This Engineer Built a Real-Life Undo Button for Pen Drawings

We've all been there. You're halfway through a drawing, your hand slips, and that perfect line becomes a catastrophic squiggle. In the digital world, you hit Ctrl+Z and move on with your life. In the real world, you're stuck with it.

YouTuber It's Triggy decided that was unacceptable, so he built a machine that can literally undo pen strokes. Not cover them up—actually make the ink disappear like it was never there.

The result is one of those projects that looks deceptively simple until you realize it's solving like six different engineering problems simultaneously. And the wildest part? It actually works.

The Problem With Being Precise

Triggy's first instinct was to borrow from existing tech. CNC mills and 3D printers use cartesian robots—those two-axis systems where motors pull themselves along timing belts to reach any point on a plane. Super precise, great for programmed paths.

But here's the issue: "When the motors are active, even if they're not moving, they're locked," Triggy explains in the video. You can't draw naturally with something that fights every curve and wants to move in straight lines. His attempt at drawing basic shapes with this setup was, in his words, "completely terrible."

So he kept the precise stepper motors but ditched the cartesian approach entirely. What he needed was a mechanism that could extend, rotate, and move fluidly while a human guides it—then retrace that exact path on command.

When Simple Mechanisms Get Complicated

The solution came from combining several classic mechanisms in non-obvious ways. The arm uses a scissor linkage—you've seen these on car jacks and scissor lifts—which converts rotational motion into linear extension. One motor controls how far the arm extends, another controls rotation.

But here's where it gets interesting: the rotation motor doesn't directly rotate the arm. Instead, it uses a gear segment to rotate the entire second motor assembly, which then controls the arm extension. This lets one motor's output become another motor's base position, giving the device two degrees of freedom from a relatively compact footprint.

To know where the pen actually is, Triggy installed potentiometers—electrical components that change resistance when rotated—on each motor shaft. The device measures their resistance 30 times per second, creating a detailed map of every stroke you draw.

Except the first version was a disaster. The gear train had so much backlash (slop between gears) that the scissor linkage would move significantly before the measurement gear even budged. The device thought the pen was in one place when it was actually somewhere else entirely.

The fix was elegant: stop measuring the motor inputs and measure the outputs directly instead. "Even if there's slop, it doesn't matter since the motors will continue moving until the output is correct," Triggy notes. This is the kind of problem-solving that separates projects that almost work from projects that actually work.

The Chemistry Part Gets Weird

Okay, so you've built a robotic arm that can perfectly retrace any path you draw. How do you actually erase ink?

Triggy needed the opposite of invisible ink—something that writes normally but becomes invisible later. Enter FriXion pens, invented in Tokyo in 2006. These use special ink that turns clear when heated, which is why the rubber eraser on the pen works: friction generates heat.

But having a robot physically rub the paper wasn't interesting or practical. Triggy needed to generate heat directly. His solution was to design a custom PCB with a copper trace so long and thin that it acts as a resistor. When voltage runs through it, it heats up instantly.

"Not having enough heat is a hilarious problem to have in electrical engineering," he points out, "since it usually feels like 90% of electronics is trying not to accidentally melt components."

The heater pattern is packed into a crazy maze that fits on the pen tip. It heats immediately when powered, cools just as fast when turned off. Install it on the pen mechanism controlled by a servo motor (for extending and retracting the tip), and you've got everything you need.

The Moment of Truth

When Triggy finally tested the complete system, his reaction says it all: "Like it was never even there. Just completely undone."

The device works by recording your pen movements as you draw. When you hit undo, it retracts the ink tip, activates the heater, and retraces your last stroke in reverse. The heated tip makes contact with the thermochromic ink, triggering the chemical reaction that makes it transparent. Clean page, no evidence of the mistake.

What makes this project fascinating isn't just that it works—it's how many different domains of knowledge had to come together. You've got mechanical engineering (scissor linkages, gear trains), electrical engineering (motor control, heating elements), software (motion tracking, path recording), materials science (thermochromic ink chemistry), and manufacturing (3D printing, PCB design).

Each piece is relatively simple. The scissor mechanism is old tech. Stepper motors and potentiometers are basic components. FriXion pens you can buy at any office supply store. But combining them into something that actually does what it's supposed to do? That's where the real engineering happens.

The device isn't going to replace Photoshop or anything. But it's a physical manifestation of something we take completely for granted in digital spaces: the ability to experiment without permanent consequences. In a world where everything is becoming software, there's something genuinely cool about bringing that capability back into physical space—even if it's just for pen drawings.

— Tyler Nakamura, Consumer Tech & Gadgets Correspondent