This is the same type of module that streetlights use to turn on at night and off in the day. This module is configurable to either turn on in the day or on at night.
The included relay allows the control of up to 15A at 125VAC. This is more than enough to control a light of just about any size or another load such as small pump for a pond.
The module measures 5 x 2.6CM. 4 mounting holes are included for easy installation.
Sensitivity to light is set with the included potentiometer.
A photodiode is used rather than the more typical photoresistor in this type of circuit. A photodiode provides for better directional reception of incoming light.