power consideration for a lock controlled from smart phone?

Discussion in 'The Projects Forum' started by rasalll, Sep 9, 2013.

  1. rasalll

    Thread Starter New Member

    Sep 9, 2013
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    Hello,

    My first post :)

    I am working on a project where a door lock will be controlled from a smart phone. The mode of communication is low power bluetooth (BT 4.0). The battery constrain is 3V (CR2032). The microcontroller will usually be sleeping except when a connection is initiated from the phone only then will it wake up and process the request and go back to sleep.


    my questions?
    - should I use solenoid or a low power dc/stepper motor to lock the door and unlock it?
    - my main concern is power, what factors should I be looking at?
    - any suggestions on low power pic microcontrollers? Is nanowatt from microchip good?
    - I would be using an led for low battery status, would that affect power?

    I am a software developer and have little experience with hardware but I am really interested in designing hardware, so please help me out here
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There is magnetic pulsed solenoids that are often used for locks where it is desirable the power is not on continuous.
    These just require a DC pulse for each direction.
    I am assuming more power will be available for the lock/unlock activation?
    Max.
     
  3. rasalll

    Thread Starter New Member

    Sep 9, 2013
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    Thank you for the prompt reply. Do these solenoids go by 'magnetic pulsed solenoids' or is there a specific name?

    Well the battery I am shooting for is the CR2032 which is only 3V rated at 240mAH. would that be an issue?
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you need to activate the lock with the low power battery, you need to discover the energy required to operate the door lock?
    3v for the pulsed locks may be on the low side?
    Max.
     
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    How many times do you want or expect it to work on a single battery?
     
  6. rasalll

    Thread Starter New Member

    Sep 9, 2013
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    well probably 3 to 4 times a day. But I guess it depends on how many times an individual opens his locker a day.
     
  7. mcgyvr

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    For 1 day or 365 days?
     
  8. richard.cs

    Member

    Mar 3, 2012
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    It can be done off 3V but I would expect the locking/unlocking process to involve charging a large electrolytic to a few tens of volts with a boost convertor before dumping it across the solenoid. It it were 20 Volts and 10,000 microfarads (this is similar to the requirements of the small solenoids used on model railways) that would be 2 Joules per actuation. A typical CR2032 is 225 mAh @ 3V so very crudely (since we're not taking into account voltage drop at end of life or under load) can supply 3*0.225*3600 = 2.4 kJ, that's 1200 actuations, more like 1000 when you account for boost converter eficiency.

    You may actually find the sleeping micro and the bluetooth dominate the battery requirements unless your solenoid is much bigger simply because they're on for more hours.
     
  9. rasalll

    Thread Starter New Member

    Sep 9, 2013
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    hmm...I can always include an on/off switch so that the user can turn it off when not in use. I didn't know about the boost converter and probably something I should look into.
     
  10. richard.cs

    Member

    Mar 3, 2012
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    Is there any reason for specifying such a small battery? The locking hardware will be quite bulky and I'd be suprised if a pair of AA alkalines make much difference to the overall size.
     
  11. MaxHeadRoom

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    The ones I have used are made by Pontiac Coil, but AFAIK they are 12vdc and up?
    Max.
     
  12. rasalll

    Thread Starter New Member

    Sep 9, 2013
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    well its the portability issue. I want to keep things small :)
     
  13. richard.cs

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    Mar 3, 2012
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    Portability? It's not something permanantly bolted to a door then?
     
  14. rasalll

    Thread Starter New Member

    Sep 9, 2013
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    nope, I want it to be portable. So that the user won't have to carry keys or remember combination lock etc. you can just unlock with smart phone.
     
  15. MaxHeadRoom

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    Jul 18, 2013
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    But surely the lock mechanism and power for it will be permanently in place?
    Unless I am missing something?
    Max.
     
  16. rasalll

    Thread Starter New Member

    Sep 9, 2013
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    well one can change the batteries when they are low, there will be an indication to the user.

    and the lock mechanism will be permanent. Just like we have it in any lock.

    I hope I answered you question :)
     
  17. MaxHeadRoom

    Expert

    Jul 18, 2013
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    For a re-cap:
    Looking at your original requirement using a CR2032.
    As I mentioned in a previous post, one of the things to discover is the energy required to operate the lock mechanism, whether this is done by motor or pulse solenoid, that average locker or any deadbolt is usually not a flimsy object.
    Are you designing the lock mechanism or operating and existing one?
    I believe the Micro power will be the least of your worries, any operating mechanism requires a driver or amplifier power at the ready, unless as you say it can be disabled, or you create a circuit to shut down after a unused period.
    Any circuit detecting low voltage requires battery power, so if the battery dies, what lights the LED?
    Are these lockers all in one place or scattered?
    If all in one place then a central power system could be set up with Gell Cells maybe?
    I can see the CR2032 being the stumbling block.
    Max.
     
  18. rasalll

    Thread Starter New Member

    Sep 9, 2013
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    for operating the lock mechanism someone mentioned a servo motor that can operate off of 3V with quite a punch (http://www.hobbyking.com/hobbyking/..._5g_3v_.html?gclid=CKqE3L25w7kCFXHMtAodI3kAAw)

    And for the battery indicator the user will be given a very early indication. And when battery is getting very low then one should not be able to lock it, thus forcing the user to change the battery. (but i have to work this out)

    it is a portable lock that people can carry with them, just like a padlock or combination lock but does not require keys and can be opened through smart phone.
     
  19. StefanS

    New Member

    Oct 1, 2013
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    Hello!
    Did you programmed the part of the program wich will do this? I'm trying to do something like this but I'm not a very good programer so I would need some help. Could you give me the piece of code that does this?

    At this moment I'm creating a new thread for asking some help for my project. I'll post here the link of the thread so you can see what I'm talking about.
    Thanks!
     
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