Sandraw is a unique machine that never fails to delight audiences whenever we present it.  It is a microprocessor controlled machine that draws pre-programmed images by use of a magnet that pulls a ball bearing through a thin layer of sand.  It was inspired by Bruce Shapiro, the originator of the concept, and you can view his machines at http://www.taomc.com/.  The magnet that pulls the bearing is attached to a rubber belt that is mounted to a piece of extruded aluminum.  At one end of the belt is a pulley, and at the other end, is a stepper motor controlled by a Propeller microprocessor https://www.parallax.com/.  Originally, long time member Paul Atkinson programmed the original Basic Stamp microprocessor, but it was later replaced by the more powerful Propeller microprocessor which was programmed by another long time member Gray Mack.

The arm itself, rotates, and is driven by a second stepper motor via a pulley and o-ring arrangement.  Between the driven pulley and the arm mounting bracket, there is a home-made commutator, or slip ring, that allows the arm to rotate in either direction continuously, and also allows the transfer of the electrical signals between the microprocessor and the motor attached to the arm.  There is an end-of-travel micro switch attached to the arm, and a “homing switch” at one end of the arm that determines where, rotationally, the machine starts to draw.

As an afterthought that always surprises the viewer the first time it activates, I incorporated an “erase” feature. The sand box is spring loaded, and at one corner, a motor is attached to an offset shaft that is connected to a bracket attached to the box.  The motor briefly spins after each drawing is completed, shaking the box to erase the drawing before starting another.  All together, there are over twenty pre-programmed images stored.