Although not a commonly-know n fact, it's possible to detect the change in temperature using a power diode. How is that possible? As a silicon diode is made from two different types of silicon, there is a change in potential voltage across the two - which alters with the temperature. Using this interesting fact, Leon Harris has documented how this is possible, but also created a lesson plan for use in schools to teach the concept.
From an electronics perspective a typical silicon diode is placed across two of an Arduino's analogue input pins - and then by using an internal pull-up resistor on one pin a small, constant current power source is enabled. By then measuring the voltage from this source through the diode - and calibrating the resulting values against known temperature - it's possible to create a very simple thermometer.
If you're interested in measuring termperature, albeit with a more reliable sensor - consider using our TEMP: DS18B20-based temperature sensor module:
The TEMP: uses the Dallas DS18B20 1-wire digital temperature sensor, with a wide measurement range of -55 to +125°C at an accuracy of +/- 0.5°C. For more information, tutorials and to order - visit the TEMP: page.