This encoders kit are designed for TT motors.  It provides the rotation degree of the wheels. Encoder uses non-contact method to convert the angular displacement signals. Best fit with Micro DC Geared Motor.It includes encoders, plastic tube, paper-mediated gasket etc. 
It uses Gravity interace which is a plug&play design to get connected with Arduino IO Expansion Shield or Raspberry IO Expansion Shield. 

SPECIFICATION

• Voltage:+5V
• Current:<20mA
• Resolution:20 PPR
• Weight:20g

DOCUMENTS

• Arduino Sample code
• Wiki (Wheel Encoders for DFRobot 3PA and 4WD Rovers)

SHIPPING LIST

• M3*5 screws     x2
• M3*50 screws    x4
• Rubber ring     x4
• Washer          x4
• Plastic cover   x4
• Grating disk    x2
• Digital cable   x2