What it shows
A plastic card between two foil contacts keeps a circuit open. Pulling the card out by a thread lets the contacts touch, which closes the circuit and sounds a buzzer.
The science
The peg is a spring-loaded switch. Foil on the inside of each jaw makes conducting contacts, and the plastic strip is an insulator that holds them apart so the circuit is open. When someone pulls the thread, the strip slides out, the spring closes the jaws and the contacts touch. The circuit is now complete and the current from the cells makes the buzzer sound.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A wooden or plastic clothes peg
- Kitchen foil
- A strip of thin plastic (cut from a bottle or a visiting card)
- About 1 m of thread
- A 3 V active buzzer
- A 2 x AA battery holder with two cells
- Two 20 cm wires, tape
Safety
For this project: Cells only. Fix the thread across a doorway at ankle height only with an adult around, so nobody falls over it.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Wrap a piece of foil neatly around the inside tip of each jaw of the peg and tape it flat.
Tape the bare end of one wire onto each foil patch so it touches the foil.
Connect one wire to the buzzer red lead and the other to the battery holder red wire.
Connect the buzzer black lead to the battery holder black wire.
Cut the plastic strip so it fits between the jaws and still sticks out at the end.
Tie the thread through a hole punched in the strip and put the strip between the jaws; the buzzer must stay silent.
Tie the other end of the thread across the doorway to a fixed hook, with the peg on the door frame.
Open the door slightly: the thread pulls the strip out and the buzzer sounds. Replace the strip to reset.
What you should see
The buzzer sounds when the thread is pulled and the card pulls free, showing a simple normally open switch closed by movement.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the plastic strip stop the buzzer?
Plastic is an insulator, so no current flows when it is between the foils.
2What closes the switch?
The spring of the peg pushes the foil jaws together when the strip is gone.
3Why use foil?
It is a good conductor and flat, so the contacts touch over a wide area.
4How can the alarm be reset?
Reinsert the plastic strip between the jaws.
Take it further
Add an LED in parallel with the buzzer, with its own 150 Ω resistor, so there is a light as well as a sound.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Science Projects for Class 6 to 8
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