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help 555 timer with 7 segment display

Discussion in 'Electronics Homework Help' started by kristofferbernardo, Mar 19, 2013.

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  1. kristofferbernardo

    kristofferbernardo

    3
    0
    Mar 19, 2013
    from this circuit the 7segment display start at 30 down to 0. and i want to make starting to 15 down to 0.
    pls help me the deadline of this project is near.[​IMG]
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,533
    2,656
    Nov 17, 2011
    All you have to do is to set the flipflops to a count of 15 when reset is applied.

    As your circuit is built, it is pure chance that the circuit starts at 30. Connecting both SET and CLEAR inputs of the flipflops to the same reset signal is not a good idea. Although the datasheet of the 7476 states a defined output for this input combination, you can never be sure what the output state will be after the reset signal goes high. Depending on the chip's tolerances it can either be 01 or 10 or the flipflop can become metastable and fall to a defined state only after some time.

    I suggest you connect all unused CLR and SET inputs to VCC using a pull up resistor. Connect only those CLR and SET inputs to reset that you need to set the flipflops to a defined count, e.g. 15.

    By the way: this is homework or some other sort of assignment, isn't it? I will move the thread to that section of the forum.
     
  3. kristofferbernardo

    kristofferbernardo

    3
    0
    Mar 19, 2013
    sir this is a removal project removal means when i make it until thursday i will pass the subject if not failed pls help me..
     
  4. kristofferbernardo

    kristofferbernardo

    3
    0
    Mar 19, 2013
    help me sir please

    can you made it for me? just have no time designing the circuit because i will make it to PCB so please.
     
    Last edited: Mar 19, 2013
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    11,533
    2,656
    Nov 17, 2011
    Sorry, I will not do your homework and please don't blame me for your tight schedule.
    I will show you how to do it yourself.

    In your schematic, U8B and U23B are set to binary 11 = decimal 3 upon reset (at least by the simulator - a real circuit will behave differently, as I explained above).

    - Remove the fixed connection from all the CLR and SET inputs of the counter flipflops to VCC (top left).
    - find the required states for all flipflops after reset to display 15.
    - create a reset line, low active. This line should be High if inactive. Use e.g. a pull-up resistor to VCC.
    - connect the SET and CLR lines off all flipflops such that
    - if a flipflop needs to be set, connect CLR to VCC using a pull-up resistor, connect SET to the reset line
    - if a flipflop needs to be cleared, connect CLR to the reset line, connect SET to VCC using a pull-up resistor.
    - Connect the reset line to a low-active signal, e.g. from a switch.
     
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