When building Mojo, I was looking for the easiest to use and cheapest motor solution to drive Mojo's two wheels. I found contiuous rotation micro servo's that were basicaly microsevos without the postitioning. But they were a lot more expensive.

Looking at how a servo works it is not that hard to trick a regular servo into continuous rotation. You need nothing extra, just a sg90 servo, a screw driver and a knife to create the easiest motor to operate straight from a microcontroler like an arduino uno or nano for just a few euro's/dollars.

Supplies

a servo (sg90 in this case)

a small phillips screw driver

a knife

an Arduino Nano, Uno or alike

a way to connect your servo to the controller (in this case a Nano expansion board)

Step 1: Open Up Your Servo

Loosen the screws at the bottom and take of the top part

Step 2: Take Out the Gears

Now take out the gears, maybe mark them with a marker so you easilly know where to put them back in.

Step 3: Search for the Flattened Bit

When you look at the axle you'll find a flat bit at the end which sticks into the potmeter inside the servo.

This potmeter basicaly tells the servo which position it is in. So if you dissconnect the potmeter the servo will not know it reached its destination position and it will keep spinning.

Step 4: Connect Your Servo to the Board

Connect the servo to the board (pin 9 in this case)

Program it to point to 90°

Use either Arduino IDE, mBlock 3 or your editor of choice.

Now check that when you power the board, the servo turns to 90° (turn of the power, rotate the servo and watch what happens when you power the board.

Step 5: Seacht for the Middle

Now you will want to have your motor stop at a fixed and known point. You use 90° to have the motor stand still so you have 91° and up for forward and 89° and down for backward rotation. This means you have to put the potmeter in exactly the 90° position. You can do this by programming your servo at 90°, sticking in the axle and turning it until the motor stops spinning. Now carefully pull the axle out.

Step 6: Remove the Flat Part and the "stopper"

Now cut of the flat part of the axle by running your knife up the axle until you hit the end of the flat part. Be careful not to break the axle of the gear

Some servo's also have a mechanical stop on the bottom of the gear (middle photo). This wil stop the gear from making the full circle we need, so if present, cut that of too.