What it shows
A switch is just a gap in a circuit that can be closed. Two foil sheets held apart by a foam ring touch when pressed, completing the circuit so current flows through the LED.
The science
Current can only flow around a complete loop. The foam ring keeps the two foil sheets a few millimetres apart, so the loop is open and the LED stays off. Pressing squashes the foam, the foil sheets meet through the hole and the loop closes. The LED needs the 150 Ω resistor so that the 3 V supply pushes only about 8 mA through it; without it a red LED would be overdriven.
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.
- Two pieces of stiff card, about 10 cm square
- Kitchen aluminium foil
- A foam sheet or thick cardboard ring with a hole cut in the middle
- One red LED and a 150 Ω resistor
- A 2 x AA battery holder with two AA cells (3 V)
- Two 30 cm lengths of insulated wire
- Glue, tape and scissors
Safety
For this project: Scissors only with an adult nearby. Keep the foil sheets from touching the battery terminals directly, which would warm the cells.
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.
Glue foil smoothly onto one face of each card, leaving a tab of foil hanging off one edge of each.
Tape the bare end of one wire firmly onto each foil tab so it makes good contact with the foil.
Cut a hole about 4 cm across in the foam sheet and glue the foam onto the foil side of one card.
Lay the second card foil side down on the foam so the two foils face each other across the hole.
Tape the edges so the cards stay aligned but can still be pressed together in the middle.
Connect the battery holder red wire to the LED long leg (anode) through the 150 Ω resistor.
Connect the LED short leg to one pad wire, and the other pad wire back to the battery black wire.
Press the middle of the pad: the LED lights. Release and it goes off. If it stays on, use thicker foam.
What you should see
The LED lights only while the pad is pressed and goes off the moment it is released, showing a working normally open switch.
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 LED stay off when nothing presses the pad?
The foam holds the foil sheets apart, so the circuit is open and no current flows.
2What is the job of the 150 Ω resistor?
It limits the current through the LED to a safe value of a few milliamperes.
3Which LED leg connects towards the positive side of the battery?
The longer leg, the anode.
4Is this a normally open or normally closed switch?
Normally open: it conducts only when pressed.
Take it further
Hide the pad under a doormat and swap the LED for a 3 V buzzer to make a visitor alarm.
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
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each