Please help

Discussion in 'The Projects Forum' started by soda, Dec 30, 2010.

  1. soda

    Thread Starter Active Member

    Dec 7, 2008
    174
    13
    Hi to all members,

    This is a mono stable timer for a alarm system. When the PIR relay is closed, pin 3 of the NE555 must stay low and when the PIR relay open the output at pin 3 must turn on or go high.

    Please look at my schematic to see if it's correctly wired.
     
  2. edgetrigger

    Member

    Dec 19, 2010
    133
    19
    Hi to you,

    Your description points to a bistable operation. In mono stable the output will go high and reset back after a time delay set by RC.

    The circuit as regards mono stable is correct and will work. The second transistor is used to pull the trigger pin low for closed PIR relay.
     
  3. soda

    Thread Starter Active Member

    Dec 7, 2008
    174
    13
    edgetrigger,

    It's not so much about the mono stable but it's more about the trigger pin 2 of the ne555 and the PIR relay contacts.

    When the PIR relay contact open, only then i want pin2 to be triggerd. So, when the PIR relay contacts is close, nothing must happen.

    You say that the 2nd transistor will trigger pin2, but i want just the opposite of that action.

    Do i need a PNP for the 2nd transistor?
     
  4. edgetrigger

    Member

    Dec 19, 2010
    133
    19
    Sorry, i wrongly analysed. When the PIR switch is closed first transistors o/p is low and the second transistor's o/p will be high. When the PIR switch is open first transistor's o/p is high and the second will output low.

    So that is how you wanted and you have built it properly. No need of a pnp here infact.


    Your ckt should work. let me update you tomorrow, it is 2.10am at my place got to sleep!
     
  5. soda

    Thread Starter Active Member

    Dec 7, 2008
    174
    13
    Ok edgetrigger,i also see it the way you see it, thanks for your help. i'm going to make a pcb and build it.
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    No, that's bassackward. SOP dictates breadboard first, if OK.. then make the PCB. This includes perforated copper clad experimenter boards. This greatly reduces the "Oh Sh_t!" factor later on. One of the first things that an electronics enthusiast/experimenter should have on his bench is a Proto-Board. No solder - no fuss plug and play. You'll never regret it. ;)
     
    PackratKing likes this.
  7. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    This would be OK if his relay has N/O contacts but not as drawn (N/C contacts) in your print. The 555 triggers on the negative going edge. If the Trigger pin is held low, as you show it, the output will remain high forever.
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    This will work with N/C or N/O relay contacts. Read the notes on my print. BTW; why do you need a relay to trigger the 555? A FET or BJT should be sufficient to pull the Trigger pin low.
     
    Last edited: Jan 1, 2011
  9. soda

    Thread Starter Active Member

    Dec 7, 2008
    174
    13
    Hi cdrive,

    I allays test all my diagrams on breadboard before i make a pcb.I already did this and everything was working the way i wanted it to be.That was after i got help from edgetrigger.

    The PIR (programmable infrared detector) relay work in such a way that when someone crosses the beams the relay open and when it's in a resting position, the relay is close.

    I could use a cmos nand or, nor-gate instead, but that would then be more complicated.

    Thanks for all your reply's
     
  10. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    You can replace the relay with a common 2N2222 or nearly any GP NPN BJT.
     
  11. soda

    Thread Starter Active Member

    Dec 7, 2008
    174
    13
    CDRIVE,

    I know you try to help me, but the relay you talking about is 5mm by 2mm in size.

    I'm definitely not going to replace that, it's just too small. It has the size of mother board components.
     
  12. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Is the relay integral to the sensor?
     
  13. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Can't see the forest through the trees scenerio.... It would appear not. Good call Alberto. ;)

    Chris
     
  14. soda

    Thread Starter Active Member

    Dec 7, 2008
    174
    13
    CDRIVE,
    I cant see any reason why i must replace the relay because the relay is part of the PIR. The PIR is a complete unit jou buy at any elect. shop.

    Do you know what a pir is. It's a infra-red sensor you use for a motion detector and when someone crosses the beams it sets off the alarm.

    On the other hand, I've already made the PCB and soldered the components to the board and using it, so i don't know why you keep on trying to help me. It's over and done.
     
  15. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    My last post on this topic was addressed to Alberto, not you. This may be your topic but you don't own the thread or the board. BTW, thank you for explaining what a PIR is, I hadn't a clue! And if you find the time do a dictionary search for the term "ingrate".:rolleyes:
     
Loading...