Timer with fixed input [duplicate]

Question

I'm using a lm555 IC in monostable mode. When my input varies from 5v to 0v the output goes from 0v to 5v. But this only happens perfectly if it happens just a pulse at the entrance, in case my input changes from 5v to 0v and continue at 0v for the rest of the time the output does not work correctly. What I want is the following, when my input changes from 5v to 0v and continue to 0v, I want my output to remain high only for the time I have determined.

What I have:

What I need:

I thought I could do that with transistor input, but I did not find a way to do it. If anyone knows something similar thank you.

Answer 1

If you add the following components onto the trigger input you will create the required trigger pulse for every negative edge. you may need to adjust the component values to suit your application.

^{simulate this circuit – Schematic created using CircuitLab}