DIGITAL THERMOMETER (8051)
Thermometer using 8051.
This article is about a simple 0-100°C
digital thermometer with 1°C resolution using 8051. The circuit is based
on LM35 analog temperature sensor, ADC0804 and AT89S51 microcontroller.
LM35 is an analogue temperature sensor IC which can measure a
temperature range of -55 to 150°C. Its output voltage varies 10mV per °C
change in temperature.
For example, if the temperature is 32°C,
the output voltage will be 32 x 10mV = 320mV. ADC 0804 is used to
convert the analogue output voltage of the LM35 to a proportional 8 bit
digital value suitable for the microcontroller. The microcontroller
accepts the output of ADC, performs necessary manipulations on it and
displays it numerically on a 2 digit seven segment LED display.
Out put of the LM35 is connected to the
+Vin (pin 6) of the ADC0804. Resistor R13 and preset R14 is used to
provide an external reference voltage of 1.28V to the Vref/2 pin ( pin
9) of the ADC0804 and with this reference voltage, the step size of the
ADC will be 10mV and span will be 0-1 V. This means that for a 10mV
input the digital out of ADC will be 1 (1 in decimal also), for 20mV it
will be 10 (2 in decimal), for 30mV it will be 11 (3 in decimal) and so
on. The microcontroller accepts this data and puts it on the seven
segment display.
Circuit diagram.
Digital thermometer using 8051
Digital out of the ADC (D0 to D7) are
connected to P1 (P1.0 to P1.7) of the microcontroller. This is the line
through which the microcontroller accepts data from the ADC. The control
pins CS, RD, WR and INTR are connected to P3.7, P3.6, P3.5 and P3.4 of
the microcontroller. This is the data path through which the
microcontroller sends chip select (CS), read (RD) write (WR) signals to
the ADC and receives INTR signal from the ADC. Data lines (a to h) of
the multiplexed seven segment display are interfaced to P0 (P0.0 to
P0.7) of the microcontroller. Activation signals for the segment driver
transistors Q1 and Q2 are available from P3.2 and P3.1 pins of the
microcontroller.
Before attempting this circuit go through these articles Voltmeter using 8051 , Interfacing ADC to 8051 , Interfacing seven segment display to 8051.