 1µs.

The electrical design consists of a PCB, battery, LCD connector, and ADS1115 analog to digital converter, MSP430G2553 MCU and LMT70 temperature sensor. The MCU is responsible for converting the LMT70 data into an accurate temperature value, which will be displayed in the LCD screen. The remote PCB is a very small PCB with only the skin temperature sensing LMT70 and supporting thermal devices such as copper pads and thermal vias are used primarily to sense skin temperature. These two boards are connected with the J2 connector. The signal path of this device starts with two LMT70’s, one for measuring the skin temperature and another one measuring for the ambient/board temperature. Both devices are connected to two channels on the ADS1115. After the analog to digital conversion is complete, the data is sent through an I2C interface to the MSP430G2553 microcontroller for further processing.

Nowadays, there are lots of smart watches that go beyond timekeeping. This device is an example of a simple smart watch that offers temperature monitoring. It can be developed by adding features like having a portable media player or can run mobile applications using mobile operating system.