A compass sensor using Honeywell HMC1052 ic.
An IR cliff sensor using Sharp GP2D120 ic. (mounts on the underside and detects if robot is about to go over an edge.)
An IR proximity sensor using Sharp IS471F.
An Eltec 442-3 Pyroelectric heat detector. (or something less pricey)
An Analog Devices ADXL202E accelerometer.
An Audio piezo.
Also a RS232 level convertor using the MAX232 ic.
bill
heres one i really want to see!
somthing that tells the robot where down is!
it could be as simple as a potentiometer hooked to a weight in a track! (Hey, i think ill go make one myself!)
that method would swing with aceleration but could be monted perpendicular to the primary direction of movement.
…first post…
EDIT: this would allow an unstable robot, like a motorcycle to adjust and stay upright.
Yeah that would be ok but I dont see a vex motorcycle going fast enough on 2 wheels to need a gyro. It woudn’t react fast enough.
A gyro sensor would still be cool since the robot would be able to sense whether or not it’s going up hill down hill and many other uses too.
Ya, i guess a motorcycle wouldn’t work. I started to build a sensor. ill post pics etc when done. I’m thinking make it kind of like a ultrasonic so programs can easily recognize it. Then again it doesn’t need to have warning to take measurements…this may need funcs. of it’s own to operate. I’ll make those too.
I like ACE:)
=$6.36 total
Thanks Speer. You’ve just given me my new task-a specially calibrated 3 axis acceleration sensor like the Wii has, only my own. You just had to mention that, but then again, I just had to read this forum at 12:11 when I’m tired 
i have used a light sensor pointed at a disk, the disk is colored half black and half white. on the black side, some sort of weight is attached. the disk is to be put on something enabling it to rotate. this would tell what direction the robot is pointed at.
I got an idea:
How about you use the compas sensor (digital) to tell whether the robot is going up or down, and then if you have a servo installed to a shiftable gear ratio, it will shift it to a lower/higher torque/speed ratio. So it would be like an auto drive for a car.
But you would need a computer to program which motor the servo’s in. And it would require a engineer that can make sure everything works. but it should work.
If you don’t know what a shiftable gear ratio, think of a car, or a bike, how gear one is really slow, but has torque, but gear 5 is really fast, but has no torque. look at Bobby C’s pics he has one.
Build one!
You don’t need an engineer. A medium dose of common sense and some simple support tools should suffice. If you hit a roadblock that you try to solve (for more than 30 seconds :))and can’t; ask some questions here.
Blake