Smoke detectors are generally used in advanced alarm systems. Most of this professional devices use gas-detectors, ionization rooms or radioactive elements as sensors. In this circuit we are not using any of this complicated components.
LM1801 special-purpose IC ,which is designed especially for smoke detectors, provides us building this circuit by using the minimum number of components. It includes one internal zener supply, two reference voltage outputs, one voltage comparator, fixing diodes and one 500mA output transistor.
R13 LDR and LED must be placed such a way that smoke particles reduce the light drop on LDR and change its resistance. On the other hand R12 LDR and LED must be placed adjacent and no air circulation occurs. Best way to do it, place R12 and LED in a separate box. So smoke won’t effect R12 and its resistance will remain same.
When the comparator detects the voltage difference between two LDRs it triggers Th1 transistor and as a result, alarm starts.
P1 is the calibration potentiometer. Hp1 is an alarm sound generator. Circuit includes one extra alarm output to drive external alarm system.
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