555 Timer help required

Discussion in 'The Projects Forum' started by VincentM, Dec 12, 2013.

  1. VincentM

    Thread Starter New Member

    Dec 12, 2013

    I have built an alarm timer circuit that is meant to work with a normall open as well as a normally closed trigger. For some reason only the normally open trigger works - I have attached a copy of the circuit diagram.
    Is this circuit designed correctly, or maybe there is a better circuit I can use. Can someone please help.

    Last edited: Dec 12, 2013
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  2. elec_mech

    Senior Member

    Nov 12, 2008
    Welcome to AAC.

    The 555 is triggered on a low signal. When you press the N.O. switch, you connect the trigger pin directly to GND causing the 555 to trigger. When the N.C. switch is pressed, you remove the high signal. The trigger input is not tied to GND and is effectively floating, so the 555 does not know what to do.

    To ensure a trigger, you must make make some kind of contact to GND.

    Below is one way that should work. R2 is used to prevent a dead short between power and GND when the N.O. switch is pressed. If the 555 triggers without pressing the buttons, try replacing R2 with a 1kΩ.
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  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    Just put a 100K resistor across the NO button so it will trigger when the NC button is pressed.
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  4. VincentM

    Thread Starter New Member

    Dec 12, 2013

    I've modified the circuit according to your diagram and it works perfectly.

    Thank you very much for your help.

  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    Any Idiot can post a schematic on the internet. It takes smart people on this forum to make them work...