Always wanted a cow, but don't have the space? Build yourself this Roomba-sized cow!

Below a quick overview of the content.

Introduction and showcase video
Movement
Control
Sound
Style
Adventure

Supplies

Raspberry Pi 3 Model B

Seeed Grove - 2-Channel SPDT Relay

Battery Holder, AA x 8

Powerbank 5v 2.1A

Adafruit Mini External USB Stereo Speaker

Wheels (2)

DC Motor - 37mm - 12V - 50RPM (2)

Swivel Wheel

Cow

Fake grass

Wooden Platform

Hot glue gun (generic)

3D Printer (generic)

Paint

Multitool, Screwdriver

Scissors, Free Fall

Step 1: Introduction and Showcase Video

My girlfriend really wants a cow. Sadly, that's a bit tricky because we live in an apartment.
I did however find a small inflatable version in the local thrift store.
This means we can make her a robot cow for adventuring, The Mooomba!

Step 2: Movement

A cow can move around, so our Mooomba needs some wheels and power to match.
Two DC motors in the front and a small swivel wheel in de back will do perfectly!

To attach the motor we drill two holes on each side, and using the motor bracket, screw them both in place.

Here's a dirty bit, the wheels I'm using don't really fit on the motor shaft.
To fix this we're going to us some hot glue, I know....

We can attach the swivel wheel in exactly the same way.

With our motors, wheels and swivel in place we can test if it has enough oempf to move around our pint-sized cow.All we need to do is attach the motor wires to a 12v power source and see what happens.

Our miniature herbivore can move, woopwoop!
It's still manually controlled though, let's change that by mo(o)ving on to the next part.

Step 3: Control

To make our lives easier and give the Mooomba a sense of self, we're going to add some electronics.

The Raspberry Pi is powered by a 5v powerbank (black block on the left).This Pi controls a two channel relay (black/blue thing on the right). The relay on its turn will open/close the connections between the 12v battery pack and the two motors. This means;

Both relays open => no power => no movement
Both relays closed => both motors powered => forward
One relay closed => one motor powered => left or right

We want to make a high quality cow, so we 3D print some braces to keep everything in place. The files for these braces are attached to this article.

All that is left to do is write some code. You can find the full code in the section "Code", but here's a overview of how it works.

Generate a number between 0 and 3 and do the following;
0 => Forward
1 => Turn right
2 => Turn left
3 => Play audio (see next section)
This is repeated until eternity, or the batteries run out.

Step 4: Sound

Every cow can make sound, so ours will too!
The first thing we need is a volunteer to do their best cow impression on camera.

We take this audio and save it as mp3, adding a USB speaker makes sure everyone can hear Mooombas majestic calls.

Step 5: Style

Last but not least, some style.
To really bring out the best we add some fake grass to our moving platform, give Mooomba a good wash and add the all important cowbell. Doesn't she look fab?