LED Flasher troubles

Discussion in 'FAQs' started by b28_82, Mar 12, 2004.

  1. b28_82

    b28_82 Member

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    Has anyone created a circuit to make an LED flash? I have been trying to make multivibrator circuits work but all they want to do is stay on constantly. I havent tried using ceramic capacitors instead of Electrolytic. could this be my problem?
  2. jon-monon

    jon-monon Active Member

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  3. ezdays

    ezdays Out AZ way

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    It could be the electrolytic if your timer was set for longer cycles. Even if you were using a 555 (great choice for this application), we would use low-leakage capacitors if we weren't too fussy about the accuracy, but tanelum capacitors if we got over 1 or 2 mfd. Electrolytics have high leakage resistance which acts like a voltage divider in any R-C circuit preventing it from ever fully charging or fully discharging. The larger the capacitor value, the smaller the leakage resistance.
  4. shamus

    shamus Registered Member

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    Here's one I made
    Shamus
    [​IMG]
    [​IMG]
  5. DWP

    DWP Member

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  6. Fred_M

    Fred_M Guest

  7. b28_82

    b28_82 Member

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    Thank you. That is just what i suspected is it was sorta just "sitting there" holding a voltage of some sort.


    Everyone else: Thank you for the input. I appreciate the links and suggestions. I have been sorta sticking with my knowledge of the basic electronics course i took back in tech school and using all the traditional components instead of ICs. BTW how much do those 555 ICs run anyhow?
  8. b28_82

    b28_82 Member

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    Also my "project" as you may have it. was to see if i couldnt try to make a more realistic, home made EOT/FRED. Something like maybe a flash every 5 sec or something. First step is to get it to flash though :D
  9. Pete

    Pete Member

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  10. jon-monon

    jon-monon Active Member

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    Peanuts, at least from a good supplier. I dunno at Radio Shanty :confused: probably cheaper to use a 555.
  11. ezdays

    ezdays Out AZ way

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    Try 40 cents U.S. here in single quantities. The have ordering on the web or with an 800#. They also have reasonably priced caps and other electronic parts. You can't beat their switch prices anywhere.
  12. b28_82

    b28_82 Member

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    You think they take peanuts at radio shack?:D
  13. b28_82

    b28_82 Member

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    well i replaced the electrolytic caps with ceramic caps and still the same effect. I would have thought that the higher the resistance the more noticeable the flash but perhaps i thought wrong. I'll draw a schematic of what i'm using and maybe one can help me a lil.
  14. ezdays

    ezdays Out AZ way

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    That would help a lot. Put down values of your caps and resistors and any information you have on the LED and any transistors you're using would help too.
  15. b28_82

    b28_82 Member

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    here we go And the LEDS are 2.1v, 25mA, 6.3mcd

    Attached Files:

  16. Pitchwife

    Pitchwife Dreamer

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    Personally I would just start over wit 555s or 556s (2 555s on one chip) if you want double flashers and use the circuits that are on the thread.
  17. ezdays

    ezdays Out AZ way

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    Just a few comments on your circuit. It's been a long time since I've built an oscillator like this, but two things, your RC time constant may be a bit short, like increase the cap size to 1 mfd or the resistor to 10K and I would suggest adding a resistor to the base of each transistor to limit the base current.

    That being said, this circuit is no less than 40 years old. 555's aren't too much better age-wise, about 30 years old, but I'd go with them simply because you can sink or source around 200 mA of current without transistors, and an oscillator only requires two resistors and a cap. Also, it has a wide operating voltage range.
  18. Fred_M

    Fred_M Guest

    556s are sometimes cheaper than 555s. A 556 is a double 555 and will work as a single 555 if you just use 1/2 of it. Downside is it's about 170% longer than a 555. Plus side is it can run 2 different circuits or, to an extent, it's its own spare :D FRED
  19. b28_82

    b28_82 Member

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    Thanks for the inputs. The 555s at Radio Shack are 1.29 ea. I'm assuming that you control the frequency of the flash with the resistors. I was sorta hopeing that i could make it work with analog components for personal satisfaction ;) I'll probably break down and get one of those 555s or since the largest ceramic caps they sell is .1uF i'll order online.
  20. jon-monon

    jon-monon Active Member

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    Freq. and pulse width are controlled by a combonation of R1, R2 and C1. There is a calculator at the first link I posted. The old way was with charts, which you might find on-line. I like to use trimpots for the resistors, so it can be tweeked because it never seems to come out quite right. Then I sometimes build a fixed resistor to match the measured value of the trimpot after getting it right. This is done by soldering together several resistors. The second link is a 555 tutorial. It's quite a fun animal to play with.