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555 timer, not gate has 2 inputs.

Discussion in 'General Electronics Chat' started by shaqywacky, Apr 1, 2009.

  1. shaqywacky

    Thread Starter Active Member

    Apr 1, 2009
    Hello everyone,

    I just started messing around with circuits, so I got a 555 timer and hooked it up.

    But when I was reading the datasheet I noticed that the not gates get two inputs. I'm not very experienced with circuits, but I could have sworn that not gates only get one input.

    I also noticed that they have plus and minus signs, but that just added more confusion.

    I've made an attachment of the pdf.(Its on the second page, first diagram)

  2. Wendy


    Mar 24, 2008
    A 555 is an open ended circuit. The NOT function (aka Inverter) is just one of them, and it has to be wired to make one.

    I cover that in this article.

  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    Actually they aren't not gates.

    They are level comparators.

    The positive input for each comparator is tied to a value related to the chip supply voltage.

    In the case of the trigger comparator it's 1/3 Vcc. In the case of the threshold it's normally 2/3 Vcc for most purposes, but this can be overridden to some extent by a control input. These 1/3 Vcc and 2/3 Vcc values are set up by the three 5K resistors in series from Vcc to supply common.
  4. shaqywacky

    Thread Starter Active Member

    Apr 1, 2009
    Oh, I get it, that fixed a bunch of my problems!