timer555 single pulse
You need to turn the input from a level to a edge trigger:



by adding a capacitor as shown to create a differentiator.  Try using 0.1 µF to start.  The pullup resistor, Rd can be 10K or so.  The diode is a 1N4148.

The 555 will not re-trigger until after the input has been restored to Vcc and then goes low again, no matter how long the original pulse is.