555 monostable triggering problem

Discussion in 'The Projects Forum' started by skj, Oct 29, 2012.

  1. skj

    Thread Starter Member

    May 20, 2010
    This is a circuit to alert housemates that some one has opened the gate without using door bell.I have used 555 IC in Monostable Mode.To prevent permanent high at output pin3 I used a RC combination to trigger pin2 for a short time which times out approx 3 sec pulse at pin3 activating a relay and in turn a bell.
    I am attaching the circuit (monostable circuit and relay at pin3 not shown).Timings are not critical.

    My problem is that if I directly connect RC out to pin 2 of 555 nothing happens
    therefore I Used buffer stage using Nand Gate ,transistor and relay.Circuit is perfectly working now.

    I don't know much calculations and have only workable knowledge of electronics.

    Please indicate flaws in the circuit, your suggestions are welcome.
  2. Wendy


    Mar 24, 2008
    Your killer flaw in no input conditioner. This article explains in detail how 555 monostables work (including the finicky details of triggering)...

    555 Monostable
  3. skj

    Thread Starter Member

    May 20, 2010
    Thanks for your comments,I have read your very useful article I will do the changes in my circuit and will come back if any query arises.