# Temperature Monitor Report by hcj

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```									                                    Temperature Monitor Report
Team: David Smith (des28@dana.ucc.nau.edu)
Submitted 11/23/99
2:20PM Tuesday class

Goal: To build a temperature sensor with visible LED gauge and flashing
alarm.

Block Diagram:

Thermistor                   Voltage              Rising                            Comparators      Final red
loses                      divider             voltage                            light LED’s      LED is
resistance as                 changes             triggers                             to visually  connected to
temperature                 resistance to       comparators                          represent the a 555 timer
rises                      voltage           one by one                               rising   to flash when
voltage        active

Circuit Diagram:
+5V
Thermistor

+5V
10,000 
100 

+5V                 +5V             +5V                                +5V             +5V
~5358                    1M       100                      1M                      100                      1M 

LM339            IRF513             LM339                           IRF513            LM339                   IRF513

+5V                                                +5V
+5V
~4570                              ~4848                                             ~5160 

+5V
+5V
1M                                                                      1,000 
+5V
100          1                 8            330 F
555 Timer

2                 7
LM339            IRF513                 3                 6
+5V       4                 5              1,000 
+5V

~5520 
Description: The voltage divider, in the upper left corner of the circuit
diagram, is used to translate the resistance of the thermistor to voltage.
The 5-volt DC current passes through the thermistor and its
compensating resistor, and then through a series resistor to ground.
This creates a voltage divider. The node above the thermistor is 5V,
the node below the series resistor is 0V, and the node in the center is
somewhere in between, depending upon the resistance of the
thermistor. The thermistor is on the upper side of the voltage divider
and not the bottom so that voltage will rise as temperature rises, and
as the thermistor’s resistance drops.

The voltage from the voltage divider is then distributed to four
comparators. The comparators then compare that voltage with a set
voltage created by using multi-turn potentiometers as voltage dividers.
If the voltage from the thermistor is greater than the set voltage, the
comparator switches. When the comparator switches, the current
stops flowing from the pull-up resistor through the comparator to
ground, and flows to the MOFSET. When the MOFSET receives
power from the comparator, it switches and opens a path from the
remainder of the circuit to ground. In the first three comparator-
MOFSET blocks, the remainder of the circuit is just an LED, which
then lights.

The fourth comparator-MOFSET block is connected to a 555 timer.
This timer is configured to blink the fourth LED at a rate of about two
blinks per second. This, of course, only occurs when the fourth
MOFSET has been switched, since the MOFSET is it’s only access to
ground.

Design Approach: The first item to decide was the temperature range to
have the experiment sensitive to. 60-90F was decided upon because
it would be the easiest range to generate in the lab for demonstration
purposes. Then, the resistance value of the compensating resistor for
the thermistor was calculated. The actual result was 10,700, but due
to a lack of potentiometers, the impracticality of a decade resistance
box, and the relatively low need for accuracy, this was rounded to
10K. The resistance value of the lower resistor in the voltage
divider was then calculated. Because the temperature range was from
60-90F, it was safe to assume 75 as a good average temperature. In
order to get the greatest precision, this average temperature would
have to create a voltage of 2.5V on the voltage divider. Simply
figuring out the resistance of the thermistor and compensating resistor
would complete the calculation. Setting the bottom resistor equal to
that value gave the desired voltage at that temperature.

The next calculation was the reference voltage for each of the
comparators. Since the desired result was for the first comparator to
switch at 60F, and then the next three at 10 intervals, I had to
calculate the voltage coming off the voltage divider at those
temperatures, and then set the reference voltages to match. I used the
equation, T=100-70*(Runknown-R100)/(R30-R100). I plugged in T
and solved backwards for the unknown resistance. Then I used the
voltage division equations to calculate the voltage coming off the
voltage divider at each switching temperature. Then, I needed to
calculate the settings for each LED’s potentiometer. I used the ratio
of the calculated voltage over 5 and set it equal to the desired
resistance over 10,000 (the max of the potentiometer).

The pull-up resistors were set at 1M because the circuit they are on
is only used for voltage, and any lost current is wasted. I put a 100
resistor in series with every LED to take a little bit of the load off the
LED and MOFSET without dimming the LED too badly.

As for the two resistors and the capacitor on the timer, it was more or
less trial and error. As I only knew the concept of the purpose of
those components, and not the method for calculating their exact
values, I picked one set to start with, and then worked from there until

Test Approach: I had the majority of the test supplies I needed in my dorm
room. I used an old AT computer power supply to provide good
constant unlimited 5VDC power to operate the circuit, and I have
access to a multimeter. I used the multimeter to test the actual
voltages and resistances of the circuit (I didn’t bother with the
capacitor). As each subsection was assembled, it was tested. Due to
the lack of an accurate thermometer, the accuracy of the temperature
readings may be slightly less than perfect, but the circuit did function
exactly as expected, under estimated temperatures. As the voltage
passed each turn-on voltage, each comparator did switch and each
LED properly lit.

Data:
Temp.              60         70             80             90
Resistance of      6366       5694           5022           4349
thermistor/comp.
Divider Vltg.      2.285V      2.424V          2.58V           2.76V
LED Pot.           4570       4848           5160           5520
Setting

Resistance of compensated thermistor at 30F: 8383
Resistance of compensated thermistor at 100F: 3677

T=100-70*(R-3677)/(8383-3677)

Voltage/5V=Resistance/10,000

Conclusions: The experiment went better than I had originally expected.
Most parts functioned on the first try. Those that didn’t weren’t hard
to fix. I did not have adequate equipment to tell exactly if the LED’s
were switching on at the right time, but they did work as they should
conceptually, and they followed estimates very closely. Discrepancies
could have originated from the use of a rounded resistor for
compensating, error or change in the potentiometers, the fact that the
voltage differences being measured were very small, or error in the
parts. What caused me the most trouble was calculating the correct
potentiometer values for each LED, and figuring out the correct
values for the timer. If I were to do this again, I would probably get a
more accurate thermometer for verifying accuracy of the circuit, I
would calculate the exact values for the timer, and I would strive for
more accuracy in the “cut corners” due to lack of parts.

```
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