By controlling how and when a circuit is triggered, you can easily turn a simple circuit into a burglar alarm – something that alerts you when something happens. By sensing light, movement, weight, liquids, even electric fields, you can trigger LEDs to light and buzzers to sound. Your room will never be the same.

Switches control the flow of electricity through a circuit. There are different kinds of switches. NC (normally closed) switches keep the current flowing until you engage the switch. The SPST and DPDT switches are NO (normally open) switches.

The pressure sensor we’re building is small, and it requires a fair amount of pressure to activate. Pressure is force (like weight) over a given area (like a footprint). If you weighed 200 pounds, and your footprint averaged 10” long and 2” wide, you’d exert about 5 psi (pounds per square inch) per foot.

However, if you walked around on stilts indeed of feet, and the ‘footprint’ of each stilt averaged 1” on each side, you’d now exert 100 psi per foot. Why such a difference?

The secret is in the area of the footprint. In our example, your foot is about 20 square inches, but the area of each stilt was only 1 square inch. Since you haven’t changed your weight, you’re still pushing down with 200 pounds, only in the second case, you’re pressing the same weight into a much smaller spot… and hence the pressure applied to the smaller area shoots up by a factor of 20.

So how do we use pressure in this experiment? When you squeeze the foam, the light bulb lights up! It’s ideal for under a doormat or carpet rug where lots of weight will trigger it.

[am4show have=’p8;p9;p11;p38;p92;p20;p47;p108;p76;’ guest_error=’Guest error message’ user_error=’User error message’ ]

Here’s what you need:

• thin sponge or foam square (about 1″ square)
• AA battery case
• 2 AA batteries
• 3 alligator clip wires
• 2 large paper clips
• scissors
• aluminum foil
• buzzer or LED

Troubleshooting: There are a few problem areas to watch out for when building this sensor. First, make sure the hole in your foam is big enough to stick a finger (or thumb) easily through. The foam keeps the foil apart until stepped on, then it squishes together to allow the foil to make contact through the hole.

The second potential problem is if the switch doesn’t turn the buzzer off. If this happens, it means you’re bypassing the switch entirely and keeping the circuit in the constant ON position. Check the two foil squares – are they touching around the outside edges? Lastly, make sure your foam is the kind that pops back into shape when released. (Thin sponges can work in a pinch.)

What’s happening? You’ve made a switch, only this one is triggered by squeezing it. If you’re using the special black foam without the hole, it works because the foam conducts more electricity when squished together, and less when it’s at the normal shape.

First, the special black foam is conducting some (but not enough) electricity when you squeeze it. It’s just the nature of the black foam included with the materials kit. Second, when you squeeze it, you’re getting the two foil squares to touch through the hole, and this is what really does it for your LED. When you release it, the foil spreads apart again because they are on opposite sides of the foam square.

Bonus Idea: Stick just the sensor under a rug and run longer wires from the sensor to your room. When someone comes down the hallway, they’ll trigger the sensor and alert you before they get there!

Exercises

1. How does this sensor work?
2. What makes this an NO switch?
3.  How can you use both the trip wire and the pressure sensor in the same circuit? Draw it out here:

[/am4show]