555 input trigger delay

Discussion in 'General Electronics Chat' started by raid3n, Jul 22, 2010.

  1. raid3n

    Thread Starter New Member

    Jul 22, 2010
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    Hello all,
    Newbie question for you. New to electronics so be kind.

    I have a 555 monostable timer set for a 2s output that works but with one problem, false triggering. I'm guessing the toggle switch being used will activate the timer falsely due to vibration.

    Is there a way to have the 555 only activate when it receives a input pulse of greater than .5ms?
     
  2. creakndale

    Active Member

    Mar 13, 2009
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  3. Wendy

    Moderator

    Mar 24, 2008
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  4. raid3n

    Thread Starter New Member

    Jul 22, 2010
    3
    0
    Wouldn't a debounce circuit still trigger the 555 even if it sees a short pulse? I think I need a RC filter of some kind to allow only a pulse greater than a 1/3 of a second to activate the timer.

    For a little more explanation, the circuit consists of 2 555 monostable timers that are triggered by a 3 position toggle switch. Position 1 (X1-3 on diagram)triggers timer 1 and pulls down the reset on timer 2. Switch in position 2 (X1-4) does the same for timer 2. Postion 3 is off (center position of switch). The false trigger occurs when moving the switch from either 1 or 2 to off, the opposite timer activates.

    I know how it could be done using software via a microcontroller but I'm rusty on circuit design.
    Thanks for the help.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    You have bigger and more fundamental problems with your design.

    Why are you using R1? The spike from either 555 switching will be transfered to the other. You desparetly need bypass capacitors, something like a 220µ for the 555 power buss and 0.1µF for each 555. I do not see how R1 contributes anything but problems, I'd get rid of it. If it is to lower the voltage to the 555s you need either a regulator or a zener (you need a stable voltage buss for the 555s), resistors suck for a stable voltage regulator. I'd use a zener.

    You also have ridiculously low values of resistors (15Ω) feeding the base of each transistor, which will increase that spike I was talking about and overdrive the 555 by a lot (they may be damaged). What are your relay coil currents? Figure a resistance 1/10 that for the base, and subtract 1.8V for the 555 and transistor BE looses. If your coil current is 0.1 amps your base current is 0.01 amps. If you are figuring 10V on Vcc (since this will be all over the place due to R1, good luck) you will have 8V for the transistor switching voltage, and the resistances to the base would be 800Ω.
     
  6. raid3n

    Thread Starter New Member

    Jul 22, 2010
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    I removed R1 and added the bypass caps as suggested. Also changed the resistors feeding the transistors to 680 ohms based on a .16 a current of the coils and a 13v supply. (Not sure what I was thinking there!)

    Thanks for the help.
     
    Last edited: Jul 23, 2010
  7. tom66

    Senior Member

    May 9, 2009
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    What is the purpose of the pots R8 & R9... they will only act as fixed resistors in that configuration.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    So, did it work?
     
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