Timer Controlled Key Safe

Dear AB,

As you know, the subject of a timer controlled key safe has been brought up a number of times. There have been a few web sites that offered one, but no one I know had been able to obtain one. One CB manufacturer has one for the European folks, but it won't work here in the United States, or any country that uses 110/120 volt 60 cycle AC power.

After looking at several ideas presented on this site, as well as some others, I decided to try and build one. There are more problems associated with this project than might initially be considered. Like what kind of material for the box? What kind of timing device? How will you control the locking mechanism? Do you want to be able to lock the device without the key? If so, how do you make that work? How do you mount the components inside without boring holes through to the exterior?

After thinking about the various types of boxes that might be available without having to fabricate one, I decided some sort of cash box might be the answer. I finally found what was called a "Security Box" at a local office supply. It was locking, fire resistive so it is double walled with insulation in between, and the lock mechanism was in the lid. I wanted this configuration so the solenoid could be mounted in the body of the box and block operation of the lock tang. The double wall construction was nice because it would allow mounting from the inside without drilling through to the exterior. It also made the box more sturdy. Granted, one could still get into the box, but not without some serious tools and the box would be destroyed. This is a sacrifice to the cost of the item. If you want one that you can't destroy at home, several safe manufacturers will sell you a time controlled safe for about $2000.00 US, that's not in my budget!

I decided early on that a solenoid that could block operation of the existing lock was necessary for cost reasons as well. Solenoids are relatively cheap compared to buying an electric striker plate, transformer and timer to run it. Those three items run about $250.00, you also then have to fabricate the box of some type of material, usually wood, to hold them. I'm not a very good wood worker and steel was out of the question.

So what kind of solenoid and what kind of power to run it? AC or DC, what voltage and how long can it be energized without over heating? I looked at a bunch of options and finally it hit me, lawn sprinklers have solenoids, transformers and timers that all work together. They also are designed to be energized for a long period of time, up to the controllers time limit, usually 99 minutes. I finally found a solenoid that would work with a slight modification so I bought it, the cheapest controller the store had, and it came with the transformer.

The sprinkler solenoid has another advantage. Since solenoids only work in one direction, some sort of return pressure is needed to push the plunger back out when the current goes off. The sprinkler solenoid has a built in spring! If I could figure it out, that would allow closing the key safe without having the solenoid energized as well.

If you look closely, you can see the end of the solenoid shaft, it's white, by the orange power cord in about the center of this picture. The key is rotated about 45 degrees and you can see the lock tang in the lid is right above it. If you finish closing the box like this, it depresses the solenoid, then when you rotate the key to the locked position the solenoid pops up and presto! it's locked by the timer. If the lid is closed completely, you can't rotate the key either direction without energizing the solenoid.

I did discover a weakness in the design while working on setting the solenoid height. Since the lock won't operate without the solenoid being energized, I had the timer in manual mode trying to set the solenoid height and it turned off. I was locked out of the thing without the timer having been set to open again! I thought I had this contingency covered by removing the nut that held the lock in place. It would come out when I tried it before installing the solenoid. But with the solenoid mounted, the tang hit the body of it and would not tilt so I could remove it. I was just about ready to destroy the box and start over when I looked at the hinge for the lid, it is a piano hinge. I took that apart and got into the box again. This can be fixed by driving out the pin a little bit, cut off the pin about 1/4 inch, drive it back in, and fill the holes with metal set, epoxy or touch them with a welder. I'm going to do that as soon as I have completed my testing. I've found out the timer is really unforgiving of mistakes in the programming sequence.

Here's the completed key safe. The blue box on the left is just a plastic AC outlet box with a receptacle and cover plate installed. The box is mounted on the side and far enough above the floor that the cover plate rests on the floor of the box and helps support the weight of the transformer. The timer is mounted on the top right with a single screw and the orange power cord can be seen running out the back of the box through two rubber grommets to protect it. It also has a large nylon tie wrap around it to prevent it from being pulled out of the box from the outside. The black object just inside the front edge of the box is the solenoid, mounted with two worm gear clamps that are screwed into the box wall. Everything is mounted from the inside with small sheet metal cap screws, nothing but the power cord is visible on the outside of the box. One advantage to the timer is that it can be un-plugged from the wall outlet and won't lose it's programming as long as the two AA batteries are installed and not dead. However, the solenoid will not work unless the key safe is plugged into an AC power source.

I think it will work out okay, you can program the timer to open the box anything from one minute a week, to three times a day for up to 99 minutes each opening. One more thing about the solenoid, because it doesn't have water pressure holding it, it vibrates some when it's energized, it sounds like a buzzer inside the key safe so you know that it can now be opened. Some folks might not like being able to determine when the box can be opened, but since this will only stay open for a certain period of time and you can't see the timer, at least you can hear it when it can be opened. Right now I'm experimenting with twice a day the box opens for two minutes. You can get the key out but may not choose to use it. The key can be replaced and the box locked anytime, of course the timer is accessible while the lid is open. Maybe some your electronically minded visitors could come up with a better timer, that would not accept unauthorized re-programming, but this is my best effort. Hope it might give some of your readers a way to get a time controlled key safe of their own for a reasonable investment of time and money. The parts only cost about $55.00 and I think that's reasonable.

Gary

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Page last updated 01-May-07 by: Altairboy@aol.com