Light dependant resistors have a very high reistance when it is dark. The resitance decreases when the amount of light falling on it increases. So when it is light the low reistance(light) means a low voltage output when there is a high reistance (dark) then there is a high voltage output.
Thermistors have switch on when they have a high voltage output or a high reistance (cold). When the tempriture rises the reistance drops and so does the voltage output. Thus causing the heating to switch off.
A data logger can be used with this to recode a continueous stream of tempritures.
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