How do I calculate the RC values to control this 555 timer circuit?
I'm building a device that squirts my cats when they cross in front of a motion detector. It works fine, but the detector runs the squirt motor for too long, so I need to make a timer circuit. I'm new to the 555, so I'm trying to figure out how to calculate R1 and C2 in this schematic to give me an approximate runtime of .5 second. I'd thought of using a potentiometer in place of R1 to give myself some ability to fine tune the pulse from .2-1 second. Here's my proposed circuit: http://i.imgur.com/B76QQGq.png