When it comes to circuits, most people are only familiar with either printed circuit board (PCBs) that use copper traces laid on top of a substrate, or a series of wires that connect components together. However, in more recent times, there has been an explosion in the number of creative ways that electronics can be integrated onto anything from fabrics via conductive threads and even directly drawn on paper with conductive inks.
But this approach comes with a few drawbacks. Namely, it can be extremely difficult to draw component footprints in the exact dimensions required and the traces can be quite messy since humans are unable to maintain straight lines over any meaningful amount of time/distance. This problem is what drove a team from Saarland University, located in Saarbrücken, Germany to create a small device, called Print-A-Sketch, that can apply conductive ink in precise patterns with minimal human intervention.
As a basis for the Print-A-Sketch, the team selected an array of parts that would work together to achieve the goal. A combination of an Arduino Mega and Protoshield were chosen due to the large number of pins and ample amounts of RAM and flash memory for holding the complex programs required. The printer is able to determine where it is located relative to a previously known position by using an optical motion sensor at the front and a wide-angle RGB camera in the middle. A small OLED display was added to the top along with a joystick pushbutton assembly on the side for user interactions. Finally, a wide piezoelectric printhead was attached to the back and connected to the conductive ink reservoir for actually depositing the ink onto a surface.
Just like with household inkjet printers, the handheld device also needed to be accurate while ensuring consistent coverage of ink, even when the user might make a mistake. To combat uneven speeds of the printer's movement, the optical distance sensor is constantly read in order to retrieve its current speed relative to what's underneath. Then depending on if it is speeding up or slowing down, the rate at which ink droplets are deposited onto the surface below is changed proportionally using a modulated firing frequency.
Now that the team had figured out how to combat changes in longitudinal movement, they then had to tackle the challenge of ensuring a straight line can be laid down even if a lateral adjustment had occurred. Based on the virtual "brush" thickness, a certain number of contiguous dots housed within the piezoelectric printed are activated to give a precise width. Therefore, by adjusting the offset relative to outside movements, consistently straight lines can be drawn.
The team also added a mode that allows for accurate 90-degree angles to be placed upon the press of a button, further increasing the printer's functionality.
Only having the ability to print lines would be quite boring and limit the number of creative uses for the device. So in addition to straight lines, users can add custom brushes/line styles, include bitmap stamps that can lay down special shapes, and copy/paste functionality. That last item is perhaps the most interesting, as components or previously drawn traces can be captured with the camera underneath and then reprinted somewhere else on the page.
The Print-A-Sketch is highly capable and comes packed with incredibly useful features for creating flexible circuits in a wide variety of places. In their paper, the team proposes that their solution can be used to build interactive objects such as yoga mats with media controls, capacitive lamps, and even flexible sensors for fitness tracking. You can see the handheld printer in action below!