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.

 

This sounds like an assignment, so I'll move it .

 

You have the right idea, but 555 monostables respond to negative going pulses. You will also need to use some gates and latches.

 

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.

 

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

 

Thanks for all the help I'm a bit of a novice at this so I really appriciate all the help.

 

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