The resistance of a thermistor experiences significant, consistent variations according to temperature. A circuit that automatically monitors, processes and reports these changes in resistance can function as a digital thermometer. A circuit like this could theoretically be built on a breadboard, but the accuracy and reliability of the measurement would be compromised, and the breadboard connections would be painfully complex. Consequently, a digital thermometer would most likely be fabricated as a printed circuit board.
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Things you need
- Analogue-to-digital converter
- LCD controller
- LCD display
Make a voltage divider out of the thermistor and a resistor by connecting your low-voltage supply (Vcc) to one side of the thermistor, connecting a ground to one side of the resistor, and connecting the open leads of the thermistor and resistor. The output of the voltage divider (Vout) is taken where the thermistor and resistor are connected. The resistance of the resistor should be similar to the resistance of the thermistor at room temperature.
Connect the output of the voltage divider to an analogue-to-digital converter (ADC). The data sheet for the ADC should suggest appropriate circuitry for conditioning and filtering the input signal. At a minimum, the input signal must be buffered so that the resistance of the voltage divider does not interfere with the ADC conversions.
Route the output of the ADC to a microprocessor. The microprocessor must determine the resistance of the thermistor according to the following formula: Rth='[R(Vout'Vcc)/Vout], where R is the resistance of the resistor and Rth is the resistance of the thermistor. You know Vcc and R, and Vout is an input, through the ADC, to your microprocessor.
Use the resistance of the thermistor to calculate its temperature. Refer to the documentation for your thermistor. Most thermistors are of the negative temperature coefficient (NTC) variety. NTC thermistors have an extremely complicated mathematical relationship between temperature and resistance. Your microprocessor might be able to adequately perform this calculation, but if not, consult the data sheet or the thermistor manufacturer for a simplified formula that can provide satisfactory accuracy. You can also calculate the temperature-versus-resistance values on a PC and store them in your processor as a lookup table.
Send the temperature data to a liquid-crystal display (LCD) controller chip, such as the ICM7211, and use the LCD controller to drive a simple seven-segment LCD display.
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