What it shows
Two single-pole double-throw switches wired together let you switch one lamp from either end of a staircase. Flipping either switch changes whether the loop is complete.
The science
Each SPDT switch has one common leg and two other legs; the common leg connects to one of the other two at a time. Joining the two throw legs of switch A to the two throw legs of switch B with two separate wires gives two possible routes between the switches. The loop is complete when both switches select the same wire, and open when they select different wires. So flipping either switch toggles the lamp. The 150 Ω resistor limits the LED current to about 7 mA.
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 SPDT slide switches (or two 3-pin toggle switches)
- One red LED and a 150 Ω resistor
- A 2 x AA battery holder with two AA cells (3 V)
- Five insulated wires about 15 cm long
- A cardboard staircase model or a plain card base
- Glue, tape and scissors
Safety
For this project: Use the cell holder only, never a mains socket or a house switch. Take care with scissors.
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.
Step 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.
Fix the two slide switches at the bottom and top of the cardboard staircase and label them Switch A and Switch B.
Find the common leg of each switch: it is the middle leg on most slide switches, or test it with the circuit once built.
Join the first outer leg of switch A to the first outer leg of switch B with a wire.
Join the second outer leg of switch A to the second outer leg of switch B with a second wire; do not cross the two wires.
Connect the battery holder red wire to the common leg of switch A.
Connect the common leg of switch B to the 150 Ω resistor, then the resistor to the long leg (anode) of the LED.
Connect the short leg of the LED back to the black wire of the battery holder.
Flip switch A, then switch B, in all four combinations and note when the LED is on. If it never lights, check the LED direction first.
What you should see
The LED lights in two of the four switch combinations and is off in the other two, and flipping either switch always changes the state.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Why do two switches control one LED?
Two parallel routes join the switches, and each switch chooses which route is connected, so either one can break or complete the loop.
02What does SPDT stand for?
Single pole double throw: one common leg that connects to either of two other legs.
03What is the 150 Ω resistor for?
It keeps the LED current to a safe few milliamperes from a 3 V supply.
04Where is this arrangement used?
On staircases and in long corridors in houses, with proper mains-rated switches fitted by an electrician.
Take it further
Add a third switch in the middle of the two-way set using a DPDT switch wired as an intermediate switch.
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
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