hello Aina,
Mostly LDR is used for making smoke detector sensors.
http://electroschematics.com/2545/smoke-detector-circuit/
http://www.sensorcircuitdiagram.com/2010/11/23/smoke-detector-circuit-based-on-the-tda2002-ic/
use the output dirctly from emitter of transistor transistor… you need not use the rest of the ckt.