Starting project by learning more about eggs..

I'd like to figure out how to create a nice 3D printed egg shape that can hold a circuit board, battery, and motor assembly. I'll start by printing off some free variants from thingiverse. Ultimately I want the egg to be quite large and at least 6 inches tall.

I also need to understand how to use a motor to cause the egg to spin and wobble. The first thought is a motor with off-center weight much like how cell phones vibrate. However this may not create an actual wobble motion. It would be useful if I could shift the center of gravity of the egg using the motor.

Features I'd like:

1. Ability to charge with a port like USB

2. Ability to re-program the motion. This will drive need for microcontroller

3. Ability to play with egg for atleast 1 hour

4. Ability to take the egg apart by unscrewing the halves

5. Total cost under $50 (circuit board, battery, motor, egg vessel)

Initial components considered:

1. ATMega328P - simple 8 bit microcontroller that could handle PWM outputs. Low power at 2.4mA@3V

2. DRV8833 - simple stepper motor controller

3. NEMA-17 (or similar) small stepper. Power draw is roughly 5W / phase (600mA max current, 7 ohm per winding) The stepper's holding torque will be useful if the egg stops moving and stays in one position. Some more calculation needed to understand if there is enough torque.

Next Steps:

1. 3D print a representative egg. Once I have a decent shape I can tape weights to it to understand how moving weight around the egg can change the center of gravity to cause it to wobble. I can use this to back out torque requirements for motor selection.

2. Pick components and breadboard stepper driver circuit

3. Design circuit board and refine egg CAD.

4. Put it all together!