We used three Sharp IR sensors (
http://www.robotshop.ca/sharp-gp2y0a...sensor-1.html).
We desoldered the three JST input terminals and soldered a servo cable in their place. Then we plug each sensor directly in to an analog input port, and point the rangefinders forwards, looking out just beneath our "ball magnet" roller.
Now we not only know when there is a ball in our possession... we also know when we're getting close to a ball (or a wall... it ain't perfect) and can start up our ball magnet automatically. One less thing for the driver to do... or one less power drain as opposed to leaving it on all the time.
I guess we could even use it to check to see if two balls are in our ball magnet... although we haven't written that code yet.
Right now we're using our practice bot to try and see if we can come up with a dashboard display to help the driver find balls in the far end of the field, or use the rangefinders to help with auto mode.
Jason