Hardware: head, electronics
Ok, so the motor-assembly now is in place. We still need some mechanics to
lift the pen from the board, or else it'd draw over everything that already is
on the whiteboard. A simple electro-magnet can do that, and I decided to make
a nice perspex carriage for it and the pen:
The idea is that if I want the pen to be off the whiteboard, the electromagnet
gets released and the spring will push a perspex rod on the whiteboard, thereby
pushing the whole carriage, including the tip of the pen, away from the surface.
If the pen needs to be 'down', the electromagnet will actuate and gravity will
push the tip of the pen down to the surface of the whiteboard again.
The electronics aren't that complex: the main task is to make the stepper motors
and the solenoid controllable by a PC, and in theory, not much more than a couple
of driver ICs or mosfets or something like that are needed for such a task. I
opted for a slightly more complex approach: the driver-ICs are controlled by
a microcontroller, which accepts the amount of steps the motors need to be moved
on a serial port. It then moves the steppers in such a way that they start and
stop at the same time, varying the velocity accordingly. For short distances,
that gives a neat straight line on the whiteboard. This way, the PC can be relieved
of a bit of work. I used an FT232-board I had lying around for for the interfacing:
the computer I was planning on connecting it to didn't have a serial port to spare.
As stated before, the drivers were scavenged off one of the scanner pcbs (STA401A) and the
pcb of an old laserjet (L298). The rest of the components filter the incoming 12VDC and
turn it into 5V for the microcontrollers power demands.