Download THERMOMETER KIT: PART NO. 2191242 Time Required: 1 to 2

THERMOMETER KIT: PART NO. 2191242
Time Required: 1 to 2 hours | Beginner Level
We always use thermometers and hear about what the temperature is, but often times we do
not think about how we actually know the temperature. Many thermometers use different means
of measuring temperature (like a mercury thermometer), but many new thermometers we use
today measure a change in resistance within a circuit. This is how the thermometer in this kit
works.
How does this circuit work?
This circuit works based off of a variable resistor called a Thermistor. As temperature increases
or decreases, the resistance of the Thermistor changes. In this circuit, the Thermistor has a
negative temperature coefficient (NTC). This means that as the temperature rises, the
resistance of the Thermistor decreases by a certain percentage every unit of temperature. This
is useful because, in our circuit, we have a potentiometer in series with the Thermistor and, as
the resistance of the Thermistor drops, the voltage drop of the potentiometer rises in relation to
the voltage drop of the Thermistor. This is because of a certain give and take between
components in a series configuration. In series, the components together have a voltage drop
equal to the power sources voltage. However, each component drops only its share based off of
resistance of the voltage. For example, our potentiometer and Thermistor have a combined
voltage drop of 9v from the battery. When the potentiometer is at 5,000 Ohms and the
Thermistor is at 5,000 Ohms, each component drops 4.5v. Now if the temperature increases,
the Thermistor’s resistance may drop to 2,500 Ohms while the potentiometer is still 5,000
Ohms. This means that the potentiometer will now drop 6v, a 1.5v increase. We can then
measure the voltage drop across the potentiometer to see changes in temperature. The LED
array is built into a voltmeter circuit using the LM3914. The two potentiometers can be used to
calibrate the readout and sensitivity of the LED array.
The green LED is simply used to state if the circuit is on or off.
The schematic for our thermometer circuit is:
What is in this kit?
In this kit, you will receive all of the components needed to build the circuit and a PCB designed
for this circuit. The components include 2 potentiometers, 12 resistors, an LED array, a LM3914
driver, a green LED, a Thermistor, a 9v battery, and a 9v battery plug. The PCB is about 1 inch
by 5 inches and is the rough shape of a normal oral thermometer (but do not use this circuit as
one).
What else will you need?
RequiredThis circuit requires a lot of soldering. It is necessary to have a soldering iron with a fine tip to
get into the small spaces. Other than soldering supplies, you will need a pair of wire cutters to
clip the excess ends from the leads of components after soldering. Examples of the soldering
equipment and wire cutters you will need are:
Description
(P/N)
Qty.
1 Soldering Iron
116572
1
2 Soldering Iron Stand
36329
1
3 Solder
170457
1
4 Wire Cutters
34658
1
RecommendedIn the PCB directly above the two potentiometers, we added a small notch to keep a rubber
band from slipping if you decide to use one to hold your battery. We have noticed that a small
rubber band or other way of securing your battery to the PCB makes the thermometer much
more usable.
We also recommend having a multimeter on hand because it is handy if trouble shooting is
necessary. One multimeter we recommend is Jameco P/N 1928476.
Helpful Tips
1. LEDs have a specific direction they go (long side to positive). This is important because
if the components are installed backwards, the circuit will not work and pieces may fail.
2. Do not trim leads before board is soldered and tested.
3. Be very careful of what is touching and what should be touching.
4. Keep you soldering nice and neat, act deliberately. Soldering is a skill mastered over
many years. Don’t get frustrated it your soldering is not a piece of art just yet.
5. Solder the smallest components first. This is much easier than soldering larger
components in first and then squeezing in the small pieces.
6. Don't solder things outside of the PCB first. I know this is obvious, but if you accidentally
damage a component, the whole circuit will not work as designed and the component
may not fit into the PCB.
What can you measure with this thermometer?
While you can measure anything with temperature, this thermometer works best when being
exposed to room temperature and then something that is much different in temperature (either
hot or cold). Keep in mind that the Thermistor takes a few seconds to reset, so you will see a
gradual changing in the display when new temperatures are presented. In testing, the quickest
ways we have been able to change the output of the LEDs is to pinch the Thermistor in our
fingers or set the entire thermometer in the refrigerator or freezer. Maybe you can keep yours
near a nice hot (or cold) cup of coffee or hot chocolate to know when the temperature is just
right?
Remember to be creative with how you use your thermometer, but always remember what is
safe to do with the thermometer. To be safe, do not use the thermometer above 50 degrees
Celsius (122 degrees Fahrenheit). Also, like all circuits, keep the thermometer dry at all times.
Directions
It may become useful to refer to the pattern for the PCB after you have started soldering:
*NOTE: The silkscreen pattern for the LED array is reversed (the notch should be on the
opposite side). Install the LED array with the notch near the outside of the PCB.
Step 1Check to make sure you have all of the pieces for the kit. You should have 12 small resistors, 2
potentiometers, a Thermistor, a green LED, a LM3914 chip, a 10 LED array, a DIP switch, a 9v
battery clip, and a PCB.
Step 2Insert the components and solder them into place one by one. Insert the components on the top
of the PCB and solder on the bottom. The placements for the components on the PCB are:
BAT1
9v Clip Leads
D1
Green LED
100k Ohm
Potentiometer
R1
R2
10k Ohm Potentiometer
R3
220 Ohm Resistor
R4
220 Ohm Resistor
R5
220 Ohm Resistor
R6
220 Ohm Resistor
R7
220 Ohm Resistor
R8
220 Ohm Resistor
R9
220 Ohm Resistor
R10
220 Ohm Resistor
R11
220 Ohm Resistor
R12
220 Ohm Resistor
R13
1k Ohm Resistor
R14
330 Ohm Resistor
R15
5k Ohm Thermistor
U1
LM3914
U2*
LED Array
U3
DIP Switch
Note: Make sure you have the LM3914, LED, LED Array, and battery placed in the PCB in the
correct direction to their polarities. The positive side (anode) of an LED is the side with the
longer wire lead. The positive side of the LED array is the long side with the notch taken out of
the corner. The red wire from the battery pack is positive. To identify the pins of a LM3914, look
at it with the pins facing away from you and the side with writing facing you. Rotate the LM3914
so that the notch in the plastic is at the top of the component. The pins will be numbered counter
clockwise as follows. When mounting in the PCB, note that the notch in the LM3914 is marked
in its outline on the PCB.
*REVISION 1 NOTE: The PCB silkscreen pattern for the LED Array is incorrect. The notch
indicating the side with pin 1 should be on the other side (toward the sensing end of the
thermometer).
1
2
3
4
5
6
7
8
9
LM3914
18
17
16
15
14
13
12
11
10
Note: The different values of the resistors can be noted by the colored bands on their bodies.
The 330 Ohm resistor has bands: ORANGE, ORANGE, BROWN, GOLD. The 220 Ohm
resistors have bands: RED, RED, BRONW, SILVER. The 1000 Ohm resistor has bands:
BROWN, BLACK, ORANGE, YELLOW.
Step 3Clip excess length from the leads of the components on the back of the PCB. Only do this after
the pieces are soldered in securely.
Step 4Connect the 9v battery once everything is soldered to the PCB. Using the DIP switch on the
PCB, turn on the thermometer. The green LED should light. By adjusting the potentiometers,
you can calibrate the thermometer.