Here's a fire alarm circuit using LDR as light detector. It warns the user against fire accidents. It relies on the smoke that is produced in the event of a fire. When this smoke passes between a LED and an LDR, the amount of light falling on the LDR decreases. This causes the resist of LDR to growing and the voltage at the base of the transistor is pulled high due to which the supply to NE555 then activated the alarm.
