From DC Circuits to Toasters to Computers

Rachel Berger

Horace Mann School, Bronx, NY

Summer Research Program for Secondary School Science Teachers

1998

Objective: To learn to wire circuits and to make the translation from circuit diagrams to pieces of equipment. [9-12 Content Standard E- Understandings about science and technology] This is a discovery lab. Students have had no DC circuit theory and need to be guided through the steps. The equipment is not expensive.

Materials: The following electronic equipment can be found at Radio Shack.

Wire Set and Resistor Set

Many LEDs, capacitors, and switches

Multimeter

Power Supply (this item is expensive)

Toasters (the cheap ones that you can open and examine)

Tool set to open a computer

[Teaching Standard D- Make accessible science tools]

Method:

After introducing the students to the different components of a DC circuit, ask them to use the wire set and an LED with the power supply and breadboard to find out how the breadboard is wired. Using the LED as a logic switch, it will tell them when the circuit is closed by lighting up. Be sure the power supply is set to low and that the voltage is not too high for the LEDs.

Now introduce the resistors and capacitors. [Content Standard Unifying Concepts- Form and function] They should already know about the theory behind them. The important things to mention are how to read the color code on the resistors and that the bump on the capacitor needs to be hooked to the positive terminal. Then, let them explore different ways to connect the components, using the multimeter as a probe for current and voltage across them. For example, the LEDs can be use with the capacitors to show discharging. After the capacitor is charged, turn the power supply off and the LED stays lit until the capacitor discharges. The multimeter can be used to find the exact voltage at which the LED turns off.

After they are comfortable with how the components work, give each group of 4 to 5 students a toaster. Ask them, "How does a toaster work?" [9-12 Content Standard A- Identify questions/concepts which guide inquiry] Then tell them to open the toasters from the bottom while remembering the components they just worked with. Give them the following tour of the toaster.

On the end where the cord enters, find the main switch (DPDT) which is closed when the toast is lowered. Also find the dashpot which keeps the toaster spring from flinging the toast out when it pops up.

In the center section, find the heating elements. These are just big resistors. Along the center element on one side is located a bimetallic strip whose arm extends underneath the heating element. After being heated (the more heat, the further it bends), the tip on the arm bumps a black switch (SPST) which can be seen from both the top and bottom of the toaster. The distance the bimetallic strip needs to move to hit the black switch depends on the position of the light-dark knob. This sets how hot the strip must get before it hits the black switch.

On the end opposite the cord is the latch mechanism that holds the toast down. Find a solenoid as well as a mechanical latch. When the bimetallic strip closes the black switch, power is sent to the solenoid which releases the mechanical latch (ask them how). Once the latch pops up, the main switch opens, cutting off power to both the heating elements and the solenoid which ends the toasting cycle.

Finally, after they have seen many different circuit components and understand a little of how they work together, it is time to introduce them to the computer. Carefully open one computer case up and show them the various boards, pointing out the mother board and the main processor chip. Now is a good time to talk introduce computer chips. Then point out all of the little resistors and capacitors just like the ones they were working with. Also, all the etchings on the green boards that connect the components are analogous to the wire that they use. To finish, turn the computer on while open (careful not to touch anything) and let them see that LEDs light up, the same way they just made theirs light up. Now the circuit is complete, both for the computer and for the students.