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Remote Lockbox

I would like to build a small lockbox to which I could remotely provide access.  For example, I leave the box at a property.  At a later date, an employee is in the area and we would like him to access the property without having to make a trip back to the home office to pickup the keys.  We provide him a one-time access code and he retrieves the key.

I recently discovered how much fun Raspberry Pi DIY projects are and this seems like an ideal candidate for my next challenge but I have to confess I am still a total neophyte to all this with no real notion as to the best way to move forward.  I would welcome any suggestions or ideas.  I also realize my definition of the problem might be more vague than is ideal but given my lack of confidence vis-a-vis which details are relevant, I thought it would be more productive for me to answer questions rather than post a giant wall of useless information - which is my long-winded way of saying, please do not hesitate to ask me any questions if they'll help get me pointed in the right direction.

Jason357 (author) 3 months ago

Also, one detail I now realize would be key - the perfect world scenario is that the system works when no communication is possible. Which is to say, I download some bit of software with a particular unique seed to the lock and then the linked/matching software at the home office generates the code that will unlock the box at that particular moment as opposed to sending some communication. Or something along those lines. (For clarity, my intention here is mostly to use this forum as brainstorming tool, so if you've got slightly different ideas that you think might be better, don't be shy.)

Well then just upload a code list, similar what banks use with the TAN and PIN system.
Have one master code set that is long and permanent and a list with 4 or 5 digit codes.
You have the same list somewhere in your office or on the mobile and tick the codes off one by one.
The box uses the first code in the list only, once used it takes the next code only and ignores all previous codes.
The list on the box can contain a few thausand codes if you like as only one will be active the hacking chances are quite slim, especially if you include lock out timers after wrong code entries.

Jason357 (author) 3 months ago

Downunder - That is all very helpful and will likely get me moving in the right direction, thank you kindly for your input. But with that said, if anyone else has thoughts/ideas/details they'd like to contribute, please don't hesitate to continuing chiming in.

Downunder35m3 months ago

One way could work like this:
Microcontroller with GSM shield so you can use a SIM card.
Digital keypad for code entry.
Battery pack for power supply.
Small relay style locking bar (like on most cheap safes from home depot).

Electronics are in standby mode until a SMS is received.
SMS is read and the content used as the match for the code entered on the keypad.
If correct code is entered locking bar will be activated to open for a specific period of time.
Once the key is put back and box closed a switch in the door will reset the code, so one time use only.
In case of network problems or general GSM failure a default code of suffient length should be added to your program to allow access for battery replacement and such things.
If that is not secure enough than leave the default code out and use a manual override for the lock bolt with a key.
I suggest using an Abloy lock for this purpose and the box to be made from stainless steel of at least 3mm thickness.
GSM antenna needs to be on the outside so a proper polycarbonate cover for it is recommended.