It depends on what type of person designed the circuit and what type of person you are.

Ergonomics: The switch closest to the door first, then mid, then far, figuring the unknown user would click the switch closest, a skilled electrician would start there. However, it’s not unreasonable for the electrician to ask the owner, so this is a hit-or-miss approach.

Installation efficiency: The installer refused to mark any of the lines and instead hooked them up at random, flip in any order, when you find the right one, return the others to the original state.

time efficiency: the energy cost to flip all three switches is minimal and you’re only going in once, flip all three at the same time. you’ve done maximum effort and maximum time savings.

Error reduction, binary counter, all combinations tested in case of chained switching

Debugging: binary counter, followed by checking the lightbulb, possibly swapping for another if one is nearby, checking all the other switches near the room, breakers, power to the structure, and asking an occupant for assistance as a last resort.

Disaster recovery: locate a flashlight or use your phone’s torch/flashlight function.

Ahh crap, other room.

1. ask an occupant

2. shove a penny in the socket behind the light bulb and listen for a breaker to pop

3. turn all three on

4. slide your cell phone under the door with video recording on, stomp on the floor hard every time you flip a switch

5. turn all the switches through a binary counter looking for one that seems to do nothing.

What bothers me about this specific question, apart from it being dated, is that it breaks the rules of these kind of riddles. They’re implied to be in a sort of frictionless sphere universe, the whole preposition is silly except as an abstract puzzle. To then rely on the physical properties of real lamps is cheating. You’re supposed to ignore all the real-world aspects of the setting except that one.

Remove the switches put a microcontroller like esp32, connected via wifi to an app on your phone. Go to the other room and see which switch switches on the bulb.

If there is no wifi, why the hell do you want a programmer. I can’t work without internet.

Unlabelled switches controlling lights in another room isn’t Workplace Health and Safety approved.

Lockout both rooms and log a job with maintenance.

Go into the room and unscrew the bulb. You can now truthfully say that no switch affects the bulb’s condition, without messing with a bunch of switches whose function you don’t understand. You even know for a fact that the lack of bulb won’t cause a problem down the line, since the room is apparently no longer accessible.

The official answer to this riddle is turn switch 1 on for a minute or so, switch it off then switch 2 on. if the bulb is hot but dark, its 1, if it’s lit it’s 2 and if it’s out and cold its 3.

the adult answer is why do I have only one chance to walk in the room?

“First, I would get a label maker and ask a coworker to assist me. Then, we’d work together to quickly figure out what each switch does, and then label them accordingly. In a business of this size and reputation, documenting your work and synergistic teamwork are foundational to value and growth.”

Then, reject whatever offer they send and say that it’s because they showed you a workplace culture that enabled middle management to test employees with busywork instead of minding their own business or solving their own damn trivial problems.

Literally ask someone which switch it is. Then ask them what idiot wired them up that way

Ok. The classic answer is “turn on the first switch for five minutes. Then turn switch 1 back off, turn on the second switch and go in the room immediately. If the light is hot, it’s controlled by switch 1; if it’s on, it’s controlled by switch 2; if it’s off and cold it’s controlled by switch 3.”

Except that a light bulb in 2025 is very likely to be an LED bulb, so it wouldn’t actually get hot. At least not hot enough to feel even a few moments later. And in a corporate setting (this is classically an interview question), the switch has been more likely to control a fluorescent tube, which can get hot, but typically not as quickly as an incandescent one.

My answer, if I were in an interview, would be to ask questions (Chesterton’s Fence).

• First of all, why do we have the one-visit limit? Is this a prod light bulb? We need a dev light bulb environment, with the bulbs and switches in the same room. (While we’re making new environments, let’s get a QA and regression environment, too. Maybe a fallback environment, depending on SLAs.)

• Second, what might the other switches do? What’s the downside to just turning them all on? If that’s not known, why not? What is the risk? For that matter, do we know that only one switch needs to be turned on to turn on the light, or is it possible that the switches represent some sort of 3-bit binary encoding?

• Third, why were the switches designed this way? Can they be redesigned to provide better feedback? Or simplified to a single switch? If not, better documentation (labeling) is a must.

• Fourth, we need to reduce the length of the feedback loop. A five minute test and then physically going to touch the bulb is way too long. Let’s look into moving the switches or the light in our dev environment so that the light can be seen from the switches.

I would say, I do enjoy riddles, so this will be fun. But I am concerned that if you think my skill at riddles is critical, that it may mean your management has gotten used to not fully thinking through the objectives they give and how those objectives interact with the existing systems or other objectives. That would result in the kind of product that looks like the right hand doesn’t know what the left is doing. If that is your reasoning for the question, how is the company countering it to create a coherent product.

And the reason I might say this is tgat in my experience, companies who ask such questions aren’t the kind I want to work for.

I’d walk into the other room first and drop a mirror in the hallway on my way back so I could see lmao. My employer wouldn’t want me touching a hot bulb since that might be a workplace hazard they’d be liable for after training me with stupid riddles.

After reading the incandescent bulb solution, and problems regarding touching the bulb, i would switch first switch on for a appreciably long time, such that bulb has hit maximum luminousity (they heat up as they run, the hotter they get, the brighter they are), then turn switch off, and turn second switch on and quicky run to other room. we are trying to observe change in luminousity as time elapses. if it reduces, it was first (we ran it for a long time, there would be some residual glow, from my irl observations from when i was small suggest roughly 1 min period where i can still tell, but bulb wattage, contrat with background and distance matter). if increasing or max luminous, then second, if nothing then third.

but it was a stupid question. my naive guess was it can not be done, because with just 1 binary observation, you can not tell from 3 switches (you need atleast 2, which the solution assumes as temp and light state, i substitute heat with light state in transition). but still stupid. my natural assumption was leds, even when i head incandascent bulbs in my house somewhere for nearly half of my life. it is also stupid, because when you allow me to do something i was mentioned in question to do, i could just bend my way to do anything. like punch/drill through wall, or hack surveillance systems, or just pull out my handy multimeter that i always have on me, open switch box and see which switch is live, which is dead, or see voltage/current/wattage change across the loop, or measure resistance and guess what thing is there, or like blackmail the interviewer to extract the answer.

Ha! Easy! Go in the other room and take a picture of the bulb. Now go back to the switches and flip each one in order, while looking at the picture. When the picture of the bulb shows it lit up, that’s the switch.

if asked this I would go into a complicated explanation of how I would dismantle the switches to identify if they were functioning first because of sub-par outsourced manufacturing standards.

they’d probably attempt to move on to a different question, but I would always bring it back to those shoddy light switches.

“so do you have any questions for us?”

yeah, do you know who the manufacturer of the light switches are? it’s probably Leviton, but I’m hoping it’s Honeywell because they’re far superior in quality. you see Leviton uses brass plated contacts vs Honeywell uses full brass fittings that don’t cause resistance and increases the potential for fires. are you aware that using one brand over another could reduce your insurance costs by up to 3%?

For those that want the actual answer: