Here's an article explaining the monostable mode of the 555 timer. There are example circuits and theory explained as well:
http://www.circuitbasics.com/555-timer-basics-monostable-mode/
Here's an article I wrote explaining the 555 timer from a beginners perspective if you're interested. There are example circuit
diagrams and theory explained as well:
http://www.circuitbasics.com/555-timer-basics-monostable-mode/