controling servo using phototransistor

Hello, I would like to control servo using phototransistor.

I am using schema from arduino in a nutshell by Jan Borchers.

And I have replaced FSR by phototranzistor.

I tried to use knob script from examples, but servo reacts wierd - just a little vibrating

I can read correct values from phototransistor 0 - 1023 depending on current light condition.

My task seems simple, I would like to control servo from 0 - 180. 0 shall be darkness 180 shall be full light.

any help?
thx in advance.

Maybe servo can't get enough current if it's powered like that from Arduino?