Sorry about that, torben. I didn't meant to give my homework for others to do.
I will trye to ansuer those questions. That circuit is suposed to detect the presense of a person and then produce a sound in the speaker that is the alarm. Now i realise that the ldr detects the presence, but how? because much light or less light? other question, wich transistor acts like a switch that activates the circuit, is it TR1 or TR2? I think its TR2 but i have doubts. Now when the circuit is activated, IC1 wil receive a tension and generate the sound of the alarm, this is an IC M581A and i didn't found much information about this component. Then this sound is amplificated by TR5 and reproduced by the speaker. This is how i see this circuit, but i'm not sure. One last doubt, how does VR2 affect this circuit, what is his influence on it?
Once again, sorry about my english, hope this time i'm more specific!
Thank you