A photodiode is a semiconductor diode made as an element of a p-n or p-i-n junction in connection with an intrinsic layer. It can be used as a detector, i.e. a detection / recording device. A phototransistor is a transistor that is activated by light. It consists of three semiconductor layers that differ in type of conductivity. A phototransistor has many times higher sensitivity than a photodiode. They are mainly used in automation, measuring systems, optoelectronic links and remote control systems. A photoresistor is a semiconductor element whose resistance depends on the intensity of light. It has two leads and a photosensitive surface with comb electrodes. They are used, among others in various devices (e.g. alarm clock, night lamp), fire protection systems, alarm systems or when detecting pollution of rivers and water reservoirs.