The use of switches is a basis for many types of daily activities of living for people with physical disabilities. A successful switch adaptation is a key determinant of Quality of Life. However, the adaptation requires that clinicians, rehabilitation engineers, and caregivers have broad knowledge and experience on, for example, positioning, motor skills, and hardware and software issues related to switches. In this paper, we present an adaptation approach based on a quantitative measure. In this approach, a three-axis acceleration sensor is attached to a body part where voluntary movement is observed. To transform the motions into corresponding switch operations, we have defined “rest,” “activation,” and “in action” states. The standard deviations of acceleration are used to classify these states with a predefined threshold. The effectiveness of our approach was examined through a case study with a child with cerebral palsy who could not use conventional switches well because of his unstable upper limb motions.