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Need help with a Constant to Momentary Relay

Discussion in 'General Electronics Discussion' started by pityocamptes, Aug 7, 2013.

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  1. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012
    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. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
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    Jan 21, 2010
    To trigger a 555 from a signal that changes from one state to another (rather than produces a pulse) is typically done with a capacitor and a resistor. It may also require a transistor if the signal needs to be inverted.

    The first hi for me when I google "555 triggering circuit" is this. There's plenty more, I went no further than this. It shows you how and describes why. The basic solution here employs more than just a diode and a resistor, but not much.

    The advantage is that it consumes less power and it doesn't suffer from contact bounce or mechanical failure.
     
  3. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012
    Thanks. I tried the R/C configuration last night and for some reason I could not get it to work. Here is a schamtic of what I am trying to do. The NO magnetic switches is what should trigger Pin 2 on the 555. Thoughts? Thanks.

    0[1].jpg
     
  4. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012
  5. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012
    This schematic is a little different than mine, and would only take a little bit of re-working my existing circuit. Would this work? Thanks.
     

    Attached Files:

  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
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    Jan 21, 2010
    Your last circuit has some problems.

    Firstly it is an astable (it will generate a continuous series of pulses).

    Secondly you have a 470uF capacitor being directly discharged through the discharge pin. This value capacitor is too high to discharge without a resistor (I can't remember the rated maximum capacitance for this, but it's a lot less than 470 uF)

    It is hard to figure out the circuit you give a few posts earlier (that you tried and couldn't get working).

    I suspect your problem is that the trigger input is best held high with a resistor and pulled low by what is triggering it. I'm not sure what you were doing but it didn't look like that. Also the position of the resistor and capacitor near your relay(?) seemed weird. You also don't show any timing resistor or cap on the 555 nor what the connected pins represent.

    edit: OK, I can see what you're doing to trigger it, but the lack of the timing components is an issue. Also your siren load (depending on what it is) may not be suitable for connecting to the 555 output, or may need a diode across it to prevent damage to the 555.

    A standard circuit diagram is always helpful as it conveys information in a way we can understand.
     
    Last edited: Aug 9, 2013
  7. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012
    Ok, thanks. Basically, what I am wanting is a circuit that turns on once the door magnetic switches are interrupted. So with that in mind I'm looking (at this point) for a kit or schematic for a circuit that once turned on starts the timer and stays on for 5 minutes or so (adjustable would be better), which powers a relay, which powers a siren. Once the time expires the circuit shuts down - or at least the relay controlling the siren. Basically a one shot circuit. Any ideas? Been looking on the web for kits or completed units and can't seem to find any in the US. Thanks.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
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    Jan 21, 2010
    OK, some questions and an issue or two...

    0) presumably siren sounds as soon as door is opened?

    1) does the 5 minutes start when the door opens, or once it closes? (if the door is open for 10 minutes, does the alarm sound for 5 mins, or 15 mins?)

    The issue is that 5 minutes is rather too long for a simple RC delay. You're looking at something more complex which has a counter etc., to count up to that delay.
     
  9. pityocamptes

    pityocamptes

    79
    0
    Jul 26, 2012
    Thanks. The alarm will sound once the door opens for approx 5 min. If the door shuts during the 5 min the alarm continues to sound but end after 5 min. If the door stays open after the 5 min the alarm stops only to reset after it closes.
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
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    Jan 21, 2010
    OK, the simple answer is a 555 timer (or similar) connected to a counter (like a 4060).

    One of the outputs is connected so that when it is 0 an alarm sounds and when it is 1 it holds the 555 in a reset state.

    The signal from the door is used to clear the counter and thus allow the alarm to sound until some period of time has passed.

    Let me draw a circuit for you...
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
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    Jan 21, 2010
    [​IMG]

    This should do it.

    The alarm will go off when power is applied.

    After that, the alarm will start to sound when the switch to the left is closed and go off 5 minutes later. If the switch opens and then closes again, the time will be reset (so it sounds 5 minutes from the last time the door was opened)

    The crossed out part of the circuit would switch some load (it could be a relay) after the time delay. The alternate part below it sounds only during the delay.

    The switch between pins 3 and 13 allows you to test the circuit without having to wait the full 5 minutes. When connected to pin 3, the delay time will be 1/32nd of the full delay time.

    Adjusting the 47k resistor connected to pin 10 can be used to adjust the delay time.

    It would be worth breadboarding this up to test it.

    The idea is that the oscillator is inhibited when the desired output gets set. The trigger is a reset to the 4060 which has the effect of re-enabling the oscillator until the output again gets set.

    Pin 3 is set 16384 clock pulses after a reset. (pin 13 is 512 pulses after a reset).

    I just realised I used 32768 as the division in my calculation, so the 47k resistor should probably be closer to 100k (oops)
     

    Attached Files:

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