SG90 Animatronic Eye Mechanism

This is a bench-top animatronic eye mechanism. Two 30 mm printed eyeballs, 60 mm apart, look left/right (about ±25°) and up/down (about ±20°) together, driven by two SG90 micro servos. It is the linkage-only eye kit people keep asking for: no hand-bent wire and no ball links. Every joint is an M3 screw running in a printed clearance hole, so it is easy to build and easy to tune. Hook it up to an Arduino, a PCA9685 servo board or a Raspberry Pi and it is ready to drop into a mask, puppet head or robot face.

How it works: each eye pivots left/right in a U-shaped gimbal yoke (vertical M3 pivot) and up/down inside that yoke (two M3 side pins). Levers on the two yokes are joined by a tie bar, so the eyes turn as a pair (a parallelogram linkage). The pan servo sits in a pocket in the deck, and a pin on its horn rides in a slot in the tie bar (a scotch yoke). Because the slot takes up any difference in horn radius, the stock horn works whatever its hole spacing. For tilt, an M3x35 screw sticks out the back of each eye as a stalk and passes through a horizontal slot in a paddle. The paddle pivots between the tilt servo horn and an idler post, so when it rocks up and down both eyes tilt, while the stalks slide sideways freely as the eyes pan.

Assembly order:

1. Print the deck and fit the pan SG90 into its pocket (tabs on the deck top, shaft end toward the eyes), using M2x8 screws and nuts.
2. Screw an M3x35 stalk 12 mm into the back of each eyeball, then mount each eye between its yoke arms with two M3x12 screws. Leave a 1 mm gap on each side so the eye swings freely.
3. Stand the yokes on the deck grid holes at (±30, 0): M3x16 up from below, a washer, the yoke, then a nylock nut tightened until the yoke turns without wobble.
4. Screw the tie bar onto both yoke levers with M3x8 screws and washers. Check that the eyes turn together.
5. Centre the pan servo at 90°, fit a single-arm horn pointing at the eyes, and push an M2x16 pin (locked with an M2 nut) through a horn hole down into the tie-bar slot.
6. Bolt the tilt servo tower loosely at x≈58, insert the second SG90 from the paddle side and centre it at 90°. Screw its horn to the paddle's right arm (drill two 1.5 mm pilots through the horn holes).
7. Thread the stalks through the paddle slots, then fit the left arm to the idler post with M3x16, a washer and a nylock. Slide the idler post in its foot slot until the pivot is coaxial with the servo shaft, then tighten everything.

Tuning: limit pan to about ±35° of horn travel and tilt to about ±14° in software so nothing hits an end stop. SG90 tab pitch and shaft offset vary between brands, so the servo seats and tower/idler feet are slotted. If your pan horn sits lower than 12 mm above the deck, raise the servo with M3 washers under its tabs so the horn clears the tie bar. A drop of oil on each screw pivot keeps the motion smooth and quiet.