A photocell is lighting control that works as an on off switch which can help improve security and automation for a carefree lighting system for home or commercial applications. Photocells use semiconductor photoconductivity to analyze the lighting expose it is receiving at the sensor to turn a switch on or off. 

Photocontrols can be used with landscape lighting transformers, bulk head fixtures, junction boxes and could be fixture mounted if there is a 1/2"knockout available for installation. 

This has a "120 Volt" Input Voltage.  
Measurements 4.25" Long x 1.25" Width x 1.25" Height.
Use the diagram on the photocell for wiring instructions.

Always make sure power is off when working with electrical parts and fixtures.