One Bulb with 3 Switches Puzzle

By | December 23, 2017

One Bulb with 3 Switches:


There are 3 switches in a room. One of them is to turn bulb on in next room. You can not see whether the bulb is on or off, until you enter the room.

What is the minimum number of times you need to go in next room to determine which switch is for the bulb in next room.

As we say always, think before you read the answer.



This needs some planned steps :

First switch on the first switch, keep it on for say 2-3 minutes.
then switch of first switch and on the second switch.
as soon as u on the second switch go to the room.

  • If you found that the bulb is glowing  that means the second switch corresponds to the room.
  • If bulb is not glowing, then either it would be first switch or the third switch. You need to touch the bulb to determine now:
    • if the bulb is warm, it means that your first switch is the switch you are looking for ( since you switched on bulb for some time).
    • if the bulb is cold ( normal ), then third switch is the switch.


The answer will be 1. We can go to next room only once to determine our switch.