1 second light

Discussion in 'General Electronics Chat' started by TBayBoy, Jun 9, 2011.

  1. TBayBoy

    Thread Starter Member

    May 25, 2011
    148
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    OK while I await supplies to be delivered, I'm trying to understand a few things about 555 and flip flops.

    Here is a circuit I cobbled together based on my reading.

    So if I understand things correctly, (and please jump in if I'm wrong) C1 will charge to 2/3 of 9V (6v) in 1/2 second. This will trigger a positive pulse on pin 3 of the 555 and then reset, the pulse will enter the clock on the JK flip flop (PGT) and cause Q to go high, the next pulse 1/2 second later will be a negative going transition (NGT) and Q remains high till the next pulse 1/2 second later a PGT that will flip Q to low, then it all repeats.

    Makes sense or am I out in the weeds? :)
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Yes, that's it.

    You're only going to get about 5mA current through the LED (if it's red) but that's OK, as 4000 series CMOS IC's can't source or sink much current.

    The nice thing about using a flip-flop on the output of a 555 is that you can get exactly 50% duty cycle without trying very hard.

    Your actual output pulse times from the 555 won't be exact; don't plan on using it as a clock input for keeping time or you'll be disappointed. For timekeeping, you need a VERY accurate clock.
     
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  3. TBayBoy

    Thread Starter Member

    May 25, 2011
    148
    19
    Thanks, this is not a "Project" type circuit, I don't plan to use it for critical timing, it is more aimed at taking what I learned this year in school and trying to take it one step farther, we touched on flip flops but not on 555 yet, so I just kept reading on and came up with this.

    Now I can't wait for my supplies to arrive so I can build. I used a POT to compensate for the component variances in the cap and resistors.

    Just a complete mental exercise to exercise the little grey cells. :)
     
  4. nigelwright7557

    Senior Member

    May 10, 2008
    487
    71
    I dont believe your JK flip flop is wired correctly.
    You are clocking through a 1 each time which will just leave the led on all the time.

    You need a divide by 2 D type flip flop circuit that toggles the Q output each time.
    The /Q is fed back into the D input to change state each clock cycle.

    Also there is nothing to set the start state of the flip flop, it needs a reset pulse to ensure the LED is off to start with.
     
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  5. TBayBoy

    Thread Starter Member

    May 25, 2011
    148
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    OK, what I'm thinking is on the first pulse Q goes high Q bared is low. The next pulse will flip the clock from high to low, and the chip is a PGT chip so it does not change the state of the outputs. The next pulse again flips the clock to PGT and that in turn flops Q to low and Q bared to high.

    Am I getting this part right?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You are confusing a J-K flip-flop with a D-type flip-flop.
    http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/jkflipflop.html

    Sorry, you're incorrect. The circuit posted works correctly as posted.

    However, I'd suggest replacing the 2.5k pot with a 5k pot, as a 2.5k pot won't have enough adjustment range.
     
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  7. TBayBoy

    Thread Starter Member

    May 25, 2011
    148
    19
    Yes I have a 5 K multi turn pot the 2k5 was my estimate to the range that it should be set to for final adjustment, you have a good point, the schematic should have the part on it and then I can note to the approximate range.

    Learning lots here :)
     
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