I don't know why but I've been trying to think of this for a while (as in a few years) and it keeps randomly coming back to me.

I saw a little toy on TV once.

It was a block of wood with two wheels (one on either side of the block) and a flag popping up at the top.

The flag was connected to the wheels such that which ever way the cart was turned the flag stayed in the same orientation. (i.e. if you push the car back and forwards the flag didn't move but if you turned the car around the flag moved with respect to the car but not with respect to the floor).

I know this must be a fairly simple gear setup but I can't think of how it would work without have crunching gears.

Any ideas?

I can't quite picture it in my head, but it's something like a differential:

http://en.wikipedia.org/wiki/Differenti ... _device%29

I'm not entirely sure what you mean.

Do you mean the orientation of the flag stayed the same, regardless of the direction of the car?
Or simply that as the steering axle turned, the flagpole turned to oppose it?

Yeah, I thought it would be based around that but the differential is kind of different in that it does the opposite of what you want.

i.e. when both wheels are moving the same speed in the same direction the diff spins but if they are moving in the opposite direction at the same speed they don't spin.

This would give the overall effect the the flag would spin around all the time rather than staying still all the time.

The effect that I saw on this toy was more like a compass pointer. i.e. whichever way the car went the flag pointed in the same compass direction (but using gears, not magnets).

The flag acted a bit like a compass pointer in that whichever way the car went the flag stayed pointing in the same compass direction.

I understand what you mean, but that's in the standard configuration used in a car. It can be used to create an output equal to the difference of two inputs. Going in a strait line results in no rotation, but turning produces the exact amount to keep the flag pointing the right way.

By multiplying one wheel by -1 the diff will subtract rather than add. I'm trying to picture how to use it in my head. I think you'd just use a pair of cogs to reverse the direction onto one wheel, and then fix the flag on the drive shaft sticking in the air.

I need some Lego!

Ah, I think I can see how it's done now.

Ok, looking at this img...


You could, I think, add a reversing gear between (you would need 2) the left hand sun gear and the 2 planet gears.

This would mean that similar movement of the axles would result in no movement of the drive shaft and equal and opposite movement of the axles would result in movement of the drive shaft. It's quite hard to picture it in my head so not entirely sure if the movement is directly in the required direction though.

I think that's right?