quick 555 timer question

Discussion in 'General Electronics Chat' started by skinner927, Jan 23, 2008.

  1. skinner927

    Thread Starter Active Member

    Dec 31, 2007
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    I've never worked with 555 timers. I have a quick question to see if a 555 timer would be a good candidate for my next project.

    The 555 timer has a trigger.

    This trigger activates the timer

    if the trigger stays on, once the time is up, will it then start again?

    I'm sure i'm not using the correct terms, if anyone's confused, i'll try to rephrase what i'm trying to say.
     
  2. adrian.dmc

    Active Member

    Feb 22, 2007
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    You can see here http://www.kpsec.freeuk.com/555timer.htm that the 555 when acting as a trigger only maintains high during a defined time so when it goes low again the timer is deactivated. To prevent this I would probably use Latches but I'm sure there's something simpler. If you find a way to solve this please post the solution.

    Hope helped...
     
  3. skinner927

    Thread Starter Active Member

    Dec 31, 2007
    36
    0
    This is what I came up with. [​IMG]


    the cap would charge in less time that the time would be for the timing cap on the 555 timer

    So! if I wanted the 555 to run for 1 second, the cap would be fully charged in 0.5 seconds.

    it would then trigger the 555 after the power is pulled

    Can I get a confirmation that this would work?
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Remove the diode.
    The diode prevents the capacitor from making the trigger input low.

    The trigger input needs a resistor to the positive supply to make it high when it is not being pulsed by the capacitor. The resistor also allows the capacitor to charge for a moment when the input goes low. When the capacitor is fully charged then the trigger input is high. 0.5 seconds is too long. Make it 0.01 seconds. The 555 cannot time out when its trigger input is low.
     
  5. adrian.dmc

    Active Member

    Feb 22, 2007
    52
    0
    I can't understand what you trying to tell us...

    So that we can understand you better try to say what do you want to do.

    So far I think that you want to trigger a timer and let the timer run until the power is out. Is this what you want?
     
  6. adrian.dmc

    Active Member

    Feb 22, 2007
    52
    0
    Sorry I didn't see the new post....
     
  7. skinner927

    Thread Starter Active Member

    Dec 31, 2007
    36
    0
    I'm using it for a car radio, The radio doesn't turn off when the car turns off. It's not a problem with the radio, it's the car model years that vary, dont worry about it.

    Is it possible to make a circuit that will open a relay for one second when given power, and then open that relay again for one second when power is lost?

    So, when the car turns on, the radio would turn on, and same with off.

    I can wire up to the actual switch in the radio. When I say relay, I say relay because I've only used relays before. I guess I could use a transistor but I don't know what the voltage is on the switch, so I don't want to use a transistor so it doesn't burn it out. Again, i've never used a transistor before so I don't even know what their maximum voltage ratings are. It wouldn't be more than 12 volts, its probably lower than that though too.

    Thanks for any help or insight anyone can give me on this.

    I was thinking of running the 12v trigger through a cap, that charges up in 0.01 seconds (thanks Audioguru). which would kill the power to the trigger but store a charge in the cap. That way when he 12v is pulled, it sends another signal to trigger so it runs again, this would turn the radio off.
     
  8. skinner927

    Thread Starter Active Member

    Dec 31, 2007
    36
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    :( nobody has any ideas?
     
  9. Pope John

    New Member

    Jan 11, 2008
    6
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    Skinner927

    Why not simply run a wire from the battery or some convenient wire in the dash through a switch beside the radio. Switch on - radio on Switch off - radio off
     
  10. skinner927

    Thread Starter Active Member

    Dec 31, 2007
    36
    0
    because the switch has to be momentary. I could get it to turn the radio on, but once it's on, i couldn't get it to turn off.

    Holding the button also makes the radio turn off, so it wouldn't work. :D

    Thanks for the reply though.
     
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