Staircase lighting
Curricula: KS3, Sek I, cycle 4
Aim¶
Understand how a circuit works in which one lamp can be switched on and off from two places, like the light on a staircase or in a long corridor.
The bench¶
A 4.5 V source, two two-way switches and a 4.5 V / 1 W lamp. The positive terminal of the source goes to the common terminal of the first switch, and the common terminal of the second goes to the lamp. Terminals "1" and "2" of the switches are joined crosswise by two yellow wires: "1" of the first to "2" of the second, "2" of the first to "1" of the second. The second switch is turned upside down. Both blades are in jaw "1", the lamp is off.
Procedure¶
- Throw the blade of the first switch into the other jaw: drag the handle or choose the position in the settings panel.
- Throw the blade of the second switch.
- Throw the blade of the first again, then the second. After each step write down whether the lamp is lit.
- Trace the path of the current along the wires for each position of the blades.
Expected result
The lamp is lit when the blades are in jaws with different numbers, and goes out when the numbers are the same. Each throw of either blade changes its state.
| First | Second | Lamp |
|---|---|---|
| 1 | 1 | off |
| 2 | 1 | on |
| 2 | 2 | off |
| 1 | 2 | on |
Questions¶
- Why are ordinary switches with two terminals not suitable for such a circuit?
- Why does the lamp go out when both blades are in jaws with the same number?
- What happens if the yellow wires on one of the switches are swapped?
Answers
- An ordinary switch can only close or break one wire. Two of them in series light the lamp only when both are closed, and two in parallel light it whenever either is closed, so in some positions one switch cannot change what the other has set. A two-way switch moves the current from one yellow wire to the other, and this lets each switch turn the lamp both on and off.
- The yellow wires cross, so jaw "1" of one switch is wired to jaw "2" of the other. With both blades in "1", the current goes from the first blade along a yellow wire to jaw "2" of the second switch, where there is no blade, and the circuit is open; with both blades in "2" the same happens along the other wire.
- The wires then join jaws with the same numbers, "1" to "1" and "2" to "2". The table turns over: the lamp is lit when the numbers are the same and off when they differ, and each throw still switches it on or off.