Darrel - Unfortunately, you just about know as much as I do. The person that wants me to do this is being pretty secretive. Here is a little more... I am assuming it is a motorized vehicle and the position of the wheels will not always be the same, so they can't be used for indication. This will be in an outside environment, moving over any type of surface from very smooth to rough (grass, dirt, pavement, snow, etc.) at a pace that can range from 1/2 normal walking speed and up. Ideally, they are looking for something that they can just aim at the ground and determine that it is moving. They are not concerned about speed or direction, just movement. Height of sensor can be about 3 feet. Oh yeah, and cheap (duh) ... I am leaning towards a vibration sensor, but I don't know enough about the end system to be able to say that it would be 100% reliable - I have not tossed out the accelerometer, thanks to you... So there you have it... And again, thank you for your input. It is appreciated! Jeff