Introduction to Digital Electronic Design – Laboratory 13, Building a Digital Thermometer 1

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

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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

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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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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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FIGURE 2: THE DIGITAL THERMOMETER SCHEMATIC DIAGRAM