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driving LED with transistor

Discussion in 'LEDs and Optoelectronics' started by Kardo22, Apr 2, 2014.

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  1. Kardo22


    Mar 7, 2014
    I made this design:
    About the way I connected the LED. I sawit somewhere online, don't know anymore. I have a guestion about it.
    Q1. Why is it connected parallel to the transistor, I thougth LED should be in series with collector (or emitter in some cases). Does this even work?

    Q2. Should I even use the transistor. I have always added 1 just in case to protect the chip but is that neccessery?
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Yes, that will work, but it's not conventional, efficient, nor particularly elegant.

    What happens is that the transistor, when switched on shorts out the LED causing it to go out. Perversely, the power consumption with the LED off is higher than with it on.

    I can't think of any particularly good reason why you'd do it like this.

    I would recommend what you're suggesting, that is, to drive the LED directly from the output of the 555. In thi case connect the LED (LED1) and the 470 ohm resistor (R4) between Vcc and pin 3 of the 555. Then you can dispense with R3 and Q2
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Like this, Q2 acts as an inverter:
    Q2 = off: LED is on (current through R4 flows through LED)
    Q2 = on, LED is off (current through R4 flows through Q2)

    The LED can be connected directly cathode to pin 3, provided you limit the current according to the datasheet values.
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Just seconds away :D
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