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Need help with 555 circuit

Discussion in 'General Electronics Discussion' started by cleeve51, Nov 2, 2013.

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  1. cleeve51

    cleeve51

    1
    0
    Nov 2, 2013
    This doesn't have to be 555 as any other solution will be gratefully accepted.
    The set-up is a microswitch with NO & NC terminals, a 10 to 20 second delay circuit and a 12v relay. I need a circuit that will operate the relay just ONCE for about 10 seconds when the microswitch is toggled. For example a security light which operates for 10 seconds when a door is opened but just once regardless of the door remaining open or being closed or opened again later.
    Ideally I wanted a design in which no current would flow until the switch was operated but being unable to solve this I tried a fall-back solution.
    Design 2 comprised a 555 circuit being powered continuously but only sending a single pulse when the switch is opened; however, as the trigger has to be shorted than the output pulse, it remains on if the switch remains open.
    Any solutions?
     
  2. iimagine

    iimagine

    55
    8
    Oct 23, 2013
    How about this simple Delayed Buffer, you can set RC time constant at 10s or whatever
     

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  3. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    The trigger pulse can be shortened by passing it through a capacitor/resistor combination with the appropriate time constant.

    The main problem will be that you do not want the circuit to actuate again the next time the door is opened, thus you need a memory, perhaps a latching relay, and some means of reset.
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    Your definition is not clear. You say when the door is opened, and the microswitch's NO contact opens, you want the relay to close for ten seconds, but then you don't want the relay to activate again "regardless of the door remaining open or being closed or opened again later".

    This implies that you need a reset input of some sort, to reset the circuit so it is ready to detect another door opening.

    But then you say you wanted to use a 555 but it wasn't suitable because if the door remained open, the relay would remain activated, because the 555 needs to be triggered by a pulse that's shorter than its output pulse. It's not hard to work around that problem, but if you do, that circuit won't have a reset feature. Every time the door state changes from closed to open, it will retrigger the relay.

    Which behaviour do you want?

    There are lots of ways to do what you want. I would use a circuit based on a CMOS gate IC such as a CD4093 (or two), powered from 12V DC. This will draw practically no current when idle, and you can arrange it so that it activates the relay each time the door opens, or only the first time until it is reset.

    I will draw you up a circuit if you can tell me what functionality you want, describe the power source (is it a battery?), and tell me the current or resistance of the relay coil.

    If the load (a security light?) is powered from the same 12V battery, it may be simpler to use a MOSFET instead of a relay. In that case, tell me the power rating of the bulb.
     
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