The Credit Card-Sized Drawing Robot

Drawing robots are usually pretty large, so you might see one sitting on a desk, but fitting one in a backpack or pocket isn’t very realistic. This is because these machines generally work on principles similar to those of 3D printers. The motors and sliding arms they rely on take up a lot of space. But is that the only way to build a drawing robot? Of course not! It is perhaps the most straightforward approach, but there is always another way.

Hardware hacker Dorian Todd recently came up with a very creative design for a drawing robot that is unlike anything else out there. It is much more complicated than other robots, so it may not be right for every use case, but when size matters, Todd’s PenBot Mini is hard to beat. The unique design gives this robot a footprint roughly the size of a credit card — if you don’t count the drawing arms.

Instead of relying on the traditional Cartesian motion systems found in CNC machines and 3D printers, the PenBot Mini uses a miniature five-bar linkage mechanism. Two tiny servo motors mounted at the front of the robot control interconnected linkage arms that guide the pen tip across the page. The unusual arrangement dramatically reduces the robot’s size, though it also introduces a great deal of mathematical complexity.

To make the design as small as possible, Todd abandoned the bulky hardware used in his earlier PenBot prototype. The original machine relied on a Raspberry Pi and much larger mechanical assemblies, but the PenBot Mini instead uses a tiny Seeed Studio XIAO ESP32C6 development board as its controller. The onboard microcontroller has enough processing power to handle inverse kinematics calculations, manage drawing buffers, and communicate with a host computer in real time over USB-C.

With so many pivot points packed into such a small area, even tiny amounts of mechanical play can throw off the pen’s position. To address this, Todd integrated miniature bearings directly into the linkage joints to eliminate wobble and improve precision. The chassis and arm assemblies were modeled in CAD using accurate manufacturer component data to ensure every part fit perfectly within the credit-card-sized footprint.

The PenBot Mini’s body and structural components were produced with a 3D printer using engineering-grade filaments designed to reduce flex. Multi-color printing techniques were also used to add visual accents directly into the finished parts. A magnetic snap-on lid protects the device during transport.

The ESP32 firmware uses an iterative solver to repeatedly refine the arm positions required to place the pen accurately, compensating for the offset between the final pivot point and the actual pen tip. Meanwhile, a custom Python desktop application allows users to sketch freehand drawings or generate text using single-stroke fonts. The software also performs collision checking to prevent the linkage arms from entering impossible positions.

Although the robot is already capable of producing fairly detailed drawings, Todd says future revisions may include geared servo reductions and revised linkage geometry to improve accuracy even further without increasing the size of the device.