Resource: Designing Electric Circuits: Door Alarm

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.

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?