TEMPERATURE INDICATOR
ABSTRACT
Temperature
measurement is now more common than ever before. Just look around and you will find even
air-conditioners having built-in temperature indicators. Also, with a
temperature probe, you can measure how hot is
the water for bath. Temperature is measured either in degrees Celsius or
Fahrenheit , though the former is now standard .The ambient temperature keeps
varying during different times of the day and night of any place.
Here we describe a
temperature indicator using AT89C2051 microcontroller, temperature sensors LM35
and LM34 and other components. LM35 and LM34 are precision integrated-
circuit temperature sensors, whose output voltage is linearly proportional to
the Centigrade and Fahrenheit
temperature values, respectively. The rate of change of the output voltage of
both sensors with respect to the rise in
temperature is 10 mV/degree. LM35 and
LM34 have a full range of –55ºC to
+150ºC and –50°F to +300°F,
respectively.
Microcontroller AT89C2051 does not have any
analogue-to-digital converter to read the analogue signals (milli volts) as
digital equivalents (digital numbers). However, it possesses two pins
that have the feature of a precision analogue to voltage comparator. The comparator compares
the two signal voltage levels at this
pins. When one of this pin goes high with respect to another pin
microcontroller will make comparator
output as high.
This
technique is used to measure the voltage
rise due to the rise in sensor temperature. It is possible by applying the
increasing voltage to one pin and the sensor’s output voltage to the other pin.
When the rising voltage crosses the sensor output voltage, pin 3.6 of
microcontroller AT89C2051 will go high. Thus the instruction will understand that
the voltage of the rising signal is just above the temperature sensor
For temperature
measurement, it was important to know the value of the rising signal voltage,
which will give the value of the equivalent temperature at that moment. This is
done with the technique of charging a capacitor with constant current. When you
charge a capacitor with constant
current, the voltage will rise linearly. Synchronization of starting of the capacitor-charging cycle and
measurement of voltage and time elapsed is done through microcontroller and
analogue switch .
Microcontroller’s
one internal hardware timer counts every microsecond once it is started. This
timer was started after open the
capacitor-shorting switch and thus reckon time in microseconds till its pin 3.6
goes high. That tells you the value of milli volts in numerals.Continuous shorting and
de-shorting of the capacitor for charging
and discharging displays the
temperature measurement value online.
For this continuous activity of shorting and de-shorting, an electronic switch
was used.
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