Lock Box

[ruru67]

Here's a little project I came up with.

A little decorative wooden trinket box ...

... into which one can place small objects, e.g. keys ...

...and close the lid.

On the back, a 12v power supply input, and two buttons.

The left hand button engages a solenoid, which prevents the slide latch from opening. The right hand button releases it. But not just any time ...

... as inside is a simple timer. You can see the solenoid at the front of the box, with the power inputs, lights and buttons. The "close" button can be pressed any time, but the "open" button only activates the solenoid when the timer has expired. A jumper on the timer circuit board (a cheap kit) sets the length of time to wait before this happens, from 36 seconds, to up to 10 hours (according to the little table attached to the bottom of the box). The cardboard tray prevents metal objects like keys from causing any embarrassing short circuits.

The red light indicates that the box can't be opened; a green light lights when it can. One can usually hear the relay on the timer click when it's time.

Safety ... well, the box is wooden. It's not going to last long under serious abuse -- I figure the screws on the hinges won't stay in place long faced by a decent sized screwdriver, or the top can be broken by putting something into the decorative holes and levering it over. But you'd have to really want to destroy it ...

[Riddle]

Wow, very nice looking lock box. I may have to try to copy your idea.

Just out of curiosity, may I ask why you chose cardboard for the interior shelf?

[onestrangeguy]

You mentioned that your tmer circuit is a 'Cheap kit' but you didn't mention where it's available.

Nice job and looks like a good design.

My thoughts were of a battery power supply, or at least a backup. What happens when the box is unplugged? does the timer reset and allow you to open the box when it's plugged in again?

[Kronopticon]

good point strangeguy... does it reset it? or does it lock it permanently

still, i REALLY like that box, i think something a bit more discreet would be perfect for me, but i still really like that one, so do go on, how DID you make it? is it all self fabrication of the hardware, soldering an all?

[ruru67]

Just out of curiosity, may I ask why you chose cardboard for the interior shelf?

Function vs effort / cost / material availability. The box is nice, but not expensive. The cardboard shelf was only a few minutes' work, and it's just to keep keys and electrickery apart.

how DID you make it? is it all self fabrication of the hardware, soldering an all?

Everything on the PCB is part of the kit, apart from changing out one resistor to increase the timer range. (I think the default range was a second or so up to 30 minutes.) The switches and LEDs are off the shelf, the solenoid used to be the case lock for a desktop PC. Yes, there is soldering required. That said, I'm no electronics guru, hence the use of a kit rather than designing the timer circuit myself.

Other work involved drilling out the holes for the switches, LEDs & power input, and the hole to allow the solenoid to engage the sliding catch on the box.

You mentioned that your timer circuit is a 'Cheap kit' but you didn't mention where it's available.

Have a look at your local electronics supply / hobby store. They usually have such kits -- it's just a 555 timer, a relay and the necessary bits to make the 555 do its thing, and of course a pre-printed PCB to put it all on. You could probably knock one up on a prototype board -- the 555 is a pretty well-known timer chip, and there are lots of circuit designs out there you can use / re-target.

This kit, at http://www.apogeekits.com/start_stop_timer_vm141.htm

looks like it would be really easy to use, and wouldn't need as much fooling around to get it right as the one I used. There are simpler (cheaper) timer kits from the same supplier, but would need work to make them adjustable.

My thoughts were of a battery power supply, or at least a backup. What happens when the box is unplugged? does the timer reset and allow you to open the box when it's plugged in again?

The timer starts over after a power break. Which means if you've set it for 20 minutes, it's going to stay locked 20 minutes from the time it gets plugged back in. I did think about using a battery, and it wouldn't be hard; i just didn't have one on hand and I couldn't be bothered thinking about how it would fit in.

While it can go to 10 hours or so, I'm yet to use that long a period; I figure that if I get stuck for an extra hour or two, it's probably OK; if Ireally need to be out, breaking the box is always an option.

If the local utility power is unreliable, you could use an external power pack and battery, but that doesn't solve the unplugging problem.

You could also hard-wire the battery/power pack (instead of having an external socket on the box), but that introduces the possibility of internal damage if the power cord gets pulled -- I'd rather have it come out and be able to be plugged back in straight away (with the associated time penalty) than risk having it not work when you expect it to. The short answer is to locate the box, power supply and cable so as to minimise the risk of pulling any plugs.

As things stand, the box can be locked and unplugged; that way the keys are unavailable until after the box is plugged back in and the timer has expired. An enhancement would be to add a key switch -- then the box could only open if the key was in and the timer had passed, or (with slightly different internal wiring) it would only power the timer when the key was in (so you'd have to obtain the key and then wait for the timer).

Also, the timer is set up to be recurring -- the lock re-locks after the timer period has expired again (and unlocks again later and so-on).

[tiemeupalso]

what happened to the pics?

[bound_jenny]

I see them. Nice box.

Jenny.

[nitro]

Hi,

I like the box and nice idea .
It would be nice if it could be self contained using battery power. Some of the digital countdown timers (egg timer type units) could be modified to trip the solenoid. Something like a couple AA batteries for the timer and a 9v for the solenoid?

[onestrangeguy]

I sort pf like the idea that you have to be there at the right time to open it before it resets and you have to wait again.

[ruru67]

It would be nice if it could be self contained using battery power. Some of the digital countdown timers (egg timer type units) could be modified to trip the solenoid. Something like a couple AA batteries for the timer and a 9v for the solenoid?

I'd only consider internal batteries as backup power for the timer.

Even if the timer's power was low, keeping the relay pulled on takes more power than you think, and you risk draining the batteries. You could make it fail safe, so that the timer circuit pulls the relay open, then when the timer's power goes away, the relay closes and the solenoid (on separate power) can be opened.

Don't underestimate how much power that solenoid sucks -- it's a fairly heavy sprung thing, with quite a long throw. Not that you couldn't find a different one, but I'd be pretty un-keen on one that wasn't sprung. The spring makes sure that when the solenoid is open, it's really open, and when closed it stays closed, but it pulls a lot of power overcoming the spring. I think that part would need to be (or at least be able to be) powered externally.

For the solenoid, I'd be leaning toward a reasonable sized 12V sealed lead-acid battery with an external charger over a crummy little 9-volt thing that might close the box three times but only open it twice before giving up the ghost ...

I sort of like the idea that you have to be there at the right time to open it before it resets and you have to wait again.

Yeah. It's a pity the timer doesn't have separately configurable "on" & "off" intervals. If I had a lot more time and money on my hands, I'd be thinking about using some kind of micro-controller or single-board computer to do the timing.

[onestrangeguy]

There is a guy on this forum who is just the person to help you with that, although he hasn't responded yet. Maybe he'd take an interest in what you're doing.

[bobbi]

that lock box is pro.

must aquire timer kit.

then its just a matter of finding a solenoid that will work. i consider myself electronically inclined but solenoids still bork me.

[ruru67]

An aside, I just posted a story on the Bound Forum which involves a variation of this box:

http://www.boundforum.com/forum/forum_p ... ?TID=10170

[Dark_Lizerd]

Nice box, I would hate to destroy it just to get out early...
With the timmer reset on power loss, I would be up set after a 36 hours weekend game and have the power go out Sunday afternoon...

[nitro]

Browsing the Smarthome website I spotted a small magnetic lock. It could work to hold a box closed or a key in the air. It is rated about 85mA at 12V, 165mA at 24V. A good alkaline battery is about 500mAh, If I get my calculations right that would give you at least a 4 hour lock time using 2 fresh 9V batteries in series. (assuming the magnet will hold at 9.6V that the batteries will provide after 4 hours) The time might be longer depending on the needed pull from the magnet. The spec is 80lb magnet pull at 12V. Depending on the lid design for the box a single 9V battery might even keep it secure for a number of hours. (Of course testing is needed with the final design)

This is a failsafe design. When the battery dies there is nothing to hold the box closed. The simplest timer would be to pick a battery brand/type that gives you the lock time you want.
(eg. Lithium=10hrs, alkaline=4hrs, NiCad rechargeable = 1hr)

mlock.jpg (3.06 KiB) Viewed 8296 times

http://www.smarthome.com/7044/80-Pound- ... -80/p.aspx
It is a bit pricey but you might find some at a discount (eBay?)