hey guys i am thinking of adding a motion sensor to the faucet in my bathroom because i always seem to leave it on, the only thing i am having trouble coming up with is what or how to control stop/start the water flow ... i have plenty of room in the bottom to install something there maybe even the top, i am not too concerned about aesthetics ..price though is always an issue :)

any insights?

In many public toilets there is a sensor that tells when your hands are under the faucet and turn it on and off.

You don't have to go that far and may not for the sake of your wallet and valve seats want to. You way only want something that beeps if you go to leave without turning the water off.

Depends what type water valve you have, is it the ones you turn the nob to open the valve, or pull up and down on a lever to adjust pressure?

When looking at your question I thought of a interesting way to make a mechanical mechanism to control the turning nob style valves. Its not ideal but you could take the tap nob off and connect a stepper motor directly to the tap. With a stepper shield on your arduino you will be able to control the stepper motor. Add a sensor above the actual tap which detects movement? which will turn the stepper motor to turn the tap on or off respectivly...

Its no where near the MOST practical but I thought it would be interesting to see lol