Burglar alarms not only protect your stuff, they put the intruder into a panic while they attempt to disarm the triggered noisemaker. Our burglar alarms are basically switches which utilize the circuitry from Basic Circuits and clever tricks in conductivity.
A complete and exhaustive description of electronics would jump into the physics of solid state electronics, which is covered in undergraduate university courses. Instead, here is a quick description based on the fluid analogy for electric charge:
The movement of electric charge is called electric current, and is measured in amperes (A, or amps). When electric current passes through a material, it does so by electrical conduction, but there are different kinds of conduction, such as metallic conduction (where electrons flow through a conductor, like metal) and electrolysis (where charged atoms (called ions) flow through liquids).
Why does metal conduct electricity? Metals are conductors not because electricity passes through them, but because they contain electrons that can move. Think of the metal wire like a hose full of water. The water can move through the hose. An insulator would be like a hose full of cement – no charge can move through it.
[am4show have=’p8;p9;p11;p38;p92;p20;p47;p108;’ guest_error=’Guest error message’ user_error=’User error message’ ]Paper doesn’t conduct electricity – it’s an insulator, just like plastic. The trip wire is an NC (normally closed) switch, meaning that this circuit works until you trigger the switch. So we need a way to stop the current (flow of electrons) until we want the buzzer to activate.
When you stick the paper index card between the two tacks in the clothespin, it breaks the electrical connection and the switch goes in the OFF position. Remove the paper and your switch moves to the ON position, and electrons are flowing around and around your circuit, and you hear a BUZZZZZZZZZ!
This alarm has a thin wire that someone “trips”, which pulls out the card, closes the switch, and sounds a buzzer or lights up an LED!
Here’s what you need:
- AA battery case
- 2 AA batteries
- 3 alligator clip wires
- wood clothespin
- 4-6″ piece of steel or (uninsulated) copper wire
- 2 tacks
- buzzer or LED
Download Student Worksheet & Exercises
Troubleshooting: The trip wire is a NC (normally closed) switch. The buzzer makes noise until you ‘push’ (squeeze, really) the switch. To arm the trip wire, insert a small card between the tacks. The card works because paper does not conduct electricity. When the card gets yanked out, the tacks touch and… BUZZ!!!
Installation Tip: Hide this switch down low by the door frame and use fishing line instead of string to make this burglar alarm virtually invisible. Use a tack in the frame or tie the line to the door hinge to secure and wait for the action…
Exercises
- How does this work?
- What type of switch is the trip wire?
- Name three places you can install this alarm.
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