Hands-on Activity: Designing a Thermostat
Educational Standards :
Pre-Req Knowledge (Return to Contents)
A familiarity with circuits in electricity, including the concepts of open and closed circuits.
Learning Objectives (Return to Contents)
After this activity, students should be able to:
Materials List (Return to Contents)
Each group needs:
For the entire class to share:
Materials note: The breadboards, wires, wire strippers, resistors and multimeters can be re-used.
Introduction/Motivation (Return to Contents)
Who can name some things that use a circuit — or multiple circuits? (Possible answers: Cell phones, radios, televisions, computers, video games, cars, houses, buildings, calculators.) Those are all great answers. Everything that plugs into a wall outlet or runs on batteries contains a circuit in order to operate. Can anyone give me a reason why an engineer would need to know about circuits? (Answer: To be able to design and create things that use electricity to operate.) That's right, many engineers use circuits in the design and manufacture of all the things we mentioned. More than any other type of engineer, electrical engineers primarily design using circuits. They are responsible for the design of most of the circuitry found in the everyday devices all around us, including computers and computer chips. However, many other engineers must have at least a basic understanding of circuits and how to create a simple circuit.
Today, we are going to investigate the circuit in a thermostat. Does anyone know what a thermostat is? A thermostat is a device installed in homes and buildings to regulate the temperature of an area of the building — such as a single room, a few rooms (a zone), or the entire building. Basically a thermostat works by determining the temperature in the immediate area of the sensor and converting that temperature into an electric signal. The thermostat is programmed to perform a chosen task based on that electric signal. The electric signal tells the thermostat to turn on or off a heater or air conditioner so as to change the temperature in the room.
We all want to conserve energy and make sure we are efficient in using energy in our homes, schools and places of work. Did you know that some thermostats are designed to help save energy? Most new homes and businesses use programmable thermostats that regulate the temperature of all or part of a building. These thermostats are useful in saving energy and can be programmed for many different settings. They are often set to not heat or cool a building during times when no people are using the building, since no one is present to benefit from the energy output. These times might be evenings in offices and schools, and daytimes at homes. Can you imagine how much energy we can save if we do not turn on the furnace or air conditioner when it is not needed?
Programmable thermostats can also be set to direct the heater or air conditioner to maintain a temperature range throughout the time when people are using the building; you set a low temperature and an upper temperature that is comfortable for the building's inhabitants. If at any time the temperature at the thermostat sensor goes out of the set comfort range, the thermostat generates an electric signal to turn on either the heater to heat the room back up into the temperature range, or the air conditioner to cool the room down into the temperature range. The thermostat keeps the heater/cooler on until the temperature gets to the opposite side of the set comfort range, then sends another signal to turn it off, giving the air time to become cool or warm again before the cycle starts over.
For example, if a room goes below a set comfort temperature, the thermostat turns on the heater to warm up the room. The thermostat keeps the heater on until the temperature reaches the upper temperature boundary. Once the room temperature reaches that upper boundary, the thermostat signals to turn the heater off. Since the heater works more efficiently (saving energy) when it is not being turned on and off repeatedly, this ensures that the temperature must drop more than a degree or two before the heater is turned back on. So the benefit of having a programmable thermostat over a standard one, is that it allows you to set your own range of temperatures for the thermostat to keep the room at, rather than having the heater turn on and off repeatedly. With the more advanced programmable thermostat designs, you can program temperature ranges for different days of the week in advance, so you can accommodate differences in building use during weekdays and weekends.
As we mentioned earlier, circuits are used by different types of engineers. For example, circuits are important to mechanical engineers in the design of motors because most motors are run and maintained with a circuit. Mechanical engineers must be knowledgeable about circuits in order to effectively design and create motors to run the parts they design. Teams of engineers from with different specialties often work together to build everything from cars to roller coasters to medical instruments — devices that combine mechanical parts and electrical systems. Knowing the basic components of a circuit and how they fit and work together is important for engineers to understand if they are to design anything that uses electricity.
Today we are going to learn about the different components in a circuit and how to put them together to create a simple circuit, a thermostat. We are going to design a programmable thermostat, in which the user determines the temperature range s/he would like the room/building to remain in and the thermostat makes sure it does that in an energy-efficient way.
Vocabulary/Definitions (Return to Contents)
Procedure (Return to Contents)
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
With the Students
Attachments (Return to Contents)
Safety Issues (Return to Contents)
Troubleshooting Tips (Return to Contents)
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.
Assessment (Return to Contents)
Class Discussion Question: Ask the students and discuss as a class:
Activity Embedded Assessment
Worksheet: Have students complete the activity worksheet; review their answers to gauge their mastery of the subject.
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.
Activity Extensions (Return to Contents)
Have students research hysteresis. Find out what it means, how this circuit uses hysteresis and other examples of hysteresis.
Have students research where else hysteresis shows up. Have them prepare a paragraph describing the phenomenon they discover and how it displays hysteresis. Also have them compare it to the circuit they have just built. How are they similar? How are they different?
Activity Scaling (Return to Contents)
References (Return to Contents)
Hambley, Allan R., Electrical Engineering: Principles and Applications, Third Edition. Upper Saddle River, NJ: Pearson Education Inc., 2005.
ContributorsTyler Maline, Lauren Cooper, Malinda Schaefer Zarske, Denise W. Carlson
Copyright© 2007 by Regents of the University of Colorado
This digital library content was developed by the Integrated Teaching and Learning Program under National Science Foundation GK-12 grant no. 0338326, and the Discovery-Learning Apprentice Program at CU-Boulder's College of Engineering and Applied Science. However, these contents do not necessarily represent the policies of the National Science Foundation, and you should not assume endorsement by the federal government.
Supporting Program (Return to Contents)Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder
Last Modified: March 10, 2014