What it shows
Each lamp has its own circuit with a cell and a switch path. The model shows red, yellow and green lights and teaches how a circuit is completed.
The science
An LED lights when current flows through it in the right direction, and it needs a series resistor to limit the current. A switch decides which LED joins the circuit. When the foil contact on the turning disc touches a pad, that LED’s loop is completed and it glows. Real traffic lights use timers and controllers to do this automatically.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A cardboard box painted as a signal post
- Three LEDs red, yellow and green
- Two 1.5 V AA cells in a holder
- Aluminium foil strips
- A cardboard disc with a foil contact as a rotary switch
- A 100 ohm resistor and copper wires
- Tape
Safety
For this project: Use only cells, never mains. An adult must supervise the wiring and cut the card.
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.
Paint the box with three circular windows for the lights and cover them with coloured cellophane.
Put one LED behind each window, with the short legs connected together by a wire.
Join the common wire to the resistor and then to the negative end of the cells.
Wire each long LED leg to a separate foil pad on a cardboard dial.
Fix a moving foil arm on the dial, joined to the positive end of the cells.
Turn the arm to touch each pad in turn and see each light glow.
Practise the red, red and yellow, green and yellow sequence.
Add a small pedestrian cut-out and label the signals.
What you should see
Only the LED whose pad is touched by the arm glows, and the order of colours can be set by hand.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does the resistor do?
It limits the current to protect the LED.
2Which leg of the LED is positive?
The longer one.
3Why does only one LED light at a time?
The arm completes one circuit at a time.
4How are real signals controlled?
By timers and controllers.
Take it further
Use a 555 timer chip to change the lights automatically.
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
Best Out of Waste Science Projects
Working models from bottles, cardboard, CDs and old parts