New Member With a Question

Discussion in 'General Electronics Chat' started by ut1205, Mar 24, 2014.

  1. ut1205

    Thread Starter New Member

    Mar 20, 2014
    18
    0
    This is my first post so I will provide a little introduction. I'm from South East Tennessee and have recently retired. Back in the 70's I was a supervisor in a GTE switching office. GTE put me through four months of "Electronics" school but when I got to my office everything there was electromechanical. I knew a lot about relays, but that was about it. Now that I have the time I want to tinker with some of the solid state stuff. Enough said, now for my question.

    I have constructed various 555 timer circuits and all work as they are supposed to however, they all require a single pulse to start the timing operation. I want to actuate the 555 in monostable mode with input from a relay but all I can do is give it a constant +voltage or ground. Without the single pulse it just continues to trigger itself. Can it be triggered and run normally with a constant input? If so, I would appreciate any assistance.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,435
    3,360
    I'm not exactly sure what you are looking for. Let me give it a try.

    A relay is activated. The contacts can be either NO (normally open) or NC (normally closed).

    You want a 555 timer circuit to trigger in the monostable mode, that is, the output will go high for a preset time and then low again.
     
  3. ut1205

    Thread Starter New Member

    Mar 20, 2014
    18
    0
    That is exactly what I want to do. My relay circuit is on a loop like a burglar alarm. If the loop is broken it will actuate the relay (either open or closed) and trigger the timer.
     
  4. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    You may already know this, but just in case you don't. The 555 triggers on a low-going pulse, so if you are triggering it with a relay, you will need to choose whether to connect it to a break set or a make set depending on exactly when you want it to trigger.
     
  6. ut1205

    Thread Starter New Member

    Mar 20, 2014
    18
    0
    I found a circuit doing a Google search that changes the pulse required on the trigger from low to high. I wired it up today and it works fine so I guess I can use either a high or low pulse.

    Thanks for the link to the capacitor thread. I will check it out.
     
  7. MrChips

    Moderator

    Oct 2, 2009
    12,435
    3,360
    The capacitor in series as shown in the circuit diagram provided by inwo is the trick.

    This circuit is called a differentiator.

    The capacitor will send a low going pulse when the switch contacts close and a high going pulse when the contacts open again.

    The other trick is to properly bias the trigger input to the 555 to give the required low-going pulse. This is done with a pull-up resistor. The pull-down resistor shown is not necessary.
     
  8. ut1205

    Thread Starter New Member

    Mar 20, 2014
    18
    0
    Thought I would post a follow up. Tried a .1mf capacitor and it works great. Sometimes the easy fixes are the hardest to think of. Thanks for the help.
     
  9. MrChips

    Moderator

    Oct 2, 2009
    12,435
    3,360
    Glad we can help.

    Be careful with your units.

    .1mF is 1000 times bigger than .1uF.
     
  10. ut1205

    Thread Starter New Member

    Mar 20, 2014
    18
    0
    Wrong terminology, used .1uf
     
  11. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    Old folks like us knew 1mfd to mean 1 microfarad. A cap that had 450mfd@250VDC written on the side didn't mean 450 millifarads. A cap of that value would have been bigger than the radio it went into. A 1 farad cap was only discussed to explain the theory of capacitance and RC time constants. How times have changed!
     
    GopherT likes this.
Loading...