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Introduction to Digital Electronic Design – Laboratory 13, Building a Digital Thermometer
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LABORATORY EXPERIMENT #13
DESIGNING A DIGITAL THERMOMETER
OVERVIEW:
In this laboratory experiment you will design a Digital Thermometer. Your
digital Thermometer will have a HEXIDECIMAL read out that you will
calibrate into a HEX to Degrees C table so you can understand the
temperature read out of your thermometer. Figure 1 shows a block diagram
of your thermometer with all of the necessary blocks to sense the
temperature, convert it to a proportional voltage, scale the voltage and
remove any DC offset error voltage, convert it to a digital quantity, and to
display it on your Circuit Design Trainer Hexidecimal Displays. How the
system works is explained below in the THEORY section. After you have
successfully built the Digital Thermometer, you will record the readout when
the probe is in boiling water. Then when the probe is in ice water. Finally you
will record the readout at room temperature and use the three reading to
generate a calibration curve. From the calibration curve, you will generate a
table that records the HEX read out for all temperatures from 0 degrees C
(Water Freezing Temperature), to 100 degrees C (Water Boiling
Temperature).
FIGURE 1: BLOCK DIAGRAM OF THE DIGITAL THERMOMETER
Introduction to Digital Electronic Design – Laboratory 13, Building a Digital Thermometer
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THEORY:
The sensing element is a thermistor with resistance to temperature range of
approximately 180K ohms at freezing to 2.5K at boiling. When connected in
series with a 180 K ohm fixed resistor, the output voltage as a function of
the thermistor resistance (or the temperature), is:
(V+) 180K
Vout = --------------------Rth + 180K
This is the voltage divider equation. Vout is connected to the input of an
LM324 operational amplifier set up with a voltage gain of 2 and a DC offset
of -2.5 Volts. This tailors the Analog voltage to go from 0 volts to 5 volts.
The 0 to 5 Volt Analog signal goes into a voltage comparator where it is
compared to a voltage generated by the Binary count. An 8 Up/Down
Counter is output to an 8 bit data latch where the correct count will be
latched into the display and simultaneously to go to a Digital to Analog
converter. This DAC is made out of resistors whose weighed values are 1, 2,
4, 8, 16, 32, 64, & 128 times each other. They are followed by an
Operational Amplifier in a voltage follower design to prevent loading the
network and distorting the output. This output go to the other side of the
voltage comparator. When the count generates a voltage that is slightly
higher than the conditioned Analog voltage, a flag out of the comparator
latches the value into the display latch and causes the counter to now count
down until the count equivalent voltage drops below the conditioned Analog
voltage. In this way, the ADC locks onto the Analog signal and tracks it as it
goes up or down. Figure 2 shows the final design that you will build and test.
PROCEDURE:
1.
Obtain the parts necessary to build the Digital Thermometer.
(Note: everything you need should be in your Digital Circuit Design
Laboratory Parts Kit L.)
DIGITAL THERMOMETER PARTS LIST:
RESISTORS: (ONE EACH ) 100, 220, 390, 820, 1.5K, 3.3K, 6.8K, 15K,
100K, 220K ohms, 1/4 Watt, and a 100K Potentiometer
THERMISTOR: 2K to 100K NEGATIVE TEMPERATURE COEFFICIENT
THERMAL RESISTOR
Introduction to Digital Electronic Design – Laboratory 13, Building a Digital Thermometer
INTEGRATED CIRCUITS:
2 - 74LS169 UP/DOWN COUNTERS; 2 - 74LS174
HEX DATA LATCHES
1 - LM324 QUAD OPERATIONAL AMPLIFIER
MISC. COMPONENTS: TELEPHONE HOOK UP WIRE. .5 CM X 10 CM STRIP
OF PRINTED CIRCUIT BOARD MATERIAL TO MOUNT
THERMISTOR. 1" PIECE OF 1/4" SHRINK TUBING.
SOLDER
2.
SOLDER THE THERMISTOR ONTO THE PRINTED CIRCUIT BOARD
MATERIAL, ONE LEAD ON EACH SIDE OF THE BOARD.
3.
HEAT SHRINK THE SHRINK TUBING OVER THE PRINTED CIRCUIT
BOARD MATERIAL AND THE THERMISTOR TO SEAL IT FROM WATER.
(FURTHER SEAL IT WITH EPOXY IF IT IS NOT WATER TIGHT.)
4.
TWIST TWO LENGTHS OF TELEPHONE WIRE OF 1 TO 2 FEET
LENGTHS. STRIP THE ENDS AND SOLDER TO THE OPPOSITE ENDS OF
THE PRINTED CIRCUIT BOARD STRIP WITH THE THERMISTOR. (THIS
WILL MAKE A NEAT LITTLE TEMPERATURE PROBE.)
5.
STRIP THE OPPOSITE ENDS OF THE TWISTED TEMPERATURE PROBE
WIRES ABOUT 1/4".
6.
FOLLOW THE SCHEMATIC DIAGRAM IN FIGURE 2 AND BUILD THE
DIGITAL THERMOMETER EXACTLY LIKE IT IS SHOWN.
7.
ADJUST EH VOLTAGE AT R11 TO GROUND TO BE CLOSE TO 2.5
VOLTS.
8.
IN THE TABLE BELOW, CALIBRATE YOUR DIGITAL THERMOMETER AS
FOLLOWS:
(NOTE: FOR EACH READING BELOW, MEASURE AND RECORD THE VOLTAGE
ACROSS R10 AS YOUR ANALOG INPUT VOLTAGE.)
A.
WITH THE THERMOMETER POWERED UP, RECORD THE ROOM
TEMPERATURE FROM A CALIBRATION THERMOMETER AND RECORD
THE HEXIDECIMAL DIGITAL READ OUT.
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Introduction to Digital Electronic Design – Laboratory 13, Building a Digital Thermometer
B.
OBTAIN A PLASTIC CUP FULL OF ICE WATER AND INSERT THE
THERMISTOR PROBE. BE CAREFUL NO TO WET THE CONDUCTIVE
SURFACE OF THE PROBE LEADS OR PRINTED CIRCUIT BOARD. THIS
WILL DISTORT YOUR READINGS. AGAIN RECORD THE RESULTS.
C.
IN A COFFEE CUP, USE A SOLDERING IRON TO HEAT HATER TO
BOILING. AGAIN MEASURE THE HEXIDECIMAL READ OUT AND
TEMPERATURE.
D.
DRAW A CURVE OF THE TEMPERATURE VERSUS THE VOLTAGE
ACROSS R10.
E.
CALCULATE THE CHANGE IN THE HEXIDECIMAL READINGS AS A
FUNCTION OF THE CHANGE IN TEMPERATURE.
F.
MAKE A 256 STEP OR COUNT TEMPERATURE TO "HEX" CONVERSION
TABLE.
G.
USE YOUR TABLE TO MEASURE THE OUTSIDE TEMPERATURE.
CONCLUSIONS:
Speculate on how you might improve the accuracy of your digital
thermometer without increasing the number of bits. How might you speed
up the conversion time. How can you make it read out directly in degrees C
or F? Also explain briefly things you learned about design, troubleshooting,
etc.
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Introduction to Digital Electronic Design – Laboratory 13, Building a Digital Thermometer
FIGURE 2: THE DIGITAL THERMOMETER SCHEMATIC DIAGRAM
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