Quick question on resistor change to make LEDs flash faster...

Discussion in 'General Electronics Chat' started by critiera119, Jan 26, 2009.

  1. critiera119

    Thread Starter Active Member

    Nov 21, 2008
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    I have this circuit constructed, alternating LED flasher. They flash too fast. I want each LED lit for only half of a second. Can I just replace the 47k with a higher resistor?
     
  2. Wendy

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    Mar 24, 2008
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    Yes, that should do it.
     
  3. critiera119

    Thread Starter Active Member

    Nov 21, 2008
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    Ok, so I went up to a 120k in place of 47k and I could not see any difference. Should I go even higher?
     
  4. bertus

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    Apr 5, 2008
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  5. critiera119

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    Nov 21, 2008
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  6. Wendy

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  7. critiera119

    Thread Starter Active Member

    Nov 21, 2008
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    Flashing LEDs for a small project. Will add 3 more to each side after getting flash rate correct, already have the resistors for those additionals to share the current correctly. I do have a few 555 and 4017 ICs. Thought maybe I could keep it more simple than going to chips.
     
  8. Wendy

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    Mar 24, 2008
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    The 555 is much simpler than transistor, and will do 50% duty cycle with 4 resistors, 1 cap, and 1 IC. If you don't need 50% exactly it will do it with 3 resistors, one cap, and a IC. I would class it a lot simplier myself. That and it can be designed much more precisely, the values will vary according to the parts, where the transistor model is a bit hit or miss. You can also use a 1 variable resistor and have both LEDs adjust in sync.

    The 4017 will allow you to do fancy flash patterns, but for straight back and forth the 555 is king.

    Check this out...

    http://forum.allaboutcircuits.com/showthread.php?t=15342
     
    Last edited: Jan 26, 2009
  9. bertus

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  10. Audioguru

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    Dec 20, 2007
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    The transistors in the multivibrator have a collector current of 26mA if the LEDs are 2V red ones. Then the base current should be 1.7mA to 2.6mA for the transistors to saturate well. Then the base resistor value should be 8.3V/2.0mA= 4150 ohms, not 47k ohms.

    If darlington transistors like the MPSA14 are used then the base resistor value can be as high as 300k then the flash rate will be very slow.
     
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