i would like to know why it is needed to put a 10k resistor in series with the phototransistor.
As far as i could find out a transistor is something like an amplifier which can open the circuit with a very small current from the basis to let more current flow through the transistor emitter. So a small current flow from the base can amplify the current which flows from the power supply through the emitter with a gain factor proportional to the base current.
So far so good i think... but why the 10k resistor?
Thanks in advance
For questions like this, which aren't directly related to the starter kit, you'd probably be better off asking in the General Electronics section. But bear in mind that not many people have the starter kit and won't be familiar with Project 4, so add a drawing of the part of the circuit you are referring to.
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.