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This is a very simple to
implement Temperature Sensor. It uses LM35DT as a semiconductor
temperature sensor which operates with a +5 volt DC.
It produces an analog output
voltage, proportional to the change in surrounding temperature in
Celsius scale (2mv/C). The analog output of the sensor is then
passed to the ADC0804 IC which produces an 8-bit binary output
(digital output) correspoding to the analog input voltage. The
digital output from ADC is then used to glow the LED which
indicates the high/low logic (LED ON: Logic 0, LED OFF: Logic 1).
The output of the ADC can be
interfaced to a 7-segment diaply using a 7-segment driver or the
digital output can be interfaced to a PC / microcontroller. The
bottom portion of the schematic shows a fixed and a variable power
supply which inputs 220 volts AC from the wall outlet in your
house, the transformer then steps-down it to 18 volts AC (9-0-9
centre-tapped), which is then converted to DC using bridge
rectifier.
The fixed regulator IC
(7805) produces a +5 volts regulated output which is used to
operate the Sensor and the ADC0804 IC. It also outputs a variable
voltage controlled by a 5K variable resistor which is used to
adjust the scaling of the ADC0804 (normally for full scale, it is
set to 2.5 volts).
Further modification may
include an automatic control circuitry interfaced to the ADC which
automatically ON/Off the
device whose temperature is
to be control/monitor. The automatic control can be achieved by
OP-AMP based comparators or using
a
microcontroller/microprocessor.
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