Wilson and Atkeson  discussed several constraints and issues of sensors used in activity recognition system. The two main factors are sensor cost and sensor acceptance. Cameras and microphone are not suitable because inhabitant would feel uncomfortable living in the house. RF tags, badge, beacon or marker type of sensors including any wearable sensors potentially inefficient because inhabitant forget, change clothes, or are not sufficiently clothed when at home. In particular, elderly people are very sensitive to small changes in environment. In conclusion, the sensors should be ‘invisible’ and do not perceived as invasive, private and reveal sensitive information, inexpensive and easy to install. In addition, sensors should be low-maintenance and low-power and able to run as long as possible without external power.
 D. H. Wilson and C. Atkeson, “Simultaneous Tracking and Activity Recognition (STAR) Using Many Anonymous, Binary Sensors,” in Pervasive Computing, H.-W. Gellersen, R. Want, and A. Schmidt, Eds. Springer Berlin Heidelberg, 2005, pp. 62–79.