MG90S Animatronic Desk Lamp Robot

This is a four-axis animatronic desk lamp with a weighted base, a yaw turntable, a shoulder, an elbow and a tilting head with seven LEDs. It's for hobbyists who want a lamp or creature neck with personality. You can drive it from an Arduino or a PCA9685 board to make it look at you, nod, shrug or follow a sensor. It's also built to fix the common complaint about servo-driven arms, the wobbly joint that hangs off the servo horn alone. Every pitch joint is a clevis. The servo horn drives one side of the child link, and an M4 bolt through a 624 bearing supports the other side. The servo only supplies the rotation, and the bearing takes the bending load.

All three pitch joints use the same design (I2–I5). The parent part ends in a clevis with 36 mm between the cheeks. The left cheek (side A) carries a bolt-on Servo Mount Plate, and the right cheek (side B) holds a press-fit 624 bearing. The child part ends in a 32 mm wide tongue. You screw the servo horn to the tongue's side-A face, and on side B an M4 bolt passes through the bearing into a captured nut. The four Servo Mount Plates are identical, and one of them also mounts the yaw servo inside the base. The published servo-horn geometry varies between suppliers, so two adjustments are built in. The plate's M3 slots slide ±3 mm so you can line up the spline with the bearing axis, and M3 washers placed under the plate set how deep the spline sits in the horn.

Assembly order:

1. Press 624 bearings into the side-B cheeks of the turntable, lower arm and upper arm. Melt M3 heat-set inserts into the two base posts and the three side-A cheeks. Fill the base's ballast ring with about 200 g of steel washers or BBs.
2. Mount each servo in a Servo Mount Plate: pass the body through the window with the spline toward the plate's short-offset end (I1) and fix the tabs with M2x10 screws and nuts.
3. Screw the yaw plate to the base posts with M3x10 screws. Screw a servo horn to the turntable's underside pad, drilling 1.5 mm pilots through the horn's own holes. Set the turntable on the base rim so the horn fits over the spline, then drive the servo's centre screw down through the 7 mm access hole.
4. Screw a horn to the side-A face of each child tongue (lower arm bottom, upper arm bottom, head). Put a tongue between its parent's cheeks and fit an M4x16 bolt with a washer through the 624 bearing and the tongue's boss into its M4 nut. Tighten until it's snug but the joint still turns freely.
5. From outside cheek A, push each servo plate in so the spline fits into the horn. Slide the plate along its slots until the joint turns freely, add M3 washers under it if the spline bottoms out, then tighten the two M3x10 screws.
6. Push the 5 mm LEDs into the head face. Run the LED and servo wires along the channel links and through the turntable's wire slots into the base, then connect them to your controller.

Tuning notes: centre each servo at 90° before fitting its horn. Keep the pitch joints within about ±80° so the tongues don't hit the parent webs. A little PTFE grease on the base's thrust rim makes the yaw axis quiet. Use metal-gear MG90S servos for the shoulder. SG90s are fine for the head and yaw.