Build A Thermometer Kit

Assembly Time: 1 to 2 hours
Difficulty: Beginner
Designer: Austin Mier

Build a modern thermometer that measures a change in resistance within a circuit.

Required Tools and Components:

soldering iron
soldering iron stand
solder
wire cutters

Qty.	Part Description	Part Number
2	10KΩ Potentiometer	3386P-1-103/63P103
2	100KΩ Potentiometer	3362P-1-104LF-VP
10	1KΩ Resistor	CF1/4W102JRC
10	330Ω Resistor	CF1/4W331JRC
10	220Ω Resistor	CF1/4W221JRC
1	LED array	LTA-1000HR
1	LM3914 driver	LM3914N-1
1	Green LED (10 pack)	LG3330
1	Thermistor	NTC-502-R
1	9V battery	ALK 9V 522
1	9V battery plug	BC6-R
1	PCB	—
1	DIP Switch	206-2-VP-R

How does this circuit work?

This circuit works 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); as the temperature rises, the resistance of the thermistor decreases by a certain percentage for 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 source 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 resistance may drop to 2,500 Ohms while the potentiometer is still 5,000 Ohms. This means 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 below:

Click for large image

Electronics Projects Recommendations

If you decide to use a rubber band to hold the battery, add a small notch in the PCB directly above the two potentiometers to keep it from slipping.

We also recommend having a multimeter on hand because it is handy if troubleshooting is necessary. One multimeter we recommend is Jameco digital multimeter with LCD display.

Helpful Tips
1. LEDs have a specific direction - long side to positive. 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. Keep the soldering neat, but don't get frustrated with this project if your soldering is not perfect.
4. Solder the smallest components first. This is much easier than soldering larger components in first and then squeezing in the small pieces.
5. 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?

This thermometer works best when measuring mild temperatures, from 0-50 degrees Celsius. The thermistor takes a few seconds to reset and will display change gradually.

Directions

Refer to the PCB pattern below*.

Click for large image

Insert the components on the top of the PCB and solder on the bottom. The placement for the components on the PCB is as follows:

BAT1
D1
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R13
R14
R15
U1
U2*
U3

9v Clip Leads
Green LED
100kΩ Potentiometer
10kΩ Potentiometer
220Ω Resistor
220Ω Resistor
220Ω Resistor
220Ω Resistor
220Ω Resistor
220Ω Resistor
220Ω Resistor
220Ω Resistor
220Ω Resistor
220Ω Resistor
1kΩ Resistor
330Ω Resistor
5kΩ Thermistor
LM3914
LED Array
DIP Switch

*Note: The first revision of the PCB Silkscreen shows U2 (LED array) incorrectly. The notch should be at the opposite side. When installed, the notch on the LED array should be near the outside edge of the PCB.

LM3914

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 are numbered counterclockwise. When mounting in the PCB, note the notch in the LM3914 is marked in its outline on the PCB.

Identify the different values of the resistors by the colored bands.
• 330Ω resistor has bands: ORANGE, ORANGE, BROWN, GOLD
• 220Ω resistors have bands: RED, RED, BROWN, SILVER
• 1000Ω resistor has bands: BROWN, BLACK, RED, GOLD

Clip excess length, from the leads of the components on the back of the PCB. Only do this after the pieces are soldered in securely.

Connect 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.

Austin Mier, an engineering student at University of New Mexico, recently worked at Jameco Electronics as an intern.

READER COMMENTS:

The description should have mentioned how to calibrate. An easy, and pretty good, way is: Ice and water slush to set 0°C and boiling water (if the thermistor will stand it) for 100°C. If there was interest I could even give a formula that would minimize the back and forth. There are also much higher grade thermistors accurate to .1°C that are used in the medical thermometers.

Respectfully, using a thermistor for a temperature sensor is not terribly accurate or stable. Perhaps you would consider using the LM24 for this purpose. The OPA342 is actually optional, but adding a little gain to the LM34 will make the entire system less sensitive to noise, and to thermal effects on the potentiometers. With these settings, the output of the OPA342 will be 200mV/°F. While the LM34 can go to negative temperatures, doing so will require a dual supply.

The fundamental theory behind the design is flawed. The accuracy of the measurement is wildly dependent on the supply battery voltage, the device itself used to measure temperature is so non-linear with temperature as to be absolutely worthless as a thermometer. Not intended to replace your thermometer.