Unwanted trigger during powering up 555 monostable multivibrator

Discussion in 'The Projects Forum' started by GirishC, Apr 1, 2015.

  1. GirishC

    Thread Starter Active Member

    Jan 23, 2009
    58
    0
    Hi All-
    Capture.JPG
    I am working on a project and 555 is one of the part. The idea is 24vdc is powering the circuit. It has a switch which need to be monitored. The relay is bistable relay to remember the previous state. So I have a 555 based circuit which will be powered up by 5v derived from 24v. E output of switch going to load/relay is sensed and fed to 555 monostable multivibrator trigger. Whenever the switch is off it will trigger the 555 enabling transistor to switch the state to off position.

    This works fine as per required. But when I power cycle the 555 timer is triggered (internally)triggering relay to switch off. To ensure the power up isn't triggering the 555, I also tied trigger pin to 5v and also to 24v.

    Are there any pointers to address this problem. The schematic is added.
     
  2. pwdixon

    Member

    Oct 11, 2012
    488
    56
    You could add an RC to the reset input to hold the 555 in reset just after power up.
     
    ErnieM likes this.
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,059
    3,821
    @GirishC
    Eliminate Q1 and r14. Connect pin2 of the 555 where the base of Q1 is currently connected. Change R2 to a 33k (this is the pull-up resistor. 555 triggers when you bring pin2 low.

    R9 can be changed to a plan wire and eliminated.

    Note: the 555 triggers when pin2 is dropped to low level. Since you start with it low, it triggers on power up.
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    i would put a power reset on pin 4, and remove all component on pin 2 and connect it like this, when the 24V is dropped the timer will trigger, and reset when the 24v is supplied.

    555.png
     
  5. GirishC

    Thread Starter Active Member

    Jan 23, 2009
    58
    0
    @GopherT: I need to detect low state on switch SW2. Hence I had to invert with Q1. In normal condition Q1 will continue to trigger the 555 which is not the case.
     
  6. GirishC

    Thread Starter Active Member

    Jan 23, 2009
    58
    0
    That's great...thanks for advic. I will try this mod.
     
  7. GirishC

    Thread Starter Active Member

    Jan 23, 2009
    58
    0
    @Dodgydave: I really appreciate the block diagram and reply. I will try this mod.
     
Loading...