Designing a Thermostat
Your Activity
Investigate circuits and their components by building a basic
thermostat.
Material
1 breadboard
1 (or more) LM35 temperature
sensor chip
1 LM324AN operational
amplifier integrated circuit
1 9V battery and battery
holders
Thermostat Worksheet
1 jumper wire kit
Small wire strippers
Multiple ¼ watt resistors of
various sizes from 500 Ohm
up to 10K Ohm
A few multimeters to make
various measurements
Breadboard and Circuit
Diagram Basics Handout
Create
1.
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5.
6.
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8.
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16.
If using the insulated wire, strip about ¼ to 3/8 of an inch from both ends of each of the wires.
Set up the circuit as shown in Part 1 of the worksheet.
Turn on the multimeters and set them up to measure voltage in mV.
Place the battery in the battery holder, or tape the ends of the two pieces of 3-inch wire to both the positive and
negative terminals of the battery.
Connect the wire coming from the positive terminal (denoted with a + on the side of the battery that the
positive terminal is on) to the power row on the breadboard.
To complete the circuit, connect the wire coming from the negative terminal (denoted with a (-) on the side of
the battery) to the ground row on the breadboard.
Complete the circuit check part of the worksheet.
Complete the modeling the circuit part of the worksheet.
Use multimeters to measure the voltage from the output of the temperature sensor and record the value on the
worksheet.
One member of your team will continue to measure the voltage from the output of the temperature sensor
while the others cool the temperature sensor using a Ziploc bag containing ice.
The light should turn on when the temperature (voltage) reaches the low point of our set temperature range.
Record this value of the voltage on the worksheet.
After the light comes on, warm the temperature sensor by blowing on it or pinching it between two fingers.
As the upper temperature (voltage) of our range is reached, the light should turn off. Again, record this voltage
value on the worksheet.
Work out the redesigning the circuit part of the worksheet.
Calculate on the worksheet the new voltages corresponding to the new temperatures.
Identify which resistors must be changed for their new design and have them change them to coincide with the
new cut-off points.
17. Model the new circuit by repeating steps 10-15 with the new, optimized circuit (Part 5 on the worksheet).
18. Complete the analysis part of the worksheet.
Science Topics
Electricity, Electrical Engineering
What’s going on?
Key parts are necessary for the circuit to function, and you can alter the circuit to optimize the thermostat temperature
range. Circuits are pervasive in the modern engineered world. Most engineers have a good understanding of electricity
and basic circuitry so as to better design everything from cars and houses, to cell phones and computers.
Designing a Thermostat
Activity Lead Notes
Introduction
Circuits have become an essential part of our everyday lives. Circuits are found everywhere — in cars, TVs,
computers, phones, homes, schools, etc. Their impact on our lives is immense and much of our society would not be the
same without the circuit. Most every electrical circuit contains the same basic components — resistors, integrated
circuits, capacitors and inductors. Each of these components performs a certain task (sometimes different components
are combined to do the job of one of the other components) and are used by most engineers, especially those working
with electricity or products that use electricity.
Thermostats are useful devices to regulate the temperature of a room, area or an entire building. They work by
using a temperature sensor — generally an electronic chip designed to change its resistance depending on the
temperature. As the temperature of the chip changes, the resistance of the chip changes and alters the voltage drop
across the chip. The chip is internally calibrated to produce a linear relationship between the temperature and the
voltage output of the sensor. After the sensor determines the temperature, the resulting electrical signal (output
voltage) is sent into another portion of the circuit designed to interpret the incoming voltage and select an outcome
based on the signal. This part of the circuit can be performed in many ways; however, the least complicated way is to
use an operational amplifier (op amp).
Using an op amp permits the introduction of hysteresis into the circuit — or memory. In this activity, students
take the output signal from the sensor and compare it to a predetermined voltage that is manually set. If the voltage
from the sensor measures lower than the voltage the students set, indicating that the temperature sensor is reading a
temperature that is colder than what we want it to be, the heater (an LED) turns on to "warm up" the room. Once the
heater (LED) turns on, the hysteresis of the op amp forces the heater to stay on until the voltage goes above the second
or high voltage set in the desired comfort range. This keeps the thermostat from rapidly turning the heater on and off if
the temperature is hovering around the desired initial temperature. By forcing the heater to stay on until the second
voltage, the circuit demonstrates path-dependence, which means that it remembers where it has been and uses that to
inform what it will do next. It will not turn off after going above the low-set voltage because it "knows" that it just
recently went below that mark. It forces the heater to stay on until the second voltage mark is passed.
The circuit the students create contains a LM35 temperature sensor, which has a linear relationship between
the temperature of the sensor and the output voltage; the relationship is 10mV for every degree Celsius. Therefore, at
room temperature (~20-23 °C), the LM35 should have an output voltage of 200-230mV. As the temperature rises or falls,
the output voltage rises or drops 10mV for each degree of temperature change.
Before the Activity
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If using insulated wire, cut it into sections for each group.
Distribute copies of the Thermostat Worksheet and the Breadboard and Circuit Diagram Basics Handout.
Class Discussion Question: Ask the students and discuss as a class:
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Why might it be a good idea to be able to control at which temperatures a heater and/or air conditioner turns
on and off?
During the Activity
1. Divide the class into groups of two or three students each.
2. Distribute the materials to each group along with the worksheet and handout.
Attachments
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Thermostat Worksheet (doc)
Thermostat Worksheet (pdf)
Thermostat Worksheet Answers (doc)
Thermostat Worksheet Answers (pdf)
Breadboard and Circuit Diagram Basics Handout (doc)
Breadboard and Circuit Diagram Basics Handout (pdf)
Safety Issues
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Working with electricity is always dangerous. To make sure that a component does not overheat, remind
students to double-check their circuit with the circuit diagram and image provided on the worksheet before
connecting the circuit to the battery.
Attention to detail is important. Remind students to take care to make sure the components are placed where
they should be. The wrong connections to ground and/or power can cause these chips to overheat, smoke, and
(potentially) become permanently damaged
Troubleshooting Tips
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Make sure students do not leave the battery connected to the breadboard if they are not actively taking a
measurement, debugging or observing the circuit. Keeping it unconnected most of the time prolongs the life of
the battery and ensures that the circuit components do not get too hot by being "left on" for a while.
If a team's circuit is not working, disconnect the breadboard wires coming from the battery and double-check
the circuit diagram and circuit. Make sure that the pins from the LM35 and LM324AN are connected and
oriented correctly. If everything looks good, reconnect the battery and debug the circuit using the multimeter.
Check the input and ground pins of the temperature sensor and the LM324AN to make sure they are connected
properly. The multimeter should read 9 volts (or close to that) for the input on the temperature sensor; the
ground for both should read zero volts (or close to that). The input of the LM324AN should be the same as the
output of the temperature sensor. Also check the connections to the LED; make sure there is an input when
there should be one, and that the ground pin of the LED reads zero volts.
LEDs can easily burn out if they are left on too long, or if too large of a current is sent through them. This is why
the output from the LM324AN goes through a resistance before reaching the LED. If a circuit is not working, and
everything else seems to be in the correct place, try a different LED.
Make sure that none of the resistors touch another resistor, which makes those two resistors in series and thus
changes the value of the resistance, and consequently the voltage going through that section of the circuit.
After the Activity
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Worksheet Discussion: Review and discuss the worksheet answers with the entire class. Use students' answers
to gauge their mastery of the subject.
In Reverse: Have students either brainstorm or research ways to allow the thermostat hysteresis work in
reverse, so that the circuit turns on at the higher temperature and turns off at the lower temperature — like an
air conditioner, instead of a heater. To do this, the students would rewire the circuit to turn on and off at
different temperatures.
Source
www.teachenggineering.org
Contributed by: Integrated Teaching and Learning Program and Laboratory, University of Colorado at Boulder