LED question

Discussion in 'The Projects Forum' started by MrEd, Nov 23, 2008.

  1. MrEd

    Thread Starter New Member

    Nov 22, 2008
    I have an issue similar to this so I thought this the best thread for the question.

    Furthermore, I'm not sure this is even possible on the scale I need so this is what I need to find out.

    I need to know how to charge a capacitor to run an led flashing, or full on for 1-5 minutes, 30 sec maybe. Charged with a 9v batt or other power source.

    Right now I have what I believe to be 3mm leds, but would like to use
    0.25mm Ultra-Thin 0603 SMD LEDs

    I'm thinking 3 parts possibly. LED, resistor, and a small cap. Otherwise I'm looking at 2 button cells, or a 3v batt that I'm not sure will be small enough.

    Here is the problem. I need to fit this little sucker inside of a cone shape approx .578 dia and 1.25" long. Weight and size is the issue.

    Is this possible?

  2. SgtWookie


    Jul 17, 2007
    Hi, MrEd - Welcome to the Forums.

    It's really best if you start an entirely new thread when you have a question. Tacking on questions to an existing thread is called "hijacking", and is generally frowned upon.

    Starting a new thread helps things from becoming confused. Otherwise, people might respond to the original poster in between others trying to help you.

    As far as your question, you'll get far more energy out of a small pair of button cells than you would be able to store in a capacitor. Battery voltage (under light load) drops off slowly until the battery is nearly depleted. Capacitor voltage drops off quite steeply, even for large-value caps.

    For your size constrains, you may be able to find a cap large enough for a second or two of light, whereas a pair of small button cells will keep you going several hours, depending upon the current you're drawing from them.

    Safety issue: Please do NOT attempt to solder wires or tabs to button cells, as they WILL explode, possibly causing personal injury!
  3. MrEd

    Thread Starter New Member

    Nov 22, 2008
    whoops, sry about the hijack thing. I must have misinterpreted what I read in the posting rules.:(

    TY for answering my question though so I know which way to go.
  4. beenthere

    Retired Moderator

    Apr 20, 2004