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Template for Computational Science Related Lessons
Title of Lesson: Electrical Engineering and Circuits
Grade level / course: Middle School
Objectives / Concepts
Lesson Abstract:
- Learning about more complex circuits, including using capacitors and Integrated Chips to
build a circuit
Standards Addressed:
Objectives:
- Students will understand the concept of a capacitor and how its size affects how it performs
- Students will be introduced to a Integrated Circuit (IC) chip, the 555 Timer
- Students will be able to build a circuit using the 555 Timer which makes a LED blink
- Students will understand how to change the speed at which the LED blinks using the
capacitor
Prerequisite knowledge:
- Students should have learned about basic circuit elements including
o Batteries
o Wires
o Resistors
o LEDs
o Breadboards
- Ideally the Students learned about all these things through an analogy with a water wheel
Students should have built a simple circuit with battery, resistor, and LED on breadboard
Teacher preparation:
- Assemble bags and breadboards with all necessary components
- Explore the applet site to help students find the right example circuits
Required Materials
(What and how much)
- Circuit Components (for each
student)
o 1 x Breadboard
o 1 x LED
o 1 x 555 Timer
o 1 x 10 F Capacitor
o 1x 100 F Capacitor
o 3x 10 k Resistors
o 1x short (5 cm) red wire
o 1x short black wire
o 1x short blue wire
o 1x 9V battery
o 1x 9V battery leads
- Circuit Diagram Layouts
Media (software, websites,
overhead, transparencies,
video, etc)
- Falstad circuit simulator
Equipment (computer, display,
OHP, VCR, etc)
-
computers
projector
Safety (Precautions and reminders)
Do not work on the circuit with the battery connected for safety of the circuit and the builder
Be careful of polarity with capacitors and LEDs and with the connections to the 555 timer. All of these
components are damaged by incorrect polarity or high currents and can smoke, or get very hot.
Time
Introduction/
Review
Presentation Outline
Event
You just learned about several different components of a circuit and built a
simple circuit yourself. Lets go over the different components we used.
Ask the student which components they used and what each component
corresponded to in the water wheel analogy (if the analogy was used).
5 min
Should Recall:
Breadboard is like the landscape
Battery is like the Hill
Wires are like the pipes or channels
Light bulb/LED is like the water wheel
Introduce a new circuit element, the capacitor.
Capacitor
10 min
Explain that the capacitor is like a water tank of glass
- As water comes in the tank fills up
- The faster the water comes in the quicker it fills up
- Can only fill to a certain point
- A larger (wider) tank fills up more slowly than a narrow
Likewise for a capacitor:
- Current flows in and fills the capacitor, raising the voltage of the
capacitor
- It fill up faster with higher currents
- Only will fill up to the supplied voltage
- Larger capacitors take longer to charge
Show the circuit diagram representation of the capacitor
- Be sure to emphasize the polarity of the capacitor and how the long
lead is the positive one. Have students look at the markings
themselves.
Next Introduce the 555 timer
Explain how it is an Integrated Chip or just a chip, which is very complicated on
the inside.
555 Timer
10 min
It is used for timing in many things because it turns things on and off at certain
times:
Examples : Car windshield wipers, interior lights, etc
The chip is controlled by the voltage on the capacitor. When the voltage is high,
the output is off; when the voltage goes lower, the output turns on.
Also the chip controls when the capacitor charges and discharges, so it keeps
everything going in a loop.
Show them a chip and explain how the pins correspond to different functions,
but we don’t need to worry too much about them.
The chip needs power just like our circuit needs its own power since there’s just
more circuits inside the chip
Computer Applet
10 min
Have the students go to the applet which shows the circuit they will build:
žhttp://falstad.com/circuit/
žChoose Circuits → 555 Timer Chip → Square Wave Generator
Get the applet up on your screen so you can show it to them.
Ask the students to identify each component on the diagram.
Point out how the power and ground act like the positive and negative
terminals on our batteries
Point out the graphs at the bottom showing the voltage on the capacitor
(green graph on left) and the output voltage. You can see the capacitor
charging and discharging as the output goes on and off.
Slow down the animation so they can watch the current flow in to the
capacitor and see the voltage rise. Then point out how when it gets to a certain
point, the chip changes the output to low, then the capacitor discharges
through the chip. Try to make sure they understand. Tell them we are going to
build this circuit but with an LED on the output of the chip
Building the 555
Circuit
30 min
Everyone should already have the parts they need and a circuit diagram
handout. Pull up the circuit diagram on the projector.
Step the students through the wiring process one step at a time. Make sure
almost everyone is caught up before continuing. First start by explaining how
to use the rails or buses on the breadboards to supply power. Plug the battery
connection wires into the right red rail (the red wire) and the leftmost blue rail
(the black wire). Remind students not to attach the battery yet.
Next have the students carefully put in the 555 chip, making sure the little dot
on the top is pointing up (having everyone do it this way not only assures
consistency when we are explaining the circuit, it also puts the correct power
buses on the right sides of the chip)
Next have them wire the power and ground to the chip, using the red wire for +
and black wire for -. Explain how we follow this convention to make things look
simpler. Then make connections between the chips pins using the resistors and
the blue wires. As you make each connection, cross off or check the
corresponding wires on the diagram on the screen.
Then put the capacitor (have everyone use the smaller one) into the circuit,
making sure it’s put in the right way. Finally the resistor and LED should be put
in, reminding the students how to use their breadboards and that the LED can
only go one way. Also remind students of how this is almost exactly like the
circuit they built earlier.
Finally the students can connect the battery and test the circuit. Help any
student’s whose aren’t working, making sure to check that the wires are
pushed in enough, they are to the right places, the LED is not faulty or burned
out. If the 555 timer feels hot to the touch, it probably needs to be replaced.
Exploring the 555
Circuit
10 mins
When the students are done building their circuits, ask them how they could
change how fast their light blinks. Make sure they understand that a bigger
capacitor will take longer to charge and thus the light will remain off longer and
on longer.
Have them come up with a guess, then unplug the battery and change the
capacitor. Where they right?
Ask them if there is any other way to change how long the light is on. Hint at
other ways to change the time, like giving it less current. If you have different
sized resistors and want to have them try it you can.
Reflections
15 min
Extra Time
Reflections
The students can explore other circuits on the applet site if there is extra time.
Also, if there’s a lot of time, a brief introduction to digital electronics could be
useful.
Bibliography
Part of this lesson, including the 555 circuit was originally designed by Professor Dr. Rhett Davis of NC
State University