Hi all,
I am very new to Arduino. I have an idea for a school project. I need to have a sensor that can detect an object. For example, this project involves a motor, I need to have a sensor that once detects an obstacle, such as putting a hand in front of the senso , the motor will stop or reverse direction.
I am wondering what kind of sensor is best to accomplish this type of task.
Any guidance will be greatly appreciated. 
Ultrasound?
IR?
Hands are quite soft, and don't necessarily reflect either very well.
Other options include infrared , but this only works at relatively short range and is dependant in the surface
There are also laser range detector with serial output that could be used
Passive infrared could also be an option
Also radar based motion are available
But basically, I think AWOL suggestion is the one you probably need to go with as its cheap and reliable
something like this is probably best for you to start out with. There are lots of examples online, if you do a search.
One way is to shine a light across the space into a retroreflector (ie, a bicycle reflector) and back to a phototransistor. Lens on both the light source and phototransistor to make it a tight beam and tight pickup.
The reflector reflects the light almost entirely back at the source, so put the source (LED) and detector right next to each other.
Use IR with shield tubes with small holes in them. Pulse the source and filter what is received to make it nearly insensitive to interference.
Thank you all for your replies.
I have decided to go for the ultrasound route because IR relies on a reflector,(am I correct?) Laser? Wow it's way out of my budget.
I ordered the ultrasound sensor. I will experiment with it, I think it will work.
because IR relies on a reflector,(am I correct?)
FYI...not always
Good luck with the project
A $3 laser diode is out of your range?