ne555 timer

Discussion in 'Homework Help' started by Ghosth, Mar 30, 2010.

  1. Ghosth

    Thread Starter New Member

    Mar 30, 2010
    4
    0
    Hi I have a problem with the ne555 timer.

    What I'm supposed to do is that I receive an positiv input PULSE that I am supposed to measure the length of with one or several ne555 timers. I'm only supposed to measure if it is longer, shorter or as long as 5ms. I'm supposed to have two outputs u0 and u0 and the truth table shuld look like this:

    0 0 - "Measure in progress"
    1 0 - "Shorter pulse than expected recieved"
    0 1 - "Error"
    1 1 - "Excpected or longer pulse recieved.

    I had a thought about using only a monostable ne555 and som inverters but I haven't really been able to implement it. Please help.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    This sounds like an assignment, so I'll move it .
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    You have the right idea, but 555 monostables respond to negative going pulses. You will also need to use some gates and latches.
     
  4. Ghosth

    Thread Starter New Member

    Mar 30, 2010
    4
    0
    Thanks. I was thinking about doing something like this.But to have an inverter on the Input to.
    My thought is that if input is longer or as long as I want ( in this case 5ms) then out will stay high until input changes to a high state. That will give me 1 1. I don't know if I should do something with a latch on out.
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    Again, the 555 responds to negative edges, not positive ones. You also have to have a capacitor / resistor coupling them, the 555 can not handle ground on the input for any duration.

    I explain why here.

    http://www.allaboutcircuits.com/vol_6/chpt_8/4.html
     
    Ghosth likes this.
  6. Ghosth

    Thread Starter New Member

    Mar 30, 2010
    4
    0
    Thanks for all the help :) I'm a bit of a novice at this so I really appriciate all the help.
     
  7. Ghosth

    Thread Starter New Member

    Mar 30, 2010
    4
    0
    Now I've come up with this. It kind of gives me the right output. I think that I have to put a capasitator by the and-gate to prevent it from acting wierd :D
     
Loading...