524
Op doesn't have time for interviews (programming.dev)
submitted 10 months ago by to c/programmer_humor@programming.dev
138 comments fedilink hide all child comments
 

Post:

You have three switches in one room and a single light bulb in another room. You are allowed to visit the room with the light bulb only once. How do you figure out which switch controls the bulb? Write your answer in the comments before looking at other answers.

Comment:

If this were an interview question, the correct response would be "Do you have any relevant questions for me? Because have a long list of things that more deserving of my precious time than to think about this!

you are viewing a single comment's thread
view the rest of the comments
[–] 2 points 10 months ago* (6 children)

If it's an LED or flourencent bulb they usually have a small amount of glow after turning them off from the phosphor coating. You might be able to catch that instead of the residual heat, but generally it dissipates pretty quickly, and it might be hard to see with one of the other lights on.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 6 child comments
    •
  • [–] 1 point 10 months ago (5 children)

    Worse, if the LED is wired to the hot side it will just barely glow at all times.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 5 child comments
    •
  • [–] 1 point 10 months ago (4 children)

    I think your house might be wired wrong if this is happening... The only thing I can think of is maybe if you've got some smart switch and no neutral, so the wifi in the switch has to power itself by leaking current through the light, which is a pretty unusual setup. I don't see how this could ever happen on a regular dumb switch.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 4 child comments
    •
  • [–] 3 points 10 months ago (3 children)

    LEDs are so efficient that even microamps can power them. If your LED driver is cheap, it'll run on basically nothing, or charge up enough to start for a fraction of a second.

    The microamps come from a hot wire running next to a switched wire behaving as a capacitor when carrying AC voltage, letting microamps leak through. (It's not required that the light is on the hot side of the switch as I said previously, my bad).

    This can happen if the switch box is a terminal box with hot and switched wires in the same cable, which is rather common. Probably some other configurations too.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 3 child comments
    •
  • [–] 1 point 10 months ago (2 children)

    Well, I can't say I've ever seen it happen, but I could see how it could happen in certain scenarios, especially if the LED has some weird driver in it. Maybe the capacitors in the driver would be allowed to charge up in some designs before getting dissipated through the LED in a flash?
    The simplest form of LED light (just a rectifier and a bunch of LEDs in series for a 120V diode drop), idk if you'd ever see any glow or flashes, since LEDs don't turn on until a certain voltage, and if you're getting like 50V on an open circuit that seems to me like you've accidentally built a transformer in your walls.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 2 child comments
    •
  • [–] 2 points 10 months ago (1 child)

    A very low current transformer, more of kess yeah.

    Some lights will charge op and flicker, others have a constant glow. The speed/brightness depends on how long the wire is, so most residential lamps are unnoticeable even when it happens, but large rooms and weird wiring can make it more obvious.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 1 child comment
    •
  • [–] 2 points 10 months ago

    I guess that makes sense it'd happen more in big buildings. The runs in most houses wouldn't be long enough to have a noticeable induced current without the electrician adding a few extra loops for fun :)

    Thanks for humoring my skepticism, it's been interesting to think about how this would happen.

  • permalink
  • fedilink
  • source
  • parent