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Do i need a relay?

by manshuk89
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Jan7-10, 10:46 AM
P: 7
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?
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Bob S
Jan7-10, 11:05 AM
P: 4,663
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
Jan7-10, 11:44 AM
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sophiecentaur's Avatar
P: 12,197
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.

Jan8-10, 11:32 AM
P: 7
Do i need a relay?

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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