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Suggestions for a cheap, simple robot unicorn build Answered

My 5yo daughter has figured out that her mad-Scientist daddy can make crazy things come true.  For her birthday, she got a real unicorn and a princess crown (see picture attached).  Now for Christmas, she has her heart set on a ROBOT unicorn, and I think I'm going to try to tackle this, but I need to keep the scope tight so that when Christmas arrives, I'll have a deliverable for her.

(My instinct always leap to grandiose ideas like actual quadrupedalism that would balloon the time/effort/money inputs into the stratosphere.   My biggest obstacle to make this work for Christmas is to stick with K.I.S.S. principles.  I can do basic welding, woodworking, fabricating, stitching, etc.  I'm reasonably good at electronics, and am a professional programmer with a degree in Aerospace Engineering.)

Here is my rough  build plan:  (Completely open to improvement suggestions)
1)  Build frame out of 1/2" square steel tubing and weld together.  Leave large torso cavity for electronics and batteries.  Feet will have wheels concealed by a hoof shroud.  Feet will be upgradable (see tech features below)
2)  Use blue foam to rough out body shape. 
3)  Cover with about 1/2" of soft foam covered with felt.  Use yarn for mane and tail.  Cover should be removable and washable.

I'm trying to figure out tiers of work so that as long as I get the basic frame and covering down, I'll be able to increase/decrease the scope to ensure I'll have a deliverable for Christmas.  Here are the tiered robotic features I am envisioning:
Tier 1: Make the saddle bounce at about .5Hz with an amplitude of roughly 1/2".  Possibly make head/neck bob up/down in sync.
Tier 2:  Add horse sound effects
Tier 3:  Build sensors for the reigns and stirrups.  Monitor if stirrups get pushed inward in a basic kick the horse motion.  Monitor if one reign or both are being pulled.  Make sounds effects based on inputs.  (E.G.  Clop, clop of hooves begins when stirrups are pushed in, and stop when reigns are pulled.)  NOTE:  Since they have access to real horses, keeping the controls reasonably close to riding a real horse will make this double as a trainer/simulator for her and her friends.
Tier 4:  Upgrade wheels to powered.  Move based on reign and stirrup inputs.   Max speed will be very slow (40fps or so), so that it can be used inside safely.  Movement is mostly just to augment her imagination, not to move like a real horse.
Tier 5:  (Not much chance I'll get this far prior to Christmas)  Upgrade software, sound effects, etc.

Current back of napkin ideas:
Frame will probably have 3 pieces: legs/torso, neck/head, Saddle/back 
Saddle/back removable for electronics access.
Neck/head will be joined to torso in 3 spots:  the top point will be anchored with a chain link between two eye-bolts, the other two points would be on the sides of the bottom, with compression springs pushing the neck and torso away from each other, limited by a mechanical linkage.  This should give reasonable motion ability, and allow the two base points to become the sensors for the reigns being pulled.
I'm thinking a small 12V deep cycle battery, and 12v salvage DC motors powering the wheels.  Depending on the motor torque, I'll either power the back two feet and keep the front feet as swivel casters, or power all four and turn tank-style.
The budget needs to stay reasonable, say $100ish.  This is a kids toy, and I've got lots of other mad-scientist projects to pump money towards.

Outstanding questions:
Motor selection, cheapest with high enough torque
Good, cheap way to make seat and possibly head oscillate.  Mechanism needs to not break even if a 200lbs rider sits on it.  (Doesn't need to work when overloaded, just can't be permanently damaged when overloaded.)
Locate inexpensive 12v charger that won't overcharge if left connected, but will charge a 3ah battery reasonably quickly.  (Or schematics to build one)  Essentially, looking for a best bang-for-buck charger.
Least expensive way to play audio clips?  I've got plenty of horse-sound clips located, but I need a way to play them economically.
Not sure how much force will be needed for movement.  I'm assuming a 70lbs rider/robot max and that it will primarily operate on flat surfaces, potentially on carpet.  Can't burn out if overloaded. 

Suggestions on simplifying this?  Suggestions on simple improvements?  Ways to keep the costs down?  Sources for economical parts?


Is there a size issue here?

I saw an advert for a toy walking horse (possibly a Barbie accessory?) - it would be a simple matter to add a unicorn horn, and maybe change the mane & tail for something glittery and magical.

Failing that, if you are after something larger, then maybe such a toy is work dissecting to examine the mechanisms.

I'm pretty sure that the toy you are referring to was my inspiration that I could pull this off in the time and budget I've got available between now and Christmas. They are pretty basic in functionality, essentially just a moving saddle and horse sounds, no controls, no locomotion. And, like many similar things, they cost about $200 more than made any sense.

But, it was just about the right size, and gave me hope that I could entertain a 5yo with just basic functionality. (Like I said, my initial ideas almost always swing grandiose.)

Wrong toy. What I saw was a simpler version of this: [Amazon link], priced at around $300, IIRC.

Her present needs to be plenty big enough for her to ride without her feet touching the ground.  (For reference her birthday unicorn in the above picture was about 36" high where she sat.)

(But the toy I linked to has some cool features I can use as ideas for future improvements.)