555 countdown timer

Discussion in 'The Projects Forum' started by Hereathome, Sep 7, 2014.

  1. Hereathome

    Thread Starter New Member

    Sep 7, 2014
    2
    0
    I am new to this forum. It looks very informative.

    I have a question about a 555. I am trying to make a countdown timer that starts when power is applied and sets off a piezeo buzzer after a duration of about 10 minutes. I have tried different thing circuits I found on the web but none of them really do what I need. Some require start up switches, etc. I just want it to start timing when power is applied.

    Could someone refer me to a page or post a circuit that would do that please? I would appreciate it.
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    You need to use it in monostable mode , and use pin 3 to pin 8 for the output, like this diagram, when power is applied the piezo will be off until the expired time, then will be on . timer.png timer.png
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,362
    Welcome to AAC. I am sure you will find useful information for all of your electronic projects right here on AAC.

    The 555 timer circuit is a simple and easy way to create short delay times of the order of seconds and shorter.
    The duration of the delay is determined by two components, a resistor and capacitor. If we use a very crude approximation for the time delay, RC time constant is given as the product of R times C.

    In the 555 timer circuit provided by Dave above, with R = 1M ohm and C = 1000uF,
    R x C = 1M x 1000u = 1000 seconds which is about 16 minutes.

    The problem with such long time delays is that large value capacitances are not very stable and accurate. For capacitances greater than 10uF, electrolytic capacitors are almost the only choice. Such capacitors suffer from a memory effect in the dielectric whereby even after complete discharge the capacitor will gradually show some additional charge.

    For long time delays using a 555 timer circuit or any other type of oscillator, more repeatable results are obtained by using an oscillator running at much shorter periods (higher frequency) and then using counter circuits to achieve the desired time interval.

    A better solution in a single package is to use a microcontroller.

    Here is a series of blog entries that began with a request for a long delay timer using a microcontroller:

    http://forum.allaboutcircuits.com/blog/msp430-interrupt-on-input-change.553/
     
    Last edited: Sep 7, 2014
  4. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    It's a standard default to say use a microcontroller, but not everyone is on the UC bandwagon. There is a tried and true, not to mention easy and low part count method that doesn't require and code, compilers, programmers, etc. Take a look at this page: http://www.onsemi.com/pub_link/Collateral/MC74HC4060A-D.PDF This will get you exactly what you want, although it doesn't include a 555, but as already been mentioned, that chip isn't the best way to go for your application.
     
    Last edited: Sep 7, 2014
  5. eetech00

    Active Member

    Jun 8, 2013
    650
    112
    hi

    If you really want to use a 555, here is a circuit I've used.
    But the comments by MrChips and the others are correct.

    eT
     
  6. Lee697

    New Member

    Aug 31, 2014
    26
    0
    I've always had pin 6&7 shorted together, and the timing resistor from the common point to Vcc - or is this circuit equivalent??
     
  7. Hereathome

    Thread Starter New Member

    Sep 7, 2014
    2
    0
    WOW! Fast responses! Thank you. The circuit is all built up and working. Thanks again.
     
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