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Return to BlogLDR & LED Pair Smoke Detector and Alarm by LM1801
Posted Jan 12, 2012 at 9:48 am
Figure 1
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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4 years ago: i want the circuits of smoke detector using LED and LDR,and also the circuit of water sensor using resistive method
4 years ago: i want to the circuit of smoke detector
4 years ago: Hello
I need 24 vdc To 220vac Convertor Curcit diagram and All Detail
3 years ago: i can hardly find lm1801... is there any substitute for the latter?
3 years ago: My daughter is doing a science project and she needs to wire a smoke detector to a red light bulb to create a smoke detector for the hearing impaired. I did this when I was in 6th grade, so needless to say, I have forgotten how. Do you have any suggestions or a diagram of the inside of a standard KIDDE detector so I can tell her where to sodder the wires to connect the bulb/socket combo to it? Thanks.
3 years ago: why does this show a 220VAC one, and not a 24VDC one? why would anyone risk their life with tornado siren voltage?
2 years ago: I had an unforgettable experience with smoke detectors when the building security went up our floor to check the alarm knowing that I was reheating my bacon on the toaster and it produces a fog in the kitchen haha!
2 years ago: HY. .
JAKARTA IS BEAUTIFUL 0NE. . .
I L?VE U ALL
2 years ago: Thanks for the detector circuit ! OGWU AUGUSTINE HERTZ
2 years ago: I NEED A WIND DETECTOR CIRCUIT. OGWU AUGUSTINE FROM G. T. C ENUGU NIGERIA !!
2 years ago: What is the basis of the detector on when to alarm? Will it sound the alarm on a coffee smoke? Because some could be so sensitive that even the coffee smoke will cause the alarm...