'NE555 P' Reset pin problem

Discussion in 'General Electronics Chat' started by vinodquilon, Feb 22, 2010.

  1. vinodquilon

    Thread Starter Member

    Dec 24, 2009
    234
    0
    I have set up the 555 astable MW to generate AF signals as shown in the attachment.
    The intended function is that When StD control signal goes logic HIGH AF signals are permitted
    to appear at + output. When StD is logic LOW, AF signals get suppressed at output.

    But in practical when StD is LOW, AF signals not suppressed at output.
    So I manually connect 4th pin to GROUND, then the signals get suppressed at + out.

    So I am going for an OR gate as a replacement of Q4 & R9. One pin is always LOW, other one
    connected to StD control.

    Does have any idea about the problem ? I think Q4 open condition doesn't bring GROUND
    condition at active low RESET pin.

    Attachment- http://www.scribd.com/doc/27308469
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    What is an MW?
    What's the StD control signal?
    Pin 4 of the 555 timer is the RESET pin. If brought high, the 555 timer will be enabled. If brought low, the timer will not function.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    Reset needs a current sinking control signal. Your NPN emitter follower cannot sink current. It needs 0.4V and 0.4mA worst case, so a 1k resistor from pin 4 to ground should work, unless R9 is excessively large in value.
    Why can't you connect StD directly to pin 4?
     
    vinodquilon likes this.
  4. andreapg

    Active Member

    Feb 2, 2010
    41
    0
    Is it an audio circuit?
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    I did a Google search. MW is an abbreviation for MultiWibrator.:D
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    Pin 4 is a direct digital input. I've found it can be as low as .7V and still work, if it is grounded the 555 stops.

    Have you looked at my notes on this site about 555s? The cookbook in particular covers several different oscillators.

    My Cookbook

    Multiwibrator!?! That made me laugh! :D
     
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