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122 Masterpiece 3 • The LEGO Turning Machine

Tape Step 11

The gear racks are the
pieces that allow movement
control from the outside. If you were
ever to build a longer Tape sub-
assembly, the gears would have to be
mounted along the entire length of
the tape.

The Direction Control

The Direction Control sub-assembly controls the right and left movements of the tape.
This should be a discrete step-by-step movement, meaning that the tape should move one
cell at a time, no more, no less.This is achieved by using a touch sensor that can be closed
by pegs connected to a 40T gear.The 40T gear is connected to a double-beveled 20T
gear that works with the gear racks added in Tape Step 11.They act as if they are 10T
gears. Every quarter of a turn of the 40T gear results in the Tape sub-assembly advancing
by half gear rack o, a single cell.
Other systems to achieve the same result can be used: for instance, a rotation sensor, a
light sensor that emulates a rotation, or a touch sensor working directly on tape.The one
limitation to remember is that the movement must be smooth enough, and never use a
timer. A timer on the motor would have to run and depend on the charge of the bat-
teries of the RCX; thus accumulating small errors on errors, and you’d soon lose the cor-
rect position of the tape.