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.

• If using insulated wire, cut it into sections for each group.
• Make copies of the Thermostat Worksheet and the Breadboard and Circuit Diagram Basics Handout.

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.
3. If using the insulated wire, have students strip about ¼ to 3/8 of an inch from both ends of each of the wires.
4. 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.
5. Have students turn on the multimeters and set them up to measure voltage in mV.
6. 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.
7. 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).
8. 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).

Figure 1. A finished circuit with battery. click for copyright

9. Have students complete the circuit check part of the worksheet.
10. Have students complete the modeling the circuit part of the worksheet.
11. 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).
12. 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.

Figure 1. Using a multimeter to take a voltage measurement. click for copyright

13. 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.
14. After the light comes on, have students warm the temperature sensor by blowing on it or pinching it between two fingers.
15. 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.
16. Have students work out the redesigning the circuit part of the worksheet.
17. Have students calculate on their worksheet the new voltages corresponding to the new temperatures.
18. 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.)
19. Have students model the new circuit by repeating steps 10-15 with the new, optimized circuit (Part 5 on the worksheet).
20. Have students complete the analysis part of the worksheet.
21. Conclude by leading a class discussion to review the worksheet answers.
22. 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.

Class Discussion Question: Ask the students and discuss as a class:

• 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?

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.