This servo rotates fully forward or backward instead of moving to a position. You can use any servo code, hardware, or library to control these
Micro 360 Degree Continuous Rotation Servo
Continuous Rotation Servo
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FT90MR Micro 12.5g Digital 360° Continuous Rotation Servo FeeTech
360 Degree Continuous Rotation Servo, Standard Size
Feedback 360 Degree - High Speed Continuous Rotation Servo : ID
Hitec Continuous Rotation Servo - 3202_0 - Phidgets
Servo Motor (Continuous Rotation) + Arduino - ESE205 Wiki