Students investigate circuits and their components by building a basic thermostat. They learn why key parts are necessary for the circuit to function, and alter the circuit to optimize the thermostat temperature range. They also gain an awareness of how electrical engineers design circuits for the countless electronic products in our world.
Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.
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- Colorado: Math
- Colorado: Science
- b. Use appropriate measurements, equations and graphs to gather, analyze, and interpret data on the quantity of energy in a system or an object (Grades 9 - 12)  ...show
- Common Core State Standards for Mathematics: Math
- 1. Use units as a way to understand problems and to guide the solution of multi-step problems; choose and interpret units consistently in formulas; choose and interpret the scale and the origin in graphs and data displays. (Grades 9 - 12)  ...show
- 3. Solve linear equations and inequalities in one variable, including equations with coefficients represented by letters. (Grades 9 - 12)  ...show
- International Technology and Engineering Educators Association: Technology
- Next Generation Science Standards: Science
- Evaluate or refine a technological solution that reduces impacts of human activities on natural systems. (Grades 9 - 12)  ...show
- Describe the relationship of a programmable thermostat to energy conservation.
- Develop a model of the circuitry in a programmable thermostat.
- Describe how engineers use circuit diagrams to design a circuit.
- List the advantages of using a breadboard during circuit design.
- 1 breadboard (EXP 350 recommended, $6 from online vendors; or RadioShack for $8+; or find used breadboards)
- 1(or more) LM35 temperature sensor chip ($1.29 each, online from www.digikey.com; get extras in case students accidentally break one by setting up the circuit incorrectly.)
- 1 LM324AN operational amplifier integrated circuit ($1.69 at RadioShack)
- 1 9-volt battery (or have a few for the class to share)
- 1 9-volt battery holder (optional, or have a few for the class to share)
- Ziploc bag
- Thermostat Worksheet
- Breadboard and Circuit Diagram Basics Handout
- 1 jumper wire kit (preferred since it is easier and reduces set-up time, $6.50 at RadioShack), or each group needs 2 pieces of 1-inch wire, 2 pieces of 3-inch wire, and 5 pieces of 2-inch wire and electrical tape
- Small wire strippers (only needed if you are using insulated wire and not the jumper wire kit, to remove insulation at wire ends)
- Multiple ¼ watt resistors of various sizes from 500 Ohm up to 10K Ohm (100-piece kit: $6.50; 500 piece kit: $13 at RadioShack)
- A few multimeters to make various measurements (such as Kelvin 50LE http://www.kelvin.com/ part # 990177, $3.65)
|Breadboard:||A reusable solderless tool used to create a temporary (usually a prototype) circuit to experiment with until a more permanent circuit is created.|
|Conductor:||A material that allows charges to move easily, such as copper wire.|
|Electric current:||The flow of electric charge through an electrical circuit or conductor.|
|Electrical circuit:||A collection of circuit elements (resistances, inductances, capacitances, etc.) connected in closed paths by conductors.|
|Hysteresis:||An electric circuit that is path-dependent and, thus, has memory.|
|Hysteresis band:||The difference in voltage between the turn on and turn off points in an electrical circuit using hysteresis.|
|Integrated circuit (IC):||Several circuit elements that are manufactured together onto a single chip by a sequence of processing steps.|
|Operational amplifier (op-amp):||An integrated circuit that contains multiple resistors and capacitors. Op-amps have many practical applications in engineering instrumentation.|
|Parallel:||Two or more circuit elements are in parallel if they are connected to the same node or junction of the circuit and have the same voltage drop across their terminals.|
|Resistor:||A circuit element that resists electric current and dissipates energy in the form of heat.|
|Series:||Two or more circuit elements are in series if the same current flows through them.|
|Voltage:||A measure of the potential energy of an electrical field to cause an electrical current in a conductor|
Before the Activity
With the Students
- Divide the class into groups of two or three students each.
- Distribute the materials to each group along with the worksheet and handout.
- If using the insulated wire, have students strip about ¼ to 3/8 of an inch from both ends of each of the wires.
- Have students set up the circuit as shown in Part 1 of the worksheet. Reference the handout for additional information, especially clarification of the circuit diagram symbols and breadboards parts.
- Have students turn on the multimeters and set them up to measure voltage in mV.
- Have students 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 (see Figure 1).
- 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 (see Figure 1).
- Have students complete the circuit check part of the worksheet.
- Have students complete the modeling the circuit part of the worksheet.
- Have students use multimeters to measure the voltage from the output of the temperature sensor and record the value on the worksheet (see Figure 2).
- Have one student in each team 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. Have students record this value of the voltage on the worksheet.
- After the light comes on, have students 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, have students record this voltage value on the worksheet.
- Have students work out the redesigning the circuit part of the worksheet.
- Have students calculate on their worksheet the new voltages corresponding to the new temperatures.
- Have students identify which resistors must be changed for their new design and have them change them to coincide with the new cut-off points. (It may be necessary to connect resistors in series or parallel to achieve desired voltages. Also, you may not be able to get the exact resistance the students calculated; if this is the case, have them get as close as possible and note it on the worksheet.)
- Have students model the new circuit by repeating steps 10-15 with the new, optimized circuit (Part 5 on the worksheet).
- Have students complete the analysis part of the worksheet.
- Conclude by leading a class discussion to review the worksheet answers.
- To further test students' comprehension, ask them how they would make 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.
- Have students complete the handout Programmable Thermostat Energy Savings Worksheet. This allows students to calculate potential energy savings by using a programmable thermostat.
- 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)
- Programmable Thermostat Energy Savings Worksheet (doc)
- Programmable Thermostat Energy Savings Worksheet (pdf)
- Programmable Thermostat Energy Savings Worksheet Answers (doc)
- Programmable Thermostat Energy Savings Worksheet Answers (pdf)
- 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.
- 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?
Activity Embedded Assessment
- For students with a better understanding of circuit analysis, have them research the formulas used to determine the resistances needed to set the on/off points (Kirchhoff's voltage and current laws, Ohm's Law, etc.).
Hambley, Allan R., Electrical Engineering: Principles and Applications, Third Edition. Upper Saddle River, NJ: Pearson Education Inc., 2005.
Tyler Maline, Lauren Cooper, Malinda Schaefer Zarske, Denise W. Carlson
© 2007 by Regents of the University of Colorado.
Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder
Last modified: November 25, 2015