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Sometimes things simply don't work, despite everything looks good. In such cases it's the best idea to work step by step to find the cause.
Resistors change their resistance with temperature. For fixed value resistors this effect is unwanted. Thermistors take advantage of this effect and do so in a predictable fashion.
A typical thermistor circuitry is shown to the right. About all RepRap controllers use the same design.
Connecting components this way gives a voltage on the signal line which depends on the thermistor's resistance and accordingly, on the thermistor's temperature. If the thermistor is very hot, its resistance is near zero, so the signal's voltage will be close to zero. If the thermistor is very cold resistance is very high, signal's voltage will beclose to the supplied voltage, 3.3 volts (or 5 volts on other controllers). In between is our measurement range.