This product is an absolute pain in the butt. I put in new batteries and screwed the cap back on, and it would flash on and then off again. It wouldn't come on. I had to adjust the tightness on the cap to get it to come on for like 1 second, and then it would go off again. It's SO incredibly sensitive, the cap has to be in the absolute perfect position for you to get it to work for maybe 10 seconds.