One of the most commonly used medical instruments nowadays is the thermometer. The thermometer is used to monitor or measure the body temperature of a sick person. The idea of creating a thermometer started from a device called thermoscope, a thermometer without a scale. Several inventors developed it until Sir Thomas Allbutt invented the first practical 6-inch medical thermometer able to sense a body temperature in five minutes. The development of the thermometer did not stop there and today, digital thermometer exists which is faster and very accurate.

This reference design is an example of a low cost non-contact digital thermometer. It only uses a microcontroller, a four digit seven segment display and an infrared (IR) temperature sensor. The concept of this design is to make the IR sensor measure the temperature of the thermal radiation emitted by the body being measured. The data acquired by the sensor will be sent to the microcontroller through the I2C bus. The microcontroller will analyze the data and then shows the body temperature on the four-digit seven-segment display.
The circuit of this reference design uses few components only and is very easy to understand. However, to make the circuit function accurately, software calibration must be implemented carefully. The whole circuit is powered by a 5V DC power supply regulated from the four 20mm coin shape batteries contained in a 120591-1 TE Connectivity battery holder. The batteries are connected in series-parallel connection to produce a 6V 480mAh source of power. With the help of a low-dropout voltage regulator, the 6V is regulated to a 5V DC supply.
For more detail: Non-Contact Body Temperature Meter