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Description:
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 transistor.
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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