Having got the chassis working, it was time to add a grab. First, though, I needed the light sensor for the sorting algorithm. So, I had to make a touch sensor which would ride up over little rods taped to the bottom of the track - one for a stop, two at close intervals for the ends. Then, I tried a number of options but I ended up using basically the same mechanism as with QuAD and mounting it sideways. These are the build-ups for those two elements:

Stick an axle through the sensor. Then use an angle crank to stick a small wheel on the end. The wheel ensures that the bottom of the dropper will ride up over the marker rods beneath the track, rather than jamming.

Next. attach a second axle to the back of the sensor. The two bushes allow adjustment of the dropper height: the green 2x1 will make an attachment point.

The bevel gear on the end of the axle is the pressure point for the sensor switch. It's held in place on a mounting plate by a bush. The mounting plate fits onto the back of a 4x2. Two 4x1's provide a guide for the sensor to run in. Meanwhile, to mount the two together, the green 2x1 block fits the centre of the front of the 4x2 plate.

And here's how it looks fitted together: the whole sensor rides up and down on the axle, making contact with the gear to go into the up=on position when it rides up.

The components for the grab are fairly similar to the one on QuAD. Two 8x1 bars side by side for the attachment point. One angle crank fits either side of them attached to the axle, with the wheel on one end. Couldn't be simpler. The light sensor fits underneath where it can see either green or yellow tube, or nothing at all.

The bars fit to the top of the motor block. The grab motor is fitted at 90 degrees to the drive motor, with a drive belt to the wheel, and that's it. Here's the finished item: click on the thing for a bigger picture and details of the software.