Here is a simple fire alarm circuit simple based on LDR and a pair of lamps for the detection of fire.The alarm works by detecting the smoke produced during fire.The circuit produces an audible alarm when the fire with smoke
When there is smoke in the light bulb will directly fall LDR.The LDR resistance will be low and therefore the voltage across it (below 0.6 V). The transistor is off and there is enough smoke happens.When to mask the light of the fall of LDR, LDR resistance increases and so do the voltage across the transistor will it.Now empowers ON.This IC1 and the power outputs 5V.This tone generator IC UM66 (IC2) to play music will be amplified by IC3 music.This (TDA 2002) to boost speaker.Resistor R6 is to protect the transistor when it becomes R4 towards the resistance values down. Resistance R2 and R1 form a feedback network for the TDA2002 and C1 couples the feedback signal from the union of R1 and R2 to the inverting input of the same IC.
The diode D1 and D2 in combination falls 1.4 V to give the rated voltage (3.5 V) at UM66. UM 66 can not support more than 4V.
