Time Delay Circuit - Door Buzzer

Discussion in 'Homework Help' started by hotbpgirl, Jul 21, 2010.

  1. hotbpgirl

    Thread Starter New Member

    Jul 21, 2010
    2
    0
    I'm trying to figure out how to get a door buzzer (12v) to stay electrified for 3 seconds after the buzzer is pressed. I figure I need some type of transistor RC circuit. Circuit needs to detect 12v for split second and send 12v to buzzer for 3 seconds. Any ideas?
     
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
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    Look up 555 monostable. That will certanly fit the bill.

    Then look up driving inductive loads with a 555.
     
  3. hotbpgirl

    Thread Starter New Member

    Jul 21, 2010
    2
    0
    will do.
    any other ways besides using the 555?
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    A time delay relay will do the trick but 555's and associated components are probably cheaper.
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    The 555 is called a timer. This is because it times.

    There are lots of ways to do it without a 555, I doubt they will be simpler though. You could do the same thing with precision using transistors for about 4X the components.

    You could also do it something like this. You will have to arrive at the values by trial and error, on of the reasons the 555 is so popular is it is predictable within the tolerance of the components.

    [​IMG]

    I may have messed up, since you put this into the homework section. We don't give direct answers, we prefer to guide you into figuring it out for yourself. This one is a gimme (though you will have to design the rest for yourself), the rest will be harder.

    The second Q1 should have been Q2. You don't want to use a prepackaged darlington (1 transistor) because they have resistors going from the first base to the second emitter, and I was going for the increased gain and high input impedance.
     
  6. Rick Martin

    Active Member

    Jun 14, 2009
    31
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    I cant imagine why you would use the 555 as a monostable to do this. As said using discrete components can achive this but in reality it would be very inpractical.
     
  7. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    How about using a simple R/C circuit, similar to this circuit which holds a relay active for an extra few seconds after switch is released... http://www.circuitdb.com/circuits/id/92

    B. Morse
     
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