Help At My Task: How to Get Started?

[Daybreaking]

Hello!
I have a very small task: I want to make a robot, more something like a device thing, that is attached to a screen (of my mobile phone let's say) that turns white or black at my will. As soon as it is white, the device does nothing. But as soon as it turns black, I want it to do something, for example rotate a servo motor. How can I achieve this? Is a light sensor suitable for this? Keep in mind that I know nothing about robots and so I would like some links or parts to be suggested for this in order to examine them.
Thanks a lot!

[jwatte]

In theory what you want to do sounds simple.
Yes, a light sensitive diode aimed straight at the screen might work. It would probably obscure the screen a little bit.
You can also use a small microcontroller, like an ATTiny85, to read the light sensor, and control a servo.
You can use a "micro" size "park flyer" servo to make it somewhat small, although it'll be bulkier than a phone still.
Then you need some way to power the thing. I think that will be the hardest part. A single LiFePO4 cell or LiPo cell could do it.

In practice, trying to tie a servo and some circuitry to a phone that you keep in your pocket or bag or whatever... unlikely to be robust :-)