A battery powered LED light source is described for illuminating the inside of a dark enclosure when the enclosure is opened in an environment with ambient illumination. Drive circuitry adapted to the LED light source turns on the LEDs when levels of ambient light above a threshold are detected, which may occur if the light source is placed in a dark enclosure that is opened in a lit room. After being opened, the drive circuitry periodically strobes off the LED light source to take ambient light measurements without the contribution of its own LED light for determining if the enclosure has been closed and the LED light source should again be shut off.