In this video I would like to show how to build a battery powered Wireless LAN plotter with parts from two old optical drives, four H bridges, a servo and a Raspberry Pi.
From the optical drives we need the mechanisms used to move the laser unit.
That carriage is driven by a stepper motor.
Unfortunately not all optical drives use a stepper motor to actuate the laser unit - my hit rate was 2 to 1.
The stepper motor has four pins:
A pair of pins is internally connected to an electromagnetic coil.
With a multimeter switched to continuity you can figure out which pairs are connected to a coil.
At this motor, pin 1 and 3 respectively 2 and 4 belong to one of the coils.
Don't remove the foil at the pins, because you will very likely also rip the copper wires off the pins by what the motor gets inevitably destroyed!
I have damaged a motor in doing so, thus we can have a closer look at the inner components of the stepper motor.
The old supply lines with the foil can be trimmed.
The new cablings have to be soldered to the pins.
This type of stepper motor needs an alternating polarity at the coils.
As a consequence, one H bridge is needed per coil, thus we need two H bridges in total for the motor.
With a lot of time you can build the H bridges from scratch using transistors, diodes and resistors.
Less time is needed when using an integrated circuit which requires only some external components...
...and the simplest way is using a prefabricated device.
The board has four output terminals:
Each pair of outputs has to be connected to one of the coils at the stepper motor.
Furthermore there are four input pins which have to be connected to the GPIOs of the Raspberry Pi.
Finally the 5V supply voltage has to be connected to the terminals "Ground" and "VCC" which is the positive terminal of the supply voltage.
A single coil of the motor draws a current of almost 500mA, which is above the maximum current provided by the 5V pin of the Raspberry Pi!
Consequently I am using an old computer power supply to provide the electric energy needed to operate the mechanism.
The motor is controlled stepwise through the GPIOs by the software running on the Raspberry Pi.
Depending on the rotational direction of the stepper motor, the laser head moves to the left or to the right.
A second stepper motor is needed to make the plotter pen move into a direction perpendicularly to the first carriage.
I have connected both mechanisms with a stripe of perforated metal and some epoxy.
The pen holder is also made of a perforated metal stripe glued at the upper laser head.
A servo is used to bend that metal stripe and so to lift or lower the pen.
The power is drawn from a lead acid battery with an output voltage of 6V which is stepped down to 5V by a linear voltage regulator.
The Raspberry Pi has a Wireless LAN stick and the plotter is controlled by a laptop through a secure shell session, nonetheless you can also connect a keyboard, mouse and display to control the plotter directly.
The mechanism is designed in such a way that the plotter draws on the plain it is deployed on - here it is a sheet of paper.
The area covered by the pen is approximately 35 times 35 millimeters.
250 steps are needed to move the pen for 35 millimeters.
Here you can see the plotter drawing a vector graphic.
The device is simple and cheap, thus it is suitable to demonstrate the functionality of a plotter.
The precision depends on the build quality of the mechanism and it is good enough to use the plotter as an educational tool.
It is practical to demonstrate the difference between a vector graphic and a bitmap:
Here you can see how a pixmap is created line by line.
In theory that graphic could be made of 250 times 250 pixels, thus 62500 pixels.
Considering the time needed to execute that huge amount of picture elements I am using a bitmap with just 55 times 55 pixels.
In doing so you can clearly notice the grid formed by the pixels.
With the special design of the plotter you can use a water resistant pen to draw on nearly any plain surface.
Here I am decorating the rear cover of my laptop.
You can download the plotter software and get the schematics at the project page.
In the next video I will demonstrate how to built a larger plotter using simple tools from a DIY store.
Thanks for watching and: "I'll be back!"
