What it shows
Three LEDs each with their own resistor and switch share one battery. Each switch controls only its own branch, which is a parallel circuit, the way the lights in a house are wired so that one can be on while the others are off.
The science
Each LED is in series with a 150 Ω resistor and a switch, and the three branches are connected side by side across the battery. Because the branches are parallel, each one sees the full 3 V regardless of what the others are doing, and closing one switch sends current only through its own LED. The resistor drops the voltage left over after the LED takes its 1.8 to 2.2 V and limits the current to roughly 5 to 8 mA, so the LEDs stay bright and cool. Pressing in the sequence red, red with amber, green, amber copies a real signal cycle.
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.
- One red, one yellow and one green 5 mm LED
- Three 150 Ω resistors
- Three push buttons or paper clip switches
- A 2 x AA battery holder with cells (3 V)
- A cardboard tube painted black as the signal post
- A breadboard or a cardboard strip with brass fasteners, plus wire and tape
Safety
For this project: Battery power only. Scissors and drawing pins with an adult present.
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.
Cut three holes in a line down the cardboard tube and push one LED through each, red at the top.
Connect the long leg of each LED to its own 150 Ω resistor.
Join the three free resistor ends together and connect them to the battery red lead.
Connect the short leg of each LED to one side of its own switch.
Join the other side of all three switches together and connect that to the battery black lead.
Press each switch in turn: only its LED lights. Press two: both light with no change in brightness.
Run the Indian sequence: red, then red and amber together, then green, then amber, then back to red.
Time each phase with a watch and write the timings on a card beside the model.
What you should see
Each button lights only its own LED, two can be on together at full brightness, and the model shows a complete signal cycle.
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 lighting a second LED not dim the first?
They are in parallel, so each branch gets the full battery voltage.
2Why does each LED need its own resistor rather than sharing one?
With one shared resistor the current would be shared and the LEDs would dim as more came on.
3What does amber mean for a driver?
Prepare to stop, or with red, prepare to go.
4What limits how many LEDs you could add in parallel?
The total current the cells can supply before they go flat quickly.
Take it further
Replace the switches with a 555 timer and two transistors to make the lights change by themselves.
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.