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Designing Electric Circuits: Door Alarm

Resource for Grades 3-8

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Media Type:
Video

Running Time: 3m 47s
Size: 1.0 MB

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Source: ZOOM

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Designing Electric Circuits: Door Alarm (Document)

Accessibility Features: Caption

Background Essay

Hair dryers, toaster ovens, flashlights, televisions sets... they all work thanks to electrons. Without electrons, or more precisely, the flow of electrons, millions of people would lead very different lives. This flow of electrons, also known as electrical current, is essentially the same whether it is generated by a nuclear power plant or an AA battery.

In order for electrical current to flow, three important conditions must exist. One of these is the presence of a material, usually in the form of a wire or cable, that conducts electricity. Electrons can move, or flow, easily through conductive material, not staying locked on any particular atom by atomic forces. Some materials, called conductors, have especially loosely held electrons, allowing for this free flow. In contrast, materials called insulators hold onto their electrons more strongly and resist the flow of current (i.e. the flow of electrons) through them.

The second requirement for electrical current to flow is a power source. Whatever its form, a source of electrical current creates what is called a voltage difference, a kind of pressure that pushes the electrons through the circuit.

Lastly, electrical current requires a closed circuit. This is a length of conductive material connected at each end to a power source passing through the object that needs the current to operate (i.e. the door alarm). The circuit allows a direct, uninterrupted flow of electrons from the power source and back -- a complete circular connection with no beginning or end.

The door alarms created by the ZOOM cast members in this video segment fulfill all of the requirements of an electrical circuit. In fact, they use the last of these three requirements to function as alarms. When the door stays closed, the circuit stays open and the alarm buzzer stays quiet. If the alarm system is designed properly, opening the door closes the circuit, causing electricity to flow and the alarm to sound.

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Discussion Questions

Choose one of the door alarms in the video and draw a sketch of it. Label the power source and the conducting material. What has to happen to complete the circuit?

Why didn't the cast members use full-size doors to test their alarms? If they didn't have the smaller models, could they have used a full-size door?

Imagine that one of these door alarms is attached to your bedroom door. What would be the strengths and limitations of its design? Can you think of a way to overcome the limitations?

See if you can find a building where a door is alarmed. How does this alarm work?

It's your turn to solve the door alarm problem. What is your design? What materials would you use?

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Transcript

(electricity crackling)

Shing Ying: You've tried everything. You've put a "keep out" sign on your door, you've piled stuffed animals in your doorway, but your little brother or sister still keeps on trying to sneak into your room. Well, today we're going to show you the perfect solution.

Kortney: We're going to design and build door alarms. Kimberly H. of Brooklyn, New York, e-mailed us the idea. We can use tape, aluminum foil, batteries, string, wire, cardboard, buzzers and anything else we can think of.

Mike: Shing Ying and I will be one design team, and Kortney and Estuardo will be the other. We're going to use smaller models of real doors, just like engineers would.

Estuardo: Let's test out these buzzers.

Shing Ying: Okay, let's just see... Do you guys want to...?

Estuardo: We'll try this one out.

Shing Ying: And we'll try this one.

Speaker: Do they work?

(buzzing)

Speaker: Oh, I like that.

(buzzer sounds with high pitch)

Estuardo: This is the one.

Shing Ying: Let's start. When you open the door, the circuit has to be connected. So when the door is closed, it has to be disconnected. So somehow when this door opens, we have to figure out how to get it to be connected.

Kortney: So I think we should position it exactly, so if we tape this here, when it opens the door, it'll hit the black wire, and the red wire will already be attached and it'll make the buzzer go off.

Estuardo: And it'll make the buzzer go off.

Mike: Now, we know that to make the buzzer, we need a complete circuit. We know the circuit won't be complete until the door opens. So this needs to be attached to something.

Shing Ying: How about we attach this to aluminum foil, and then to the aluminum foil, we attach it to the battery? And we attach the battery to the buzzer.

Mike: So this is like that, and then this is to... This is negative, so it needs to go here.

(buzzer sounds)

Estuardo: Okay, now it's working, so... Yeah, I think it's... Keep it, keep it, keep it. You have to hold it. Hold. Okay, now I'll tape the wire.

(alarm sounds)

Mike: So now when we open the door...

(buzzer sounds)

Shing Ying: Oh, perfect!

Mike: Yay!

Shing Ying: High five!

(both laugh)

Shing Ying: Try to sneak in, you guys.

(buzzer sounds)

(all chuckling)

Estuardo: That's pretty loud.

Speaker: Yeah!

Mike: Now, the two batteries supply us with the electricity. This bell needs two batteries instead of just one. We made a conductor here... (buzzer sounds) And a conductor here, and when the door is open, the two conductors touch and it sounds the bell. So when you try to open it... (buzzer ringing) it makes the noise.

Shing Ying: Do you want to try to open it together?

Mike: Sure.

(buzzer sounds)

Shing Ying: Aw! That was so cool. Cool.

Estuardo: This is sort of like an on/off switch or the light switch, because right now it's open and it's not really working, just like when you have it off--I mean the light switch--but when you put the wires together... (buzzer sounds) It's a closed circuit.

Shing Ying: Hmm. That's so cool.

Estuardo: Completing a circuit is just one way of engineering a door alarm. Here's another. This alarm runs on batteries, and here's how it works. An infrared beam shines out of the box. Mike's going to demonstrate for us. When Mike gets close to it, his body reflects the beam back to the box. The beam then hits a receptor in the box and turns on the alarm. (alarm sounding) (laughs)