555 timer bistable false trigger

Discussion in 'The Projects Forum' started by raptorjp, Nov 24, 2008.

  1. raptorjp

    Thread Starter Member

    Nov 24, 2008
    11
    0
    Hello.

    I have 2 555 timers controlling a stepper motor drive. One is acting as a pusle generator for stepping the motor and the other is in bistable mode toggling the direction (0 to 5V).

    The problem is that my bistable 555 sometimes will false trigger. I have put a 220uF cap between the power, and am using a 0.1uF cap on pin 5 to ground. Pin 2 (trigger) is also ac coupled with a 100uF cap. With or without these caps the timer will false trigger. The trigger is controlled by the NO contacts of a relay (not connected to the coil).

    Is there a way to add a low-pass filter so that only the contacts closing on the relay will trigger the timer and not - what I am assuming are - short transient dips. Any help would be great thanks!

    :)

    raptorboy
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Do you have caps on the pin 5 of the chips? That is what Pin 5 is for.

    You might also think about power supply caps next to the 555's, which is standard practice.
     
  3. raptorjp

    Thread Starter Member

    Nov 24, 2008
    11
    0
    Yes. I have a 220uF cap for the power (which is running both 555 timers - one in astable, the other in bistable). I have a 0.1uF to ground on pin 5 of my bistable. The astable seems to be acting fine (though false triggers on this timer is not as important at this time).

    Should I put a separate 220uF between the timers?

    Thanks,

    raptorboy
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Your "false trigger" may be caused by the "bouncing" of the relay's contacts.

    It may help quite a bit to add a small RC time constant across the contacts.

    Post your schematic so we can see what's going on now.
     
  5. raptorjp

    Thread Starter Member

    Nov 24, 2008
    11
    0
    The false triggers happen many seconds after the contacts have been closed and opened and do not occur often, but drastically affect the motor control when they do.

    What about using an edge-trigger with a cap between two pull-up resistors into pin 2?

    @sgtwookie
    I will try to get a schematic up if I can.
     
  6. raptorjp

    Thread Starter Member

    Nov 24, 2008
    11
    0
    Hello again.

    Just wondering if anyone can suggest a way to cause the timer to trigger on longer pulses so that short pulses are ignored. Thanks again.

    raptorboy
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Where's your schematic?
     
  8. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Maybe, due to vibrations of the structure, the relay contacts open/close for a short period and trigger the 555.
     
  9. raptorjp

    Thread Starter Member

    Nov 24, 2008
    11
    0
    Hello everyone,

    Sorry it took me so long to reply. I started working on a schematic, but it never made it to fruition b/c I didn't have any effective means (i.e. software) to produce them at this time and have been really busy.

    Luckily after 3 months in service the stepper motor is preforming fairly reliably. I have recorded data indirectly relating to its performance and only every once in a while does the bistable circuit false trigger, but not enough to cause significant problems. It is still a working prototype on a breadboard, but working well. Since I am operating another 90V DC motor there are vibration issues, as "mik3" suggested, that may be contributing to these transient triggers.

    Another factor I found that may have been inducing a higher number of false triggers, was the board on which my relays were. I scavenged a relay board used for security systems that I though had a series of 4 independent relays and terminal blocks for easy wiring. Turned out that each coil was tied to a common via a small jumper wire (which I later removed). So I guess that it possible that the additional coils were causing problems.

    `````````````````````````````````````````````````````````````````
    12V+---signal in----(coil 1)``````NC-(coil 2)`````````NC-(coil 3)
    ```````````````````````| `````````````| ````````````````|````````
    -------common------------xxx-------------------------------------
    `````````````````````NO-1```````````NO-2```````````````NO-3``````
    ```````````````````````|`````````````````````````````````````````
    ````[555-pin2]--||------|`````````````````````````````````````````
    `````````````````````````````````````````````````````````````````

    where xxx is where I cut the connection, NC is (not connected), NO is normally-open contacts of relay. The 555 is operating at Vcc +6VDC and has a separate common than the 12VDC control signal.

    Thanks again for all your help and sorry it took me so long to get back. You guys are great! :)

    raptorjp
     
    Last edited: Jan 30, 2009
  10. raptorjp

    Thread Starter Member

    Nov 24, 2008
    11
    0
    Hello again.

    SOLVED:
    I think I figured out the real reason I was having a problem. The problem would come and go, so sometimes I thought I had fixed it, but in reality did not. In all the schematics that I had seen for the 555 bistable, pin 6 was not connected. I finally came across a website that mentioned that I should hold pin 6 to Vcc. It definitely made a difference. Most websites only mentioned to cap pin 5 to ground to avoid noise causing false triggering, but nothing about what you should do for the other pins.

    It is all so obvious now that I shouldn't have digital inputs floating, regardless if the schematic draws them that way.

    Well, long story short, upgraded to a DirectLogic PLC with touch screen interface. Nice! But the 555 did a lot of work and without it I would be nowhere near where I am now.

    Thanks again!!!

    raptorjp
     
  11. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,176
    397
    Thanks for the follow-up, it happens so seldom ,it's like a breath of fresh spring air.
     
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