well project 4 uses photo resistors not transistors fed to analogue inputs lets say they are 10K in the dark using a 10K resistor to ground acts as a voltage divider and puts the analogue pin at 2.5 volts. As the light changes so does its resistance and thus the voltage on the analogue pin. The different voltage levels is what the ADC uses to tell us how bright the light is. The photoresistors could be replaced with 10K thermistors which would cause our input voltage to change with temperature instead of light...research voltage dividers. Analogue input is 0 to 5 volts.