NTC thermistor has a negative temperature coefficient that resistance decreases with increasing temperature.
NTC thermistor are made from semiconducting materials that usually consists of a sintered ceramic fabricated in a variety of shapes and sizes from a mixture of oxides chosen from the elements Mn, Ni, Co, Cu and Fe. By varying the composition and the size of the semiconducting elements, resistance values between 100 and 106ohms at room temperature can be achieved.
The fields of NTC thermistor applications are temperature sensing either for the purpose of temperature measurement, temperature control, thermal alarms or for the compensation of changes in circuit resistance brought about by variations in the ambient temperature.

CDS PHOTOCONDUCTIVE CELLS

CdS photoconductive cells are light-sensitive resistors in which resistance decreases with an increase in light intensity when illuminated. These devices consist of a thin single-crystal or polycrystalline film of compound semiconductor substances. Most commercially available photoconductive cells are manufactured from cadmium sulfide (CdS), which is sensitive to light in the general visible spectrum as the human eye, and this, together with their inexpensive, small size and ease of use, allows wide possibilities of application: street light turn on and off automatically by level of daylight, in security devices such as motion sensing lights and cameras, and in alarm systems.photometry etc.