Temperature Measurement Device

Temperature is a physical quantity, which needs to be monitored for various applications. The design was part of a larger project where the data of humidity and temperature was to be recorded using a 68HC11 microprocessor over a week. The temperature sensor need to be within +/- 0.5 °c and function properly within the range of +30°c and -20°c.

Instrumentation Design

Various temperature sensor designs such as Thermistor, and DigitKey IC were considered. However, the diode base temperature design was selected. Linearity of the device, and simple software requirements were the key advantages, while relatively more components and cost were the disadvantages.

Step1: Establishing physical quantity and electrical quantity relationship
For sensor design the first step is to clearly establish the relationship between the physical quantity and the measuring system. Here, an integrated chip which uses the temperature sensitivity of PN junciton is used. Temperature -->Temperature Sensor (LM352)

Step2: Signal Conditioning
In order to use the signal information the signal from the measurement device need to be "conditioned". Amplification and linearity correction are two typical conditioning applied. Here, amplification is sufficient.

Step3: Analog to Digital Conversion
The conditioned signal is analog and need to be converted to digital, before microprocessor can use it. Depending on the accuracy of the measurement required, the resolution of the analog to digital converter need to be selected. Here, an eight bit analog digital converter was used. Moreover, the signal and digital values were mapped as follows:

The unconditioned voltage of Vi=3.03 volts maps to conditioned voltage of Vo=0.1178 volts and 6 digital value, which in terms correspond to 30 °C.

Step4: Storage and manipulation using a microprocessor

The final hardware design is as follows:

Temperature --> Temperature Sensor --> Signal Conditioning --> Analog to Digital Conversion --> Microprocessor (Storage and Manipulation)

 

Design Assessment

Various limitations of op-amp need to be considered and taken into account for the circuit. The final results were adequate and can be summarized by the following table.