Need help with a Constant to Momentary Relay

Discussion in 'The Projects Forum' started by pityocamptes, Aug 7, 2013.

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  1. pityocamptes

    Thread Starter Member

    Jul 25, 2012
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    I saw this schematic:
    [​IMG]

    Link: http://www.the12volt.com/relays/page5.asp#ctm

    where the output last only about 1/2 second.

    I'm trying to figure out a momentary trigger for a 555 timer circuit and was wondering if this type of relay (NORMALLY OPEN):

    [​IMG]

    would work as the relay above with the R/C components? Any help appreciated. Thanks!
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    This appears to be a standard automotive relay where the normal numbering is 85 & 86 for the coil and 30 for the common contact with 87a N.C. and 87 N.O.
    It appears that all they are doing is placing a capacitor across the coil so that when the power is removed there will be a delay until the Cap bleeds off?
    At least that is the way I see it, a crude 1/2sec timer, you could do it more accurately with a time IC, 555 etc.
    Max.
     
    Last edited: Aug 7, 2013
  3. wayneh

    Expert

    Sep 9, 2010
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    That would likely stay on a bit longer than the larger automotive relay in the original.

    But why do you need a relay to trigger a 555?
     
  4. pityocamptes

    Thread Starter Member

    Jul 25, 2012
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    I'm trying to add a simple trigger for my existing 555 timer circuit. My project box is small and I really don't have any more room for another IC (pads pretty much all taken). I was hoping a could use a reed relay and an R/C combo to trigger the 555 timer. It is for a shed alarm (door). The magnetic switches (N/O), once activated (N/C position) will power the relay, but I only need a momentary (1/2 sec) to activate the 555 trigger. Hope that makes sense. Thanks.
     
  5. pityocamptes

    Thread Starter Member

    Jul 25, 2012
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    The trigger wire from pin 2 to ground (unless I use a momentary switch, which won't work for my application) has to have some sort of momentary trigger to get the 555 to fire for 5 minutes... any other ideas?
     
  6. wayneh

    Expert

    Sep 9, 2010
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    A single trigger pulse to ground will trigger the 555. It doesn't need to be allowed to go back up.
     
  7. pityocamptes

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    Jul 25, 2012
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  8. pityocamptes

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    Jul 25, 2012
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    Thats the problem, I'm not sure how to get the momentary trigger to ground once the magnetic switches are "engaged", thus activating the 555 and triggering a siren for 5 minutes...
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I might be missing something, but don't you just need a simple 555 monostable to do what you want?
    The switch engages the 555 which times out after 5 minutes?
    Max.
     
  10. pityocamptes

    Thread Starter Member

    Jul 25, 2012
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    I'm going to post a schematic. Maybe that will help. Thanks again.
     
  11. pityocamptes

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    Jul 25, 2012
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  12. pityocamptes

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    Jul 25, 2012
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    Do you have to have a push button style switch at the trigger? Or could I simply use a resistor to drop the voltage to pin 2 from the 12v main coming from the magnetic switch? Basically, I want power to the 555 to always be active (so that turning it on does not start the timer) - the switch (upper left) lights up a green (G) led when alarm is not in use. When the switch is thrown to the red (R) side the red led lights up and arms the system. The "trigger" would be one of the two magnetic switches being interrupted and starting the timer while sounding the siren. Its a one shot alarm, but that is fine... thoughts? Getting ready to go to the parts store and would appreciate how I might be able to solve the trigger issue with the fewest components required. Thanks again...
     
  13. pityocamptes

    Thread Starter Member

    Jul 25, 2012
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    Not sure if the image attached...
     
  14. wayneh

    Expert

    Sep 9, 2010
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    That's a little hard to follow. Take a look here.
     
  15. pityocamptes

    Thread Starter Member

    Jul 25, 2012
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    Thanks. Ok, How about this circuit. Its close to what I have, and would not take much to alter my existing one. Please let me know. Thanks.
     
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