Battery powered electromagnetic time lock for mailbox

[ac Smith]

Maybe someone can point me in the right direction on this DIY home project I've been considering. I have a metal mailbox with a magnet to hold the door -- similar to the magnet on kitchen cabinets. I want to replace that magnet with an electromagnet and timer -- both powered by battery. Basically I want the electromagnetic lock to activate at night (say 6pm to 6am) without my intervention.

So what I need is a very small programmable timer that can trip a circuit with the electromagnet. (I think I saw this done with a quartz wrist watch by tapping into the alarm circuit). Any ideas on timers and electromagnets? Tell me if this won't work!

 

[Pommie]

To power an electromagnet all night would require a very big battery and even then it won't last many nights. Have a look at latching solenoids as an alternative measure.

A simpler method would be to use a key when you open it or have a solar panel operate an unlock solenoid but you may have problems on cloudy days.

Mike.

 

[jpanhalt]

You could also use a hobby servo with a hook on the movable part and a staple/loop on the door. If you use a photocell or light dependent resistor (LDR) , you wouldn't need a timer. The whole thing could be run with a CMOS version of the 555 (e.g., 7555) and have minimal battery drain. The servo does not need to be powered continuously (don't get coreless, just the cheapest analog version will work). If you want to use a microcontroller, that could cut current drain even more during the idle period and provide timing.

John

 

[KeepItSimpleStupid]

Electromagnets and batteries don't get along for long times I'm afraid.

Friend has a business and he has a huge box on the wall where stuff can be inserted easily, but you need a key to get in. Furthermore, he has a motion detectuin and video surveillance,

 

[MikeMl]

One data point. I have one of the large electromagnets that is used as part of a card-operated electronic lock at a walk-through gate at an airport. It requires 24Vdc at 0.14A (3.3W) to keep the door locked... That will kill a large sealed lead acid battery in a couple of days...

 

[dr pepper]

An electromagnet would work, make the electromagnet hold the door shut when it is not energised, then have a button to energise the magnet when you want to open the door, the magnet is then only on for 1/2 a sec, to prevent unauthorised acces hack on of those plug in timers to switch dc, you can get them where the clock section runs from a button cell, its not so tricky to find where the o/p is to the switching part, or you could probably find something on ebay.