Need schematic for 1 pulse timer and OFF

Discussion in 'The Projects Forum' started by bluebear, Dec 11, 2010.

  1. bluebear

    Thread Starter New Member

    Dec 9, 2010
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    Hello,

    I need a schematic that will work like this: power on (+12v), make 1 pulse (1 second long) of 12V (+) output and after that shut OFF the output. No need for re-cycle the schematic or anything. Just 1 pulse and go OFF. Can be made with an 555?

    Thanks....
     
  2. iONic

    AAC Fanatic!

    Nov 16, 2007
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    A 555 as a monostable one-shot using a normally open momentary switch can do this. Now sure what you would be using as a trigger.

    [​IMG]


    I can't be sure this will work as I do not know what sort of application you are using it for.


    iONic
     
    Last edited: Dec 11, 2010
  3. bluebear

    Thread Starter New Member

    Dec 9, 2010
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    It's for an defrost sistem for car. Starting engine from remote and after 2 minutes 1 wire will power the heater so i want to put the circuit in paralel so my defrost sistem will power too.

    BUT the alarm system will put +12V permanent on that wire and the defrost system is working by impulse on-off button (is not mechanic button).

    So, wire will have +12v after 2 minutes engine start and the schematic have to make only 1 second +12v output on the relay defrost and remain on 0v after that (not pulsing anymore +12v).

    That will work?

    P.S: no trigger in the schematic. the trigger will be powering the schematic.
     
    Last edited: Dec 11, 2010
  4. iONic

    AAC Fanatic!

    Nov 16, 2007
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    I'm not the expert, but since I and as interested as you I am going to find out, weather it be via internet search or someone else's explanation here, so just hang tight.

    The answer might just be in the datasheet.

    Your car's defrost is activated with a 1 sec. 12V pulse or really just a momentary 12V pulse?


    Perhaps something like this:
    [​IMG]

    You may need an NPN transistor on pin 3 to source more power to turn the defroster on....not sure.

    Any better ideas anyone???

    iONic
     
    Last edited: Dec 12, 2010
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    The schematics so far would continually re-trigger since the input is +12V

    A non-retriggerable one shot is the need, If I am reading the description correctly.

    How does the current system respond to a continual +12V signal? Does the alarm manufacturer make a module for this purpose?
     
  6. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Another try...

    [​IMG]

    This should trigger the 1 sec timer from the transition from 0 to 12V, yes??
    Not too sure on the transistor values.
     
    Last edited: Dec 12, 2010
  7. bluebear

    Thread Starter New Member

    Dec 9, 2010
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    Correct:

    We need a non-retriggerable one shot output.
    Somebody tried for the same car to put continous +12v and is working BUT, the car system is made for one shot +12v on the relay defrost.
    The button is like digital (not mechanical on-off). It's push-ON, push-OFF.

    That why i'm trying to make the same with an schematic.
    So, again :): after the car engine is starting by pager and after 2 minutes one wire will have +12 from the alarm system that will power the heater, etc... Taking the +12v from that wire and powering our schematic, will make an output of +12v for just 1 second (like pressing the button) or 2 seconds long if it's hard to make the schematic. The ideea is not to output contiously 12v or pulsing by 1 sec like in the schematics above.

    Thanks guys.
     
  8. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    ionic,

    What's the point of R3, R4 and the transistor...since pin 2 is grounded? ;)
    Maybe, delete R2 and tie pin 2 to the collector?
    Or better, connect pin 2 to the collector through a 0.1uF cap.

    Ken
     
  9. bluebear

    Thread Starter New Member

    Dec 9, 2010
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    Ok guys let's make it simple: when powering schematic, output will make 1 second +12v and that's it (will stop output -> 0V). If we can cut the power after that output second by a relay or something will be better but not mandatory if the output remains low.

    The cycle ofcourse can be re-made after cutting/powering the schematic again. No counters no nothing...
     
    Last edited: Dec 12, 2010
  10. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Well, that was pretty silly, wasn't it. I tied pin 2 high with the 100K resistor, then went on to tie it directly to ground!

    The following schematic suggests your suggestions. But by removing R2, what state is pin 2....floating?

    [​IMG]


    This will still keep pin 2 low, constantly...
     
    Last edited: Dec 12, 2010
  11. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Yes, using a transistor (power transistor) or relay may be necessary. This is essentially what I am trying to do. The real issue is getting the 555 to trigger from a positive going signal as apposed to a negative going pulse.

    There are two way to wire the circuit.

    1) power both the 555 and input signal with the "magic" one wire.
    2) power the 555 with the starter, and the signal with the "magic" one wire.

    iONic
     
  12. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    No...to your last schematic.
    Leave R2 connected to pin 2.
    Ground the transistor's emitter.
    Put the cap between Pin 2, and junction the transistor's collector and R4.
    This is the common way for providing a brief negative, trigger pulse to the monostable from a the leading edge of a zero to + input signal. This allows for a signal that goes high and low, or goes high and stays high.
    Also, unless the input to R3 has a path back to ground when the +12V goes away, I would add a 10K resistor from base to ground. This is to prevent false triggering with environmental noise to a floating base.
    Next post:
    1 & 2 are referred to "Power-on reset" circuits. Many 555 versions in Google

    Ken
     
  13. iONic

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    Nov 16, 2007
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    Make and model of the remote car starter might be helpful. The signal you want to trigger from may not be the most appropriate. Pressing the starter is a momentary action and could possible start a dual 555, the first leg to start a 2 minute delay, the second to create the one second +12V pulse to the defroster.
     
  14. Wendy

    Moderator

    Mar 24, 2008
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    Sorry I came in late. This is pretty much a standard 555 schematic, and I believe it will meet your needs. Usually used for a power up reset on logic...

    [​IMG]
     
  15. bluebear

    Thread Starter New Member

    Dec 9, 2010
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    It's MAAT 7500 alarm system with remote engine start but no useful info :)

    If two 555 or one 556: first will trigger the second but what will make the first to not trigger the second again? so it;s the same problem: output 1 time and go off. AND by the way: no need for 2 minutes delay because the alarm is making this job (2 minutes delay after engine start will put +12v on one wire for accesories).

    So conclusion : schematic that will do this: triggered by alarm system wire (+12v) and output only 1 second (+12v) NOT PULSING (going output in 0v after first second) !

    @Bill_Marsden : that schematic I'm pretty sure that will pulse the output right? no good :)
     
  16. iONic

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    Nov 16, 2007
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    Thanks Bill, for helping the OP and for correcting my errors. To complete my leaning experience, could you please explain the POR portion of the circuit?

    Let me be stupid once again and guess. The instant power is applied, there is a low on pin 2, starting the timer. After the cap has charged the voltage across the cap presents a high on pin 2. The whole action sort of acts as a
    replication of a momentary push?? !


    Thanks

    ...and I hope I am not hijacking the OP's thread, it is all information that he ought to understand as well, though.
     
    Last edited: Dec 12, 2010
  17. bluebear

    Thread Starter New Member

    Dec 9, 2010
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    :D BUT where is the 1 second output and stay in 0v after? :D
     
  18. iONic

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    The 1 sec. output starts when the entire circuit gets it's power from the "magic"
    12V line and is determined by Rt and Ct. there will be a 1 sec high on pin 3 on power-up (Power-up in this case is the entire circuit). This turns on the relay for 1 second only. Vcc is the "magic" 12V line. The relay will not stay on because after 1 sec, the transistor turns it off.

    The values I had calculated for Rt and Ct were 470K and 2.2uF. As for what will make good values for R1 and C1 I am not sure.
     
  19. bluebear

    Thread Starter New Member

    Dec 9, 2010
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    Oky doky let me put it in multisim to test :) thanks.
     
  20. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Just to throw another monostable/power-on-reset circuit option into the mix.

    Ken

    Corrected error in schematic.
     
    Last edited: Dec 12, 2010
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