As has already been said, GPS units don't detect speed cameras. You load known camera positions into them as POI (Point of Interest) databases. I use a Garmin i3 and keep it up to date using the camera database downloaded from:
http://www.pocketgpsworld.com/
Their database isn't free, but costs only £19 a year. If you spot a new camera and notify them, giving a lat-long fix for the new camera. They verify it, and if you are the first to notify them of a particular camera you get free lifetime membership.