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The light sensor measures how much light is shining on it. It has 2 modes. In LIGHT mode, as the light shining on the sensor gets brighter, more signal passes through it (making lights brighter or motors turn faster). In DARK mode, the signal increases as it gets darker.
Use a purple littleBits screwdriver to turn the dial and adjust how much light it takes to change the signal. Clockwise increases sensitivity, counterclockwise decreases it. Use a bargraph to see how it's working!