Sprite_TM’s Magic Paintbrush


When it comes to hackers we love, there’s no better example than Jeroen Domburg, a.k.a. Sprite_TM. Sprite’s now working for Espressif, makers of the fantastic ESP8266 and ESP32, where he created a miniature Game Boy and turned this PocketSprite into a real product. He’s installed Linux on a hard drive, and created a Matrix of virtualized Tamagotchis. In short, if you’re looking for someone who’s building the coolest, most technical thing of sometimes questionable utility, you need look no further than Sprite_tm.

Sprite was back at this year’s Superconference, and again he’s bringing out the big guns with awesome hardware hacks. This time, though, Sprite is tapping into his artistic side. Sprite is very accomplished in making PCB art and DaveCAD drawings, but actual art is something that’s been out of reach. No problem, because you can just buy an inkjet printer and make your own art. Sprite’s doing something different, and he’s turning his inkjet into a Magic Paintbrush.

The schematic for the first inkjet cartridge

This hack began by buying a simple HP inkjet printer from China for $28 USD. There’s some interesting hardware in this cheap printer — an ARM processor that might run Linux, there’s a big ‘ol serial Flash chip with all the code completely unencrypted, and there’s a serial port that spits out debugging info. On the printer carriage itself, there’s an encoder, but the real story here is in the ink cartridge itself. There’s a reason inkjet cartridges cost more than the printer itself. Inside each inkjet cartridge, there’s a reservoir of ink, and a series of tiny holes. Right beside those holes, there’s a small heating element. When current is applied to that heating element, the ink literally boils (yep, that’s where the trademark ‘Bubblejet’ came from) and squirts out the nozzle onto a piece of paper.

The first HP ink cartridge had twelve of these nozzles, which is easy enough to control directly from a computer or microcontroller. To print faster, though, you’re going to need more nozzles, and that means more heating elements. To do this, HP engineers simply lay out the nozzles in a grid, and control each nozzle with a MOSFET and a matrix that will heat up these nozzles individually. The HP cartridge in Sprite’s printer is much more complex, and after removing the metal mask on the printer cartridge and carefully inspecting the silicon underneath, he was presented with hundreds of nozzles, all controlled through just a handful of pins. That’s alright, because oscilloscopes exist, and this is something that can be eventually reverse engineered.

After a few tries, Sprite got his inkjet cartridge squirting ink everywhere, which meant only one thing: he had to turn it into a magic marker that could draw in any color. A quick bit of KiCAD later, and Sprite had a PCB that wrapped around the ink cartridge that printed nyan cats on anything.

Of course, being able to print nyan cats on everything from skin to lattes is cool, but with the simple addition of a color sensor, Sprite’s magic marker can become a marker that will write in any color. Yes, if you hold this magic marker up to a banana, it will write in yellow. Hold it up to an apple, and it’ll do green or red. If this sounds familiar, you’re right: this is the idea behind the perpetual Kickstarter scam, the Scribble pen, but this one works.

If you haven’t watched the video, please do. Sprite’s presentations are as humorous and enjoyable as they are informative and technical. And thanks again to Sprite for the great talk!



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