Kirby LED Keychain

Overview

As another personal gift project for my girlfriend, I designed a Kirby-themed LED keychain that lights up with a sequential chase animation at the press of a button. The twist: the entire circuit runs on pure analog and discrete logic with no microcontroller, no firmware, and no programming.

I wanted to prove that a charming, interactive gadget can be built from first principles using nothing but a 555 timer, decade counters, and a handful of passives, all powered by a single CR2032 coin cell.

Circuit Design

The clock source is an LMC555 CMOS timer running in astable mode at roughly 10 Hz. Its output feeds a CD4017 decade counter that sequences five LEDs one at a time. A second CD4017 counts full animation loops; after nine cycles the circuit latches off through a PNP transistor that cuts power to the oscillator.

A momentary pushbutton resets the loop counter and re-enables the 555. Press once and the LEDs chase for about five seconds, then the circuit shuts itself down to conserve the coin cell. Fast-switching 1N4148 diodes form a simple OR gate that feeds the reset logic.

The entire design runs at 3.3 V directly from the CR2032 with no regulator. Because the CMOS ICs draw microamps in quiescent state and the auto-shutoff kills the oscillator after each animation, estimated battery life is on the order of years of typical use.

PCB Art

The board itself is shaped as Kirby and doubles as the visual centerpiece. I used four PCB fabrication layers as artistic media:

• Solder mask over copper: the main body is a copper fill covered by pink solder mask, giving Kirby his signature color with a smooth, uniform finish.
• Exposed HASL: the star on Kirby’s back is drawn as copper with no solder mask, leaving shiny exposed tin that catches light.
• Silkscreen: Kirby’s face (eyes, mouth, blush) is printed in white silkscreen ink on the front mask layer.
• Solder mask over bare substrate: the feet use colored mask with no copper underneath, creating a matte contrast against the body.

All components live on the back side, keeping the front as a clean canvas for the artwork.

Fabrication

I ordered fully assembled boards from JLCPCB. The SMD bill of materials (0805 passives, SOT-23 transistor, SOIC timer and counters, SOD-323 diodes) was chosen entirely from JLCPCB’s parts library to keep assembly turnkey. The only through-hole component is the 12 mm tactile switch on the back, hand-soldered after boards arrived.

Zero-ohm resistors scattered across the back serve as PCB jumpers to avoid routing traces on the front of the board.

Results

The finished keychain is a compact, self-contained gadget that demonstrates analog design can still deliver delightful interactions. With no firmware to write or debug, the entire two-week timeline went into schematic capture, PCB layout, and art, proving that constraints breed creativity. The auto-shutoff guarantees the CR2032 lasts for years of daily button presses, making it a truly maintenance-free gift.