trigger reset newbie question

Discussion in 'General Electronics Chat' started by mark.beard, May 23, 2010.

  1. mark.beard

    Thread Starter New Member

    May 23, 2010
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    Hi all

    i'm trying to create a circuit that uses a 555 in astable mode to flip a led on and off for a 2 minute period - this is fine and simple enough. what i need is for an alarm circuit to activate when the LED state changes and have a reset function to silence the alarm until the next change of state of the LED when the alarm circuit should be triggered again. i have no issue with the alarm circuit, again this is simple enough but i cant work out how to use a reset that wont automatically get triggered straight away by the LED state being either high or low - if the above makes sense - please help! i'm thinking i need some kinda of latching mechanism but i'm new to this so its hard to suss out (for me at least) :)

    any help very much appreciated.

    Many thanks
    Mark
     
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    I've read and reread post & still confused so made some guesses: A trigger pulse starts oneshot, OS, A, 2 min., led on, alarm sounds, trailing edge triggers OS B for 2 min. OS B inverted and used to reset OS A, pin 4 ensuring that OS A will not be able to be retriggered for 2 min. If a reset pulse is initiated within the first 2 min, alarm is reset via OS B. If it is desired to reset alarm later more circuitry will be needed. Maybe you can provede an updated timing chart?
     
  3. mark.beard

    Thread Starter New Member

    May 23, 2010
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    0
    Hi Bernard

    Thanks so much for your reply and diagram and for trying to understand my requirements. Maybe rather than trying to explain what i need from a technical standpoint a more basic approach would help you see what im trying to say.

    So i want to build a circuit that will indicate to my wife that she should run for two minutes, a LED changes state and alarm sounds (which can be reset to silence it) after a further two minutes the LED changes state again and the alarm is triggered again (which again can be silenced). The idea is for a running technique which sees the runner walk for two minutes then run for two minutes, the alarm is to alert you to the fact that a two minute period is over and the led state is to tell you whether you should now be running or walking.

    Hope the above is clearer - as i say i am new to this so i'm not sure what you mean by trailing edge, OS A/OS B (is it oscillator?) many thanks for your time and help in advance.

    All the best
    Mark
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Does the further 2 min start after the first 2min or after alarm has been reset?
     
  5. mark.beard

    Thread Starter New Member

    May 23, 2010
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    Hi again Bernard

    the further two minutes starts after the first two and repeats until power off - the alarm only needs to be triggered it isnt relevant to the timing - its just so you know a two minute segment has passed - the alarm cues you to the fact you should change from running to walking or walking to running.

    Many many thanks for taking the time to help me - i know it must be difficult with a beginner.

    All the best
    Mark
     
  6. Norfindel

    Active Member

    Mar 6, 2008
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    9
    It seems to me that you need one signal that means "state change". For example, a clock's rising edge, then you feed that to a flip-flop wired to change the state of it's output when the clock signal arrives (this feeds the led), and another one that always sets it's output to 1, and can be reset by pressing a button (this feeds the alarm).
     
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    On powerup, IC 1 starts timing 2 min, IC 2 is reset & IC 3 is held at reset for about 50 ms via C8. At the end of 2 min. IC1-3 goes low[ trailing edge ] causing a negative pulse on IC2-2, start timing a 1.7 min period causing "Sonalert" to sound for 1.7 min or untill momentary NO SW is pressed resetting IC2. When IC2-3 goes high it causes FF IC3 to change states, Q is high turning on LED, so that LED is off for 2 min & on for 2 min. R1 adjusts the 2 min time & may be replaced with a fixed resistor after measuring. R8 adjusts time S is on, slightly less than 2 min. can also be replaced. Operating V depends on alarm, S. requirenent, but at less than 10V the LED will need a driver. At 12V R7 is 1k for about 10mA.LED current - not too bad for a bright white. Short on time for now, but look over data sheets on the ICs, and come up with questions.
     
  8. mark.beard

    Thread Starter New Member

    May 23, 2010
    4
    0
    Hi Bernard

    Firstly thanks so much for your help and your wonderful diagrams and explanations. That must have taken you some time to draw out and i really appreciate your help and design skills.

    I'll build the circuit today and test it - i'm really really grateful - what a star you are !

    Norfindel - thanks also for your comments - what a nice board this is :)

    All the best
    Mark
     
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