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555 timer

Discussion in 'General Electronics Discussion' started by mudell, Dec 11, 2011.

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  1. mudell


    Dec 11, 2011
    Can someone please give me some help on what component models can i use for the comparators, transistor and the rs flipflop inside the 555.

    Thanks very much !
  2. electro_pa


    Aug 14, 2011
    Hi mudell,
    Your request seems to indicate you have a basic mis-understanding of what the '555' is.
    In electronics, we understand a '555', is an integrated microciruit, used for timing functions.
    That means it is a solid object, a component with pins to attach in your circuitry.
    You cannot meddle inside it.
    See: http://
    A 555 is an extremely useful component in basic electronics. It has been around for a very long time, 40 years in fact!
    What lead you to ask this form of question?
  3. mudell


    Dec 11, 2011
    Thanks for your reply.
    I have a basic school project and I want to build an astable circuit using the 555 IC and rebuild the same astable circuit this time using descrete components for the 555 (not using the IC itself), i.e. with the three 5K resistors, the 2 comparators etc..
    Now my problem is that I have no idea what should I connect to represent the comparators (those inside the IC). I was thinking about for example connecting 2 LM741's.

    Is it a good idea ?

  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    They should not be too critical. Juast make sure you pick something that will operate at the voltage you're using.
  5. electro_pa


    Aug 14, 2011
    Comparator suggestion.

    Hi murdell,
    Now I understand your dilemma!
    Your project is a good test for study in electronics.
    Using a LM741op-amp, for a comparator is not very good. It has a linear output circuit, not designed for 'switching' style output.
    It might work, but will be unpredictable.
    Much better to use a real comparator, say LM393, dual, low-power, comparator. It's available from Jaycar and Altronic for less that $2.
    Have a look at spec sheet. Hope you understand the terms used.
    LM393 will work OK, if not required to drive much external load on the output pin. It's not guaranteed to 'sink' more than 6 milliamps!
    A real 555 can 'sink' 200 milliamps under some conditions.
    Also you will need to put a 'pull-up' resistor on the'output' pin, since LM 393 output pins are 'open-collector'. See the schematic diagram.
    Good luck, and ask again if you need more help.
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