I have an interior Jack and Jill bathroom that is connected to two bedrooms. I'm trying to solve the problem of someone locking the other door when using the bathroom and then forgetting to unlock it before leaving.

Trying to come up with a solution, I thought of synchronizing the two locks via a light sensor. Light on, both doors lock. Light off, both doors unlock. Unfortunately, I'm unable to find a lock to achieve this.

Does a lock like this exist? What would be another viable solution to this problem?

  • So if the light switch fails in the "ON" position...?
    – DJohnM
    Jan 16, 2016 at 23:29
  • @DJohnM manual control of the locks would still be possible ideally. Jan 17, 2016 at 0:52
  • How about just entry door locks with a key, and leave the key in the lock. This prevents accidentally walking in on someone, but leaves a way to lock and unlock from either side.
    – JPhi1618
    Jan 17, 2016 at 2:58
  • For the common case of ensuring privacy against accidental entry when occupied, it doesn't need a key even. What are referred to as "privacy locksets" have a mechanism to override the lock, commonly with a thin metal pin.
    – Tim B
    Jan 17, 2016 at 16:14
  • 1
    If the doors open outwards then make a rope with two hooks to reach between the door handles. When inside hook the two doors together. You can't get out if you forget to unhook. :^)
    – Transistor
    Jan 17, 2016 at 22:14

3 Answers 3


DO NOT BUILD THIS PROJECT. It is not safe. I am leaving the description here for now to retain the comments.

For a safe version of the Jack&Jill bathroom project see my other answer, that begins with "I think I understand all the requirements:"

Shop for "Electric Bolt Locks (Fail Safe)". These sell for about $40 to $500 but you certainly don't need the expensive kind. They operate on 12VDC or 24VCD so you can wire them safely and easily. Install one on each door and control them both from a single switch inside the bathroom, so that both are locked or unlocked together.

To deal with the possibility of the locks somehow "failing secure" instead of "failing safe" you should get locks that also open with a key (or a pin -- thanks @Tim B), and you should place the transformer somewhere outside the bathroom.

You could use a switch near each door, either in a 3-way or a parallel circuit. The parallel seems less confusing because you can say "Both switches need to be down to get out of the bathroom."

Or if you really want to make the locks to operate with the light then you can just wire the transformer in parallel with the light fixture. Of course this will result in people occasionally thinking they are locked in the bathroom when they are not.

In any case make sure the locks themselves are wired in parallel.

  • 1
    I was thinking along the same lines. If the locks are lock when energised type they can be switched in an emergency off from outside by switching off the circuit breaker at the fuseboard.
    – Transistor
    Jan 17, 2016 at 17:46
  • 1
    @transistor - exactly. Locksets labeled "Fail Safe" will open when the power fails, and those labeled "Fail Secure" will lock when the power fails. Jan 17, 2016 at 20:20
  • That might depend on whether it's a bathroom or a prison! :^) I wanted to avoid any ambiguity.
    – Transistor
    Jan 17, 2016 at 20:24
  • 1
    You might have some problems with egress rules, it needs to be possible to exit a room in an emergency. Contrary to popular belief, crisis does not bring out the best in people, panic often impairs judgment. While the room is filling with smoke, people will forget "turn off both switches". Now imagine the lights are also off, which is why tying locks to light switches would not be legal either in fire code or NEC. Jan 18, 2016 at 21:02
  • 2
    @Wolf Harper - You are correct. On reflection, I believe that my deadbolt solution could be dangerous. I considered only one class of emergency, a power failure. But as you point out, a fire or similar emergency with the power still on and a panicked user could be fatal. The problem is that in a violent emergency the only natural action is to grab one of the doorknobs and open the door. Jan 19, 2016 at 3:26

I think I understand all the requirements:

  1. The natural action when leaving the bathroom is to turn one of the doorknobs and open the door. For safety reasons this action, without any other required motions, must allow egress from the room under any and all conditions. (Thanks Wolf Harper)

  2. Also, to solve the specified problem, opening the door this way, without any other required motions, must normally (ie. absent failure or emergency) unlock both doors. Requiring additional motions to unlock both doors would obviate the prupose of the project; you might as well put up a sign and be done.

  3. Locking both doors must be easy and natural, preferably with a control near the doorknob. I'm not going to try for an automatic solution, such as connecting the locks to the light, because I can't devise a reliable way to tell if the room is occupied.

  4. The house occupants must be able to override the locks from outside the bathtoom (eg. to prevent someone from locking herself in). Having a key or pin hanging nearby is probably acceptable in most households.

This can be done but the solution may cost more than the average homeowner would be willing to spend. You will need:

2 "storeroom" locksets
2 electric strike plates, "fail safe" type
2 micro switches or magnetic switches that can detect if a door is closed
2 illuminated pushbuttons, the kind with the illuminator on a separate circuit
1 stepdown transformer to power the strike plates
1 latching relay matching the voltage of the transformer and strike plates
a roll of low voltage wire, amount depending on size of bathroom etc.

A storeroom lockset always opens from the inside by turning the handle, and is always locked and operated only with a key from the outside. (Usually the outside handle doesn't even turn.) A door with this fitting can be opened from the inside even in a panic and regardless of the state of the strike plate.

The storeroom lockset satisfies requirements 1 and 4.

A "fail safe" electric strike plate will hold the door locked from the outside when energized, and release it when the current is off. The plan is to maintain the strike plate current with a latching relay.

The micro switches or magnetic switches will unlatch the relay when either door is opened.

The required latching relay can be made from a single-pole single-throw normally-open relay, since the relay coil power can be made to flow through the micro switches.

The electric strike plates, micro switches, and latching relay satisfy requirement 2.

The pushbuttons should be mounted inside the bathroom near the doorknobs. I think the best place is on the door, just above the knob, with a tasteful plaque reading "LOCK BOTH DOORS". I have shopped for lock sets and strike plates with built-in switches but have found nothing, hence the pushbuttons.

I like to put pilot lights on everything so I would buy pushbuttons with built-in lamps wired independently from the switch contacts. Failing this I would use doorbell buttons and separate pilot lights. The purpose of the pilots is to reassure the bathroom occupant that both doors are locked. The voltage of the illuminators must match the voltage of the strike plates.

The pushbuttons satisfy requirement 3. This is the weakest part of the design because you are forced to use something made for a different purpose.

This is the kind of project I like to build just for fun. And the part I enjoy most, I might say the reason for undertaking the rest of the project, is the wiring diagram:

enter image description here


I'll assume it's the normal use-case of basically honest people trying to avoid "whoops, sorry, didn't know you were in there". Locks must always allow exit, so deadbolts are out. I'd look for alternate solutions:

  • Electric strikes (retractable door-jambs), which secure with the bathroom light on. Now leave the privacy locks on both doors locked all the time. If the light is off, the strikes are released and the door can be pulled open, locked or not.

  • Wire outlets off the bathroom light, one near each door handle. Put red night-lights in them. If a user sees the red light on, that means the bathroom light is on, and they should knock. Abolish the privacy locks. This is easier wiring than the strikes.

  • Put a turkey skewer on a light chain next to the door handle, so the locked out person can easily override the privacy lock.

Now if Jack is barging in on Jill maliciously, you have a problem - he probably knows about the turkey skewer trick. When Jack gets back from military school, he can find a portable RV toilet in his room, entry-grade locks on both doors, and Jill has the only key.

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