We tried to build an automatic CD changer. We got really far using only parts from the NXT kit. In the end the CD changer needs just a little bit of assistance when picking up the CD.

We could not build a new CD feeder because we ran out of part in the end. So there is still room for extension after Christmas (hopefully:-). We still have a motor left to drive the feeder.

The CD Changer is built to put in a new CD whenever the drive opens. Thus it could be used to play music from several CDs or for data backup to feed empty CDs whenever one is full.

The CD Changer has 2 mechanical units, one to pick up and drop CDs and another one two move the changer arm to the pick up and drop places.

The pick up and drop arm is the part we are most proud of. It only needs one motor to function. With this one motor it can pick up a CD and lift it or it can drop a CD and lift the empty arm. The ultrasonic sensor is also attached to the arm to detect when the CD drive is open.

The ultrasonic sensor is used in a funny way. It detects that the drawer is open when the distance reading becomes large (255), because the drawer shields one "eye" of the sensor so that it becomes blind (picture #4).

The lifter arm is built such that when the motor turns into one direction, it opens the rubber claws to hold a CD and then move upward and when the motor turns into the other direction it closes the rubber claws to drop a CD and move upward again. There is a large middle position, where the arm stays put horizontally and only the claw moves. It is a bit tricky to adjust but it allows to grab and drop CDs with only one motor. To calibrate the arm before every use, the push sensor detects the upper and dropped position. This is the waiting position above the CD drive drawer.

The arm turner in comparison is rather simple, it just serves to turn the arm between the 3 positions: waiting for the drawer to open close to the drive, drop zone for discarded CDs, pick up table for new CDs.

The CD drive we have is not perfect for the job as the drawer does not move out very far. One cannot lift the CD straight upward from the drawer, one needs to move away from the case while moving upward. This is also implemented in the program that during initial lift of the CD from the drawer moves slightly away from the case.