LDR & LED Pair Smoke Detector and Alarm by LM1801

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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Comments (3)

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rajeshvijay said:

i want the circuits of smoke detector using LED and LDR,and also the circuit of water sensor using resistive method

March 22, 2008

rajendra prtap singh said:

i want to the circuit of smoke detector

May 02, 2008

Muhammad Azhar said:

Hello I need 24 vdc To 220vac Convertor Curcit diagram and All Detail

May 16, 2008