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555 and a lot of LEDs

Discussion in 'LEDs and Optoelectronics' started by Muhasaresa, Mar 4, 2012.

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  1. Muhasaresa

    Muhasaresa

    45
    2
    Jan 2, 2012
    Hi everyone :D

    This might be a simple question, but it has been bothering me for days. Please can someone help me?

    I have a 555 timer that causes an LED to flash once every second.
    Now I want 20 LEDs to flash at 1 Hz so I connected a transistor from the output to the 20 LEDs. Will they now flash slower because the capacitor will take more time to charge up?

    Muhasaresa
     
  2. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    What transistor are you using?

    If you are talking about a mosfet then I don't think the time it would take for the gate capacitance to charge would be even visible to the eye. You are talking about microseconds.
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    I presume you're talking about the timing capacitor.

    As long as the LEDs don't load the power source significantly (reducing its voltage) the timing capacitor is essentially isolated from the load, so it should have no effect.

    As for going from 1 to 20 LEDs, you don't mention how the single LED is connected, the supply voltage, or the LED current. (or indeed the LED type -- so we can guess Vf)

    I'll assume you're using a 9V supply and running the LEDs at 20mA.

    So at the moment you're probably connecting the LED to the 555's output pin (3) via a resistor.

    You could just connect another 19 LEDs in parallel (each with their own resistor, BUT the current would be 20 x 20 mA = 400mA which is way too high for a 555

    OK, so the next solution is to connect 10 strings of 2 LEDs, each running at 20mA, and each with their own resistor. This will draw 200mA from the 555. This is achievable, but is likely to cause some stress to the 555.

    A better alternative is to have the 555 drive a transistor (or mosfet) and have that switch the 10 strings of pairs of LEDs (each string having a resistor)

    This page has a LED Dimmer that demonstrates using LEDs this way from a 555. Note that it uses a 12V supply, allowing three LEDs in series.
     
  4. Muhasaresa

    Muhasaresa

    45
    2
    Jan 2, 2012
    Thanks for the help :D I never thought about it that way!
     
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