Free teaser · lead magnet

One finger that curls

Build one laser-cut birch finger that curls when a servo pulls a Spectra tendon—learn the joint, the tendon, and the PWM pulse that drives it. This teaser is meant to work on its own. The full five-digit pack is optional when you’re ready.

Ages 10+ with adult help · No soldering · About $12–25 in parts for one digit if you already have an Arduino and can cut a few plywood scraps.

Mini parts list (1 digit)

  • 3 × 3mm birch finger segments (~25, 22, 20 mm × ~13 mm) — laser-cut or carefully sawn
  • 2–3 × M2 screws + locknuts (revolute hinges)
  • 1 × SG90 micro servo
  • ~40 cm braided fishing line / Spectra
  • 1 × Arduino Uno (or any board with Servo.h)
  • 3 × Dupont jumpers · scrap plywood “forearm” block · drivers · optional return elastic

Mini build

  1. Cut three segments; drill/laser hinge holes; assemble the chain with M2 screw + locknut—snug but free to pivot.
  2. Knot Spectra at the tip (palm side); guide along the inside; leave a tail.
  3. Mount the servo on a scrap forearm block; tie string to the horn.
  4. Wire: signal → D5, V+ → 5V (USB OK for one servo), GND → GND.
  5. Upload the teaser sketch; tweak OPEN / CURL until it fists without buzzing.

Teaser sketch

#include <Servo.h>
Servo finger;
const int OPEN = 30;
const int CURL = 140;

void setup() {
  finger.attach(5);
  finger.write(OPEN);
  delay(500);
}

void loop() {
  for (int a = OPEN; a <= CURL; a++) { finger.write(a); delay(15); }
  delay(500);
  for (int a = CURL; a >= OPEN; a--) { finger.write(a); delay(15); }
  delay(500);
}

One “why”

Pulling the tendon shortens the path on the inside of each hinge. The plywood segment rotates around the screw (a fulcrum). That’s a lever chain—the same family of ideas as the tendons in your hand and in stage animatronics. The servo isn’t “sending curl”; it’s winding line by turning to an angle set with PWM pulses (~1500 µs ≈ 90°).

When you’re ready

Full Animatronic Hand pack — $29

You’ve proven one digit. The full pack is the complete apprenticeship—finish with a machine you keep:

  • Laser-cut 3mm birch palm + five digits + forearm tray (SVG/DXF)
  • M2/M3 joints, Spectra tendons, 5× SG90 independent actuation
  • Structure → joints → tendons → wiring → PWM → calibration
  • Wave demo + first-principles lessons + honest BOM (~$45–90 parts)
Buy full pack $29 Product page Build Club waitlist

Honesty rule: this teaser must work standalone. Frustration is not a funnel. Free content summary adapted from the pack’s TEASER.md.