4017 and leds "on time"

Discussion in 'General Electronics Chat' started by thedt, Aug 6, 2009.

  1. thedt

    Thread Starter Member

    Dec 12, 2008
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    So im using a 4017 and a 555 to drive leds. I want to know if i can increase the on time of each led. I found the latching thread but I dont need that, I would just like them to be on a little longer. Can I just use a capacitor on each led?

    using the one on the right at the moment
    [​IMG][​IMG]
    [​IMG][​IMG]
     
    Last edited: Aug 6, 2009
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Please post your schematic or a link.

    Ken
     
  3. Wendy

    Moderator

    Mar 24, 2008
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  4. SgtWookie

    Expert

    Jul 17, 2007
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    You could put a cap from each transistor's collector to GND, but you'd need a separate resistor for each LED.
     
  5. fanie

    Active Member

    Jan 20, 2007
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    This is an easy adjustable oscillator.

    The larger the cap the slower the oscillation will go. The RC time you can look up in the 555's data sheet, the graph with capacitance VS time.

    The 4017 is not really a good LED driver, it limits the current to 8mA if I remember right. If you need more current add some ULN2003 or ULN2004's, they are hammer driver circuits and can handle up to 500mA current on each output.

    The open collector transistors you have there will also work well.

    Personally I dislike the 555.

    Sorry, I added the second circuit, you can adjust the duty cycle with it, ie the on time can be adjusted wrt the off time. You may want to use a bigger cap if the timing is too fast.
     
    Last edited: Aug 6, 2009
  6. thedt

    Thread Starter Member

    Dec 12, 2008
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    I actually was using the left and just mistyped. haha
    but i got some npn laying around so it shouldnt be no problem.

    "SgtWookie You could put a cap from each transistor's collector to GND, but you'd need a separate resistor for each LED."
    Thanks ill give the collector to gnd cap a try.

    "The 4017 is not really a good LED driver, it limits the current to 8mA if I remember right"
    It shouldnt matter for the transistor circuit, but yea the leds were a bit dim when i had the left-hand circuit built.
    And yea I have a pot instead of the 47k R or the RC of the 555, so I can get the timing how I like.

    One last question. For the circuit on the left i tried to put a cap in || with with the led and resistor on an output pin and the best I got was just a quicker on/off of the led it was on. Shouldnt that of worked as well or does the output pin ground when not on or was it not enough milli amps to charge both the cap and light the led?

    thanks
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Are you wanting the LEDs to stay on after the 4017 has clocked? This is what Wookie is assuming (I think), and what I was asking before? The 555 Hysteretic Oscillator is an old design, well understood, and works quite well.
     
  8. thedt

    Thread Starter Member

    Dec 12, 2008
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    Yea i wanted the Leds to stay on for longer then just after the pin clocked. so instead of like 0.5 of a sec maybe like 1-2 sec on time. 555 Hysteretic Oscillator would work but i am using more then 2 leds so id have to chain them together im guessing.

    "You could put a cap from each transistor's collector to GND, but you'd need a separate resistor for each LED"
    Yeah that worked kinda. Just played the whole slow drain (led would get dim and slowly fade), even playing with didnt cap values didnt get it good enough.

    The original operation wasnt too bad so i guess ima just stay with that, until i get around to building a PIC programmer. haha

    thanks for the help guys.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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