555 Trigger Help

Discussion in 'The Projects Forum' started by FlamingMonkey, May 3, 2011.

  1. FlamingMonkey

    Thread Starter New Member

    May 3, 2011
    Hi, new to the forums and electronics in general so I apologise if i don't make sense or seem a little stupid. Additionally I have searched the forums & google with little luck.

    I am doing a project at the moment which consists of a monostable one shot NE555 controlling how long a a stable NE555 chip flashes a led light for.

    The monostable one shot output (pin 3) sends a 20 second on period to the astable 555 which flashes at approximately 1 flash per second.

    My question is, how can I trigger the initial monostable circuit without the need of a physical push switch. My understanding is that the switch is there to briefly allow current to flow which initiates the process. Is there some sort of simple circuit or simple component that would help me in having a circuit that doesn't rely on a physical switch in that part of the circuit. I am trying to make a simple door alarm that is triggered using a reed switch. I hope this makes sense.
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    Welcome to the forum.

    I got a better suggestion.
    Try a motion sensor for a change. Simple and reliable
  3. FlamingMonkey

    Thread Starter New Member

    May 3, 2011
    A motion sensor is a good idea, although a solution would be good just to put my mind at ease !
  4. DerStrom8

    Well-Known Member

    Feb 20, 2011
    You could use a comparator and a LDR to sense changes in light or dark--connect the output of the comparator to a transistor that connects pin 2 of the 555 to ground. Another way would be to simply have a sort of antenna connected to pin 2, and you could touch it with your finger to trigger the 555. There are endless possibilities :D
    Good luck!
    Der Strom