I'm trying to think of different ways to build a device that will allow me to mount it to any moving object so that the moving object will be able to discern it's speed (It DOES NOT have to be very accurate)
I just need to know a few levels of speed: not moving, slow, medium, fast, etc.
I want to be able to mount it on a bike, maybe a scooter, maybe a skateboard, etc.
Cheap - I want to be able to make this thing for
Bike computers work with a magnet mounted on a wheel and a hall effect sensor to pick up each rotation.
A velocity can be calculated by discerning the distance traveled given time. (the bike computer method is great, but it doesn't allow me to use it on a skateboard, for example)
Are there optical sensors that can be pointed at the ground?
An infrared distance sensor won't work (right?) because it is always the same distance from the ground.
What about a sound sensor? Can it measure some sort of doppler shift?
GPS is too expensive
I potentially can use an accelerometer. How would I know the difference between a stand still vs. no acceleration.