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flashing led help

Discussion in 'LEDs and Optoelectronics' started by misterinley, Apr 11, 2012.

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  1. misterinley


    Apr 11, 2012
    hi guys,

    need help on the basics.

    Ive wired up a 3v LED to a 3v battery (so not needing resistors) and put a capitor in before the led in a series circuit but no light at all.

    what am i doing wrong?

    any help will be much appreciated.


  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    What is a 3 V LED? Any LED has a I-V characteristic where the voltage drop rises when the current rises (although the relationship is nonlinear).

    Anyway: putting a capacitor in series with the LED will allow current flow only for a very short time (microseconds?) until the capacitor is charged. If you measure the voltage across the capacitor you will find the whole 3 V across the capacitor. Remove the series capacitor and replace it by a resistor. You max be able to use a small resistor, but do use one, otherwise you will drain the battery too fast and even run the risk of destroying the LED. See the LED tutorial in this forum for details.

  3. markm6164


    Jan 22, 2012
    Buy some 555 ic's. You'll have hours of fun making leds flash :)
  4. timothy48342


    Nov 28, 2011
    I take it you want to make the LED flash?
    A capacitor is not a flashing circuit all by itself. It does let current flow until it fills up, but then it stops and that's the end. You could get the LED to flash just once. If you hooked up a switch to dump the capacitor, then you could get the LED to flash once each time you press the switch, but not on it's own. You have to keep pressing the switch each time.

    You can probably buy flasher circuits that do exactly what you expected form the capacitor, but you can easily make your own.

    A 555 timer is a very easy chip to work with. (And pretty cheap, so it's not the end of the world if you destroy it.). Wikipedia has a great page on it. It can work a number of different ways, but, if you look in the "modes" section, one mode is "Astable." That's the one for making the 555 turn on and off over and over. It only takes the 555, 2 resistors and 2 capacitors to make it work. In wikipedia's schematic, it has a little red squiggely line next to "out." You would hook your diode and a 3rd capacitor from there to ground.

    There are other ways, of course, but that's what I would use.

    On the other hand.. if your just trying to learn about capacitors and how they work... Well, you just learned that they don't make an LED flash all by themselves. :)

    Edit:I agree with markm6164 :endEdit
    Last edited: Apr 11, 2012
  5. misterinley


    Apr 11, 2012
    thanks for the help guys, im new to this as you've just seen, and I will look into these 555 chips.

  6. CocaCola


    Apr 7, 2012
    Depending on your end goal there are solid state flashing LEDs that require no additional circuit to flash...

    As Harald Kapp touched upon, use a resistor for the LED in almost all cases, even if the max supply voltage is in the 'safe' range for LED based on the datasheet, even if it's just a 1 Ohm resistor in your case... There are instances when you can skip the resistor when you are using a current regulated supply, or when using a button or coin cell battery that has a high enough internal resistance and low max discharge rate that will almost certainly never overcome the LED...

    Also Google up 2 transistor LED flasher, for a simple flashing circuit...
  7. selanac


    Apr 15, 2012 has a Flashing LED circuit with an FM transmitter. Don't think they use 555 IC's at all.

    I've been away from Basic Electronics for a while, but thought 555's were Timing chips.

    Really amazed how they've come along or maybe how I'm seen them used now days. This compared to my beginning stages of electronic knowledge.
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