Circuit Design: "Random Waveform"/Trigger/Relay/Timer

Discussion in 'The Projects Forum' started by JimFlan24, Nov 23, 2010.

  1. JimFlan24

    Thread Starter New Member

    Nov 23, 2010
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    I am trying to design a circuit but am having some trouble. My knowledge of electronics is somewhat limited, but I have taken a couple of Intro courses during my bioengineering coursework and done some work in building power supplies. I will describe the circuits inputs and desired outputs.

    I have a sensor which returns 0-10 V or 4-20 mA (prefer working with Volatge-controlled stuff since that all I've ever really worked with).... and I want to have a cycle timer run one cycle upon having a threshold voltage being input, lets say 4V. I want the timer to complete its cycle regardless of the change in the input. It will generally drop based on the actions of my system, but it won't always... and I want it to be able to handle the condition where the input stays constant or even rises.

    I've read about multistable, monostable, and bistable vibrators... relays... triggers... but can't seem to find the perfect combination to handle this. A lot of these systems want to have inputs as impulses... but I have this time-varying waveform which could potentially be anywhere and send multiple triggers which is not what I want. Ideally, once the threshold is reached the cycle will run unstopped... and only upon completing its full cycle will it respond to another input.

    I figured this was a decent place to post to help solve this problem?
    Thank you for your time.
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    If your 'process' takes a specific amount of time to complete, every time it runs. then a timer circuit to 'blank' the inputs during that time would be the way to proceed.

    The trigger would activate the 'function' and start the timer as well. When the timer times out, then the inputs are enable again.
     
  3. JimFlan24

    Thread Starter New Member

    Nov 23, 2010
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    Neat trick... that makes a lot of sense. I am still unsure of specific names for each component. For example, what type of time-delay relay would allow for a long ON period (20-30 minutes) as that is the duration of my cycle. What type of trigger would you recommend? Would a monostable multivibrator be a good trigger-response component to drive the relay? If I'm not mistaken, once triggered, these components stay on for a certain period of time as long as the trigger is an impulse. Would the "blanking" be achieved by another relay which creates an open circuit? Thanks for your help, I know these are probably newbie questions... I'm just now starting to get into my own projects that are not recipe-style coursework.

    Jim
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I think you are looking for a non-retriggerable monostable multivibrator. The 74HCT221 is an IC for this.

    It will change output once the trigger is met, then run through the sequence while ignoring additional triggers/events

    If I am way off base or not even close to what you are looking for, let me know, I didn't quite understand the question perfectly.
     
  5. JimFlan24

    Thread Starter New Member

    Nov 23, 2010
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    Thank you for your valuable input.

    I had thought of using some sort of component which would send out a pulse when reciving an input over its threshold. This pulse would trigger a normally-open timed-open relay (NOTO) which would drive my cycle while it was in the closed position. Given that my input does not necessarily return to zero, a second relay with normally-closed, timed-close properties would be used to turn off the input to the sensor/trigger.

    If I'm reading its description properly, your device would act as both the sensor, trigger, and NOTO relay? And it would also eliminate the need for the NCTC relay. But I'm not sure about timing... if my cycle is to last 30 second on, 15 minutes off... or 1 minutes on, 30 minutes off, is this feasible? I know some components are designed for much shorter time periods and some for longer and have had issues/trouble/ignorance in using them in the past.

    Not sure if the additional picture helps at all as to what I was thinking...
     
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Timing circuits are varied and plentiful. My thought was a 555 type timer. They are stable up to about 5 or 10 minutes delay times. Then you can add on a decade or hex counter, or a string of flip flops and extend the duration to 100's of minutes.

    Lots of ways to do timing and triggering, but it's easier if you plan out what your process timing is exactly, first. The circuit can be built to conform to any timing needs.
     
  7. JimFlan24

    Thread Starter New Member

    Nov 23, 2010
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    So I'm not sure I follow... how can a counter be used to extend the time of the OFF part of my cycle... My familiarity with these parts is a bit limited, so if there is a site giving more info on counters/timers and how they work together that would be great.
     
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