Digital Thermometer using

  
The most common appl icat ion for the NTC thermistor is temperature measurement. Accurate temperature measurement can easily be accomplished by interfacing a Wheatstone Bridge, 6K/30K ohm thermistor network and a digital voltmeter integrated circuit as illustrated in Figure 5. The IC consist of an analog to digital converter with built-in 3-1/2 digit LCD driver providing resolution of 0.1°C. Using the 6K/30K ohm thermistor network makes it possible to achieve an overall system accuracy of ±0.4°C from 0°C to 100°C. This digital thermometer can easily be interfaced with additional circuitry to provide a temperature control circuit with a digital display.
Digital Thermometer using  - schematic

The advent of low cost micro controllers used with precision interchangeable NTC thermistors, provides the design engineer with unlimited design possibilities for temperature measurement and cont rol systems. These systems are relatively inexpensive to produce yet offer very high temperature accuracy and var ious software cont rol led outputs. For example, a micro controller system utilizing remote thermistor sensors can monitor and control the temperature in several locations in an office building. For this case, the micro controller is comprised of a built-in microprocessor, analog to digi tal conver ter, RAM and several digi tal inputs/outputs. The complete system Figure 6 utilizes the micro controller, multiplexer, EPROM, digital display, keypad and display driver. The micro controller is programmed in assembler language. The temperature measurement is calculated within the micro controller using the resistance versus temperature algorithm and the a, b and c, constants for the specific thermistor resistance and curve material. Refer to the Steinhart Equation on page 5. An alternative method to convert the thermistor resistance to temperature is to program a lookup” table in EPROM. After programming, the micro controller tells the multiplexer to send back temperature data from a particular zone (room in the office building) and converts the resistance of the thermistor into a...



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