# smoke detector ?

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#### manz

##### New Member
hi there.

Does anyone have/know a simple circuit to detect smoke.What are the main part/detector name? or maybe it just can be done by just using a LED/IR LED and LDR that connected to NPN and a relay what so ever.

Note that, this is not really a critical project that is to really detect smoke in case of fire/emergency.No life will be harm if it fails to work : )

thanks.

#### Hero999

##### Banned
IR tends to be scattared by smoke less than red so shorter wavelengths are probably a better choice.

You can either have the LED directly opposite the sensor and monitor it for a decrease in light or position it so not much light from the light is shining on it from the LED then look for an increase intensity caused by the light being scattered by the smoke.

##### Banned
Have you thought about using an ionization detector? This is what most smoke detectors are supposed to use and the sensativity is superior to that of an LED setup. I unfortunatly have no idea where you'd get the sensors from, aside from perhaps a junk smoke detector.

#### Hero999

##### Banned
You could always, just use the entire smoke alarm PCB with the piezo transducer removed and the output driving your other circuit.

##### Banned
If you're lucky, the piezo transducer is one of the built in oscillator designs, so the signal would be straight DC going to the buzzer.

#### ikalogic

##### Member
The LED + LDR configuration, is not that bad, it is acutally used in many sensors..

you can also combine a heat sensor + smoke sensor, this how the sensors work in my university.. and BOY we allways mess around trigerring the alarm in the whole building...

#### manz

##### New Member
Hero999 said:
IR tends to be scattared by smoke less than red

Hm I didnt know that,thanks 4 the info.

Hero999 said:
You can either have the LED directly opposite the sensor

hm yup i can see the point but it would be nice if the sensors LED light is not visible to the naked eyes so thats why I tried to used the IR LED.

Have you thought about using an ionization detector?

Hm that a good idea but i just dont know where to get it though,do you think i need to buy a smoke detector to get it or can i just get the part only?

ikalogic said:
The LED + LDR configuration, is not that bad, it is acutally used in many sensors..

you can also combine a heat sensor + smoke sensor, this how the sensors work in my university

heat sensor - may not be suitable in my project though because i dont think theres an increase in heat in the sorrounding eg smoking.

I have tried the setup using the IR LED and LDR :

1.the smoke that pass through the setup jus increase 200 ohm at the LDR.
2. the set up isnt so stable because it really affected by the ambient light, even i had put the LDR in a cylindrical black housing (about 2 cm) made from paper.

so maybe i need to use an IR detector instead of LDR that is to reduce ambient light effect ?? do you have any idea ?

thanks to all.

Last edited:

#### ikalogic

##### Member
hello,
i've seen a cute idea on a magazine long time ago..
the idea was to put 2 sets of leds and LDR, (2 LEDS and 2 LDR)
one of the 2 sets, will be used to detect smoke, the other, will be encapsulated in a transparent matter, to detect what ever ambiant light variations..

now as far as i remember they just used an opamp with a coupt of resistrs to substract the light variations from the smoke detector...

this is the main idea.. you have to dig inside..
if you'r really interessted, i'll search in my 250 electronics magazines.. but it may take some time...

##### Banned
You could probably do that with a single LED and two LDR's using a tiny plastic prism. Keep in mind too, the distance between the LED and the LDR is going to effect your results pretty dramatically. The farther away you place it the more a light smoke is going to interfere with the light.

#### manz

##### New Member
Hmm i dont know what to say still.but thanks anyway.maybe the idea proposed by ikalogic have something to think of, but still waiting the search result from the 250 magazine anyone else have any idea ?

thanks again guys.

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