I made a little car which changes direction whenever it hits a wall, using only one motor.
There a seven gears involved: 4 (or 3) red gears, 2 yellow gears, and 1 blue gear.
The car is 1 grey rod long, 1 red rod wide, and 1 red rod high (due to the motor).
The design uses the principle that turning a gear left has different meanings depending whether you're looking at the top or bottom of the gear.
Step 1: Bottom Frame and Wheels
This is the bottom framework.
There are 2 axles connecting this frame which hold the wheels and the gears that turn in the same direction of the car. Having 2 gears is recommended, and a rubber band must be put on any wheels connected to a gear.
NB this framework isn't entirely symmetrical.
Step 2: Main Axle
Here is where the magic happens.
At no time can both red gears mesh with the yellow gear. To achieve this, align a blue spacer next to the orange connector like in the fourth picture. You will have to readjust this when adding it onto the car.
Step 3: Top Frame
Build these for holding the motor... and aesthetics.
Step 4: Putting It All Together
First put in the Main Axle in between the bottom frame. The white rod (holding the yellow gear) slots into the yellow connector. It doesn't matter which way the other yellow gear points.
The top frame then is added. Make sure the yellow connectors from the main axle and top frame are on the same side.
Step 5: Motor and Done!
The motor holds a blue gear which meshes with the side Yellow Gear. Attach the green rods to the dark grey connectors and tighten this with the tan clips.