Mobile sensors are useful in many environments because they can move to increase the sensing coverage. In this paper, we present a mobile sensor prototype in which the Mica2 sensor node is used to control the movement of the robot built with commercial off-the-shelf (COTS) components. We use a sensor relocation application to demonstrate the feasibility of our design. In the sensor relocation application, after a sensor node failure creates a coverage hole, a mobile sensor node is relocated to cover the hole in a timely and energy-efficient way. We present a distributed sensor relocation algorithm and provide novel solutions to implement this algorithm in our mobile sensor platform. Experimental results show that our relocation algorithm can reduce the sensor relocation time and balance the energy consumption of the mobile nodes.