LDR & LED Pair Smoke Detector and Alarm by LM1801

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. Instead of these, we are using two LDRs and one LED.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

Circuit is connected to the mains directly. D1 rectifies the
alternating source and R7 reduces it to the level that the circuit can
operate with. C2 capacitor stabilizes the voltage and zener inside the
IC regulates it. Two identical LDRs are connected in bridge type so this connection prevents the circuit from temperature changes and ageing effects.

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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