Using 1N4148 As A Temperature Sensor

[yamadanao1]

Can I know which component senses heat or acts as heat sensor in the following circuit? In the given diagram, it is said that the 1N4148 diode acts as the sensor. But basically it is a zener diode and I don't think that it is able to sense temperature shift. This circuit provides a working model but the sensor remains as a mystery for me. Can you please help it for me?

Then how to test 1N4148?

 

[audioguru2]

A zener diode is used with reverse bias voltage. A temperature sensing diode is used with forward voltage that is completely different.

Look at the datasheet of a 1N4148 diode. Its forward voltage is shown at -40 degrees C, at +25 degrees C and at +65 degrees C. The forward voltage drops as the temperature of it rises.

Test it at a current of 0.1mA to 10mA at different temperatures.