Why is this LED needed?

Discussion in 'The Projects Forum' started by EwanKodak, Jul 10, 2014.

  1. EwanKodak

    Thread Starter New Member

    May 17, 2014
    Hi all, I'm making a circuit based on this schematic:
    http://www.qsl.net/yo5ofh/hobby circuits/circuit pictures/555leds.gif

    It works, but I don't really understand the lower transistor functions.
    What really confuses me is why the LED attached to the pnp transistor base is needed, and more to the point why is it when I replace the LED with a diode it doesn't work.

    Many thanks,
  2. bertus


    Apr 5, 2008

    The led on the base of the PNP transistor will create a voltage drop.
    Depending on the used led this can be from 2 to 4 volts.
    A single diode will have a voltage drop of about 0,7 volts.

  3. ClayMarley

    New Member

    Jul 6, 2014
    I think the trick here is to note the output pin-3 of the 555 is about 1.7V lower than the supply voltage. So when pin3 is high, there would still be enough voltage drop at the base of the PNP to conduct. The LED causes a voltage drop sufficient to prevent current flow. A typical diode however only has a voltage drop of 0.7V, so there is still a 1V drop at the base, causing current to flow. The LED voltage drop is much higher. Try replacing the LED with 2 or 3 diodes in series.