The XR1891SN is a type of misc part that is used in electrical and electronic circuits. It is a light-sensitive resistor, which is used to measure the amount of light that is present in a given environment. It is composed of two terminals and is often used as a variable resistor in circuits. It can be used to sense light levels and detect changes in the ambient light level. This can be especially useful in applications such as security systems or motion detectors.