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.
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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 with battery connectors (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)
|A reusable solderless tool used to create a temporary (usually a prototype) circuit to experiment with until a more permanent circuit is created.|
|A material that allows charges to move easily, such as copper wire.|
|The flow of electric charge through an electrical circuit or conductor.|
|A collection of circuit elements (resistances, inductances, capacitances, etc.) connected in closed paths by conductors.|
|An electric circuit that is path-dependent and, thus, has memory.|
|The difference in voltage between the turn on and turn off points in an electrical circuit using hysteresis.|
|Several circuit elements that are manufactured together onto a single chip by a sequence of processing steps.|
|An integrated circuit that contains multiple resistors and capacitors. Op-amps have many practical applications in engineering instrumentation.|
|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.|
|A circuit element that resists electric current and dissipates energy in the form of heat.|
|Two or more circuit elements are in series if the same current flows through them.|
|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 1/4 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 using the breadboard, jumper wires, 9 V battery, and the LM35 Temperature Sensor chip. The circuit is shown in Figure 1. 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 measure the voltage across the 1 kΩ resistor. They should get a positive value around 0.23 V.
- Have students rub their hands together and touch the top of the LM35 sensor. Note what happens to the voltage reading.
- Have students measure the temperature (ºC) and voltage (V) of the room and their palm using a thermometer and the circuit they just built. Record data in the space provided in the Thermostat Worksheet.
- Using their data, students should develop an equation to determine the voltage across the 1 kΩ resistor in terms of temperature (ºC) assuming a linear relationship.
- Set the thermostat in cooling mode. Instruct students to choose a "set" temperature that is lower than their palm temperature, but higher than the room temperature.
- Determine the LM35 voltage output corresponding to the set temperature.
- Have students divide the 9 V supplied by the battery so that part of the circuit equals the LM35 output voltage for their desired temperature. Do so by moving the 1 kΩ resistor (R2) over and combining it with another (R1) as shown in Figure 2.
- Have students answer the questions under Part 2 (question 5) in the worksheet to identify the relationship between current and voltage.
- Also in their worksheets, tell students to determine an equation to quantify the value of R1.
- After obtaining an equation for R1, plug in the known values (VBat=9 V, R2=1 kΩ, V2 set point) in order to determine the value needed for R1.
- Have students connect the LM35 sensor and voltage divider to an operational amplifier (LM324). Add the LM324, 2 kΩ resistor (R3), light emitting diode (LED) and connections to their breadboard as shown in the worksheet and in Figure 3 below.
- Instruct students to connect the 9 V battery (see the following steps) to the circuit to measure the voltage across R2.
- 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.
- 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.
- Remind students to disconnect the battery between measurements.
- Have students continue to follow in their worksheets to answer the questions provided.
- Have students answer the questions in their worksheets to redesign their thermostats for the case when nobody is home.
- Have students adjust their circuit so it accommodates for heating by following instructions in their worksheets.
- Have students use their heating circuit to measure the temperature and voltage when it turns on and turns off and record in their worksheets.
- 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.
- Have students add one resistor (R4) to create different "on" and "off" points for the heating circuit. The heating circuit should resemble that in Figure 4 below.
- Have students measure the voltage drop from pin 14 to ground when the LED is on to later figure out the value of R4.
- Following along in their worksheets, direct students to develop an equation for R4 using Ohm's Law (V=IR) and properties of series circuits.
- Using the appropriate R4, have students test the circuit and comment on how it behaves.
- Conclude by leading a class discussion to review the worksheet answers using the Thermostat Worksheet Answer Key.
- 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, Aaron Osowiecki
© 2007 by Regents of the University of Colorado.
Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder
Last modified: February 11, 2016