project help~

Discussion in 'Homework Help' started by xiaoboi, Jan 19, 2011.

  1. xiaoboi

    Thread Starter New Member

    Jan 19, 2011
    2
    0
    I need to design a DC fail safe supply system.

    My main voltage is 24V. When my main voltage fails below 24v (22v and below), a LED will be lighted up and a buzzer will sound. After 3 seconds, the supply will be switch to the standby voltage and the buzzer will go off (using a relay).

    anyone? :(
     
  2. RRITESH KAKKAR

    Senior Member

    Jun 29, 2010
    2,831
    89
    Use ic like 741, by this you can setup yours ref. voltage like 22V less than it the o/p will be high then using 555 stable mode you can set time.
     
  3. xiaoboi

    Thread Starter New Member

    Jan 19, 2011
    2
    0

    i did tried this. but i'm not sure about the connections for the LM555 timer.
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    Post your schematic ...
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    This circuit will sense low voltage, the second will sense whether it is above or below a setpoint.

    [​IMG]

    Something I drew up for this thread.

    Indicator circuit
     
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