Pre-calibrated silicon junction devices: LM34D (F) or LM35D (C) or LM335A(K) are very nice (10 mV per F or C or Kelvin). Cost about $1. It's cheap, accurate, small and calibrated. Its the best solution to this kind of problem, so long as you have an ADC channel available.
Non-calibrated silicon junction devices: LM34/35 type sensors can be constructed out of a diode or a transistor wired as a diode. You'd have to calibrate it (~ 1 mV per C?). Cost $.06. The second best solution. The difference in voltage drop across a diode at two known currents is also related to temperature and is independent of anything except the physical properties of the junction (ie - all silicon diodes give the same result). A practical circuit will involve a switchable current source and an amplifier. A quad package of low cost 324 opamps would very probably suffice for all the active electronics.
Thermistors: used in a voltage divider circuit. Cost about $.50 for the thermistor. You'd have to calibrate it and program a linearization routine. It you have to do it with a digital pin, thermistors are easier because you can read it as a capacitor charge time.
Characterizing a thermistor with the Steinhart-Hart equation
Thermocouple wire: (type T is easiest). You'd have to have an amplifier with a gain ~ 200. Cost of the wire $.01 for a 1 cm piece. Type K & N can stand 1000 deg C. in the right atmosphere and packaging. Use a lot of shielding as they are very sensitive to noise
Home made thermocouple: You can twist a copper and iron or other metal wire together to make a thermocouple. You'd have to calibrate it. It will rust.
Microchip PIC uP Temperature senseing
Ubicom SX uP Temperature sensing
The method is based on the same principle which the band gap reference is based: The CHANGE in voltage across a silicon junction with a known change of current is a property solely [tm] of temperature and the properties of silicon.
For practicle purposes, this is: Delta V_junction = K.T.ln(Ihigh/Ilow) where T = absolute temperature. As Ihigh and Ilow are fixed this reduces to: Delta Vjn = K.T
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