A sensor is a detector and a reporter of the presence (containership) and change in state of the stimulus it is designed to sense (detect). For example, the human eyes detect the presence of light and whether or not the light is bright. On sensing, the sensor relays its information to a processing system for analysis and appropriate response.
All living organisms contain sensors (primarily specialized cells). Some of these biological sensors are designed to sense physical aspects (e.g. light,sound, odor, taste, touch) of the external environment of the organisms containing them while others are designed to sense the internal environment (e.g. metabolic indicators such as glucose level, oxygen level and signal molecules such as hormones, neurotransmitters and cytokines) of the organisms. As a result of their knowledge of biological sensors, humans have created various mechanical and electronic sensors that can sense various physical aspects of the environment (e.g. light, sound, motion, temperature, pressure, volume, vibration, magnetic fields, electrical fields, etc).
The stimuli sensed, the sensors, the sensings (they are behaviors), the transfer of the information to the processing systems, the processing systems, the analyses of the information and the responses, are all expressions of Pj Problems. Consequently, sensors sensings are expressions of Pj Problems.