Place your sensor in the light or noise environment that it will experience while the program is running. Then use the “view” option on the NXT display screen to see exactly what value your sensor is reading. To use the “view” option on the NXT, choose View from the home screen, select either 'light sensor' or 'sound dB', and then select the proper port. The NXT window will show the current reading for the selected sensor. This should help you determine the threshold values needed to run your program.