After I moved into my current house, I noticed that the doorknob on my back door had a very annoying "feature". The doorknob on the interior side would turn and open the door whether the knob was locked or unlocked. My family has always had the sort of knobs that won't turn at all when the door is locked, and has grown accustomed to interpreting a turning doorknob as one that will not lock behind them. Within a few short months, various members of my family have succeeded in locking themselves out of the house using this new type of knob. Instead of completely replacing the knobs (zero fun) or hiding a spare key outside (big security hole), I set out to discover how these knobs work and how to modify them to be more user-friendly.
The doorknob set in question has an interior knob with a rotating tab (see photo) and an exterior knob with a keyhole. The door's primary security device is a deadbolt, and we never intentionally lock the handle. Unfortunately, the lock tab does get turned by accident, usually from being bumped or getting caught on someone's clothing. I figured that I could solve the problem by preventing anyone from unknowingly locking the door by accidentally bumping the tab.
A Note About Safety: This feature is viewed by many as a safety issue. In the event of, say, a house fire, you can quickly exit without needing to fumble around and unlock the door. This modification does not impact the safety aspect of this model of knob; you can still open the door from the inside without needing to unlock it. You should probably not make this modification on knobs that have keyholes on both knobs.
Step 1: Remove Doorknob
The doorknob will be held to the door by two screws. Remove these screws and the doorknob should pull apart into three pieces: the interior knob, the exterior knob, and the latch mechanism that sits inside the door. Take a good look at how the knobs fit together when removing them, since you'll need to put everything back together later.
Try to keep a hand on both knobs when removing the final screw. Without the screws in place, they tend to fall out of the door and can get scratched or dented if they hit hard pavement.
Step 2: Modify Lock Mechanism
The locking mechanism is part of the exterior knob. That knob has a square shaft attached to it, which causes the latch to retract when the knob is turned. A long, flat rod sits inside the shaft and protrudes from it several inches. This rod is permanently attached to the locking mechanism and when the doorknob is fully assembled, the far end of it will connect to the rotating tab on the interior knob. Turning the tab will rotate this rod, thereby engaging or disengaging the lock.
This flat rod is the source of all of our problems. Cut off the protruding end of the rod using a rotary tool or a hacksaw. Cut the rod close to the end of the shaft, but be careful not to cut the shaft itself. By cutting the rod, you are severing the rotating tab's connection to the lock.
Step 3: Re-assemble the Doorknob
Carefully put the doorknob back together. This generally involves attaching the exterior doorknob first - sliding it through the holes in the latch mechanism - and then re-connecting the interior knob. Replace the two screws, then check that both handles still turn and engage the latch.
You will notice one key difference with your modified doorknob: the rotating tab on the interior knob now spins freely. Cutting the rod severed the tab's connection to the lock mechanism, and the tab no longer serves any function. The only way to lock or unlock the knob is by using a key on the exterior knob, ensuring that both you and your key are on the same side of the door. There are still some rather elaborate scenarios where you can lock yourself out, but they're far less likely to occur since they all require you to knowingly and intentionally lock the door.
You lose the ability to lock the knob from inside the house, so this shouldn't be done if the knob is your only way to lock the door (knob locks aren't very secure so if this is your situation, you *really* need to consider adding a deadbolt). We use a separate deadbolt for security, not the knob lock, so it wasn't a factor for us. Making this one simple change has completely halted our pattern of people locking themselves in the back yard, without having to re-knob the house or introduce any new security problems.