The PT42-21B-TR8 is a Phototransistor manufactured by Everlight. It is an infrared (IR) phototransistor, which is a type of transistor that uses infrared radiation to determine current flow. It features a wider dynamic range of up to 5V and has a high Luminous sensitivity and a very low dark current. The device also features a wide viewing angle and a low noise factor, making it suitable for a range of applications. The PT42-21B-TR8 is ideal for use in switching circuits, surveillance systems, and detection of environmental changes.