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Do i need a relay?

  1. Jan 7, 2010 #1
    i have just purchased a digital lock that works on 12v dc...
    the lock has a transformer and a rectifier , hence i connect it directly to the mains....
    when i switch the button on, the lock opens...

    now i want to connect a keypad to this digital lock such that it opens only when a correct code is entered......
    my question do i need a relay to operate this mechanism..? if yes then why?
  2. jcsd
  3. Jan 7, 2010 #2
    If the keypad runs on 12V dc (regulated), then you could use an npn open collector (common ground) to operate the lock. A relay does provide an extra layer of isolation, however, in case the lock solenoid requires a lot of inductive current.
    Bob S
  4. Jan 7, 2010 #3


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    A relay is an old fashioned device which does a lot for you. It provides very good isolation (it can switch AC, for example) and, effectively, very high signal amplification. You may need a diode and capacitor to deal with switching spikes with an inductive load but that is not a problem. The only thing wrong with relays is reliability, because they have moving parts (and, of course, they are non-linear). Nonetheless, they are a very effective low-cost way to achieve just what you seem to want. Reliability is not really an issue.
  5. Jan 8, 2010 #4
    thanks a lot for the reply guys.....
    i have successfully made a password protected lock using a relay........ i think i will add the diode for protection against spike just in case....

    i am now thinking about takin this module to the next step.....
    i want the password to change every 2 days and a notification 2 be sent via an sms to the mobile phones of some predetermined individuals.....

    i know a few mechanisms of password generation....
    can anyone tip me as how to incorporate a gsm device for the sms purpose?
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