A gravity-based angle sensor apparatus includes an outer housing (which may be incorporated into a carpenter's level or other reference frame) that contacts or is fastened to a workpiece, and a sensor assembly rotatably mounted within the outer housing. The sensor assembly includes a frame having internal cavities for supporting a LED light module and a sensor responsive to the orientation of the sensor assembly. The sensor electrically bridges two terminals of the LED light module to activate an LED indicator lamp when the sensor assembly has a reference horizontal orientation.