Smoke Detector

Make? Why (rhetorical questions)? They can be had in most home improvement outlets for less than £5 Sterling already tested and ready to go.

How much sterling can you get for £5 Sterling these days anyway? Surely it's not 5 pounds.

That's bellow the belt bhankiii:!

Is this is a homework project?

The simplest use infra red devices and a small blacked out tube. the comercial types use a special capsule containing a radio active isotope. The rest is a cicuit to detect the electrical changes brought about by the chemicals within the smoke. This will point you in the right direction but you will have to look up the rest. Ionising, Schmit trigger, bistable, look these up. All the best.

you can get close with the schmitt trigger and such that was mentioned, but actual smoke in real smoke detectors on the market today use small amounts of radio-active particulates to differenciate the smoke particles from the normal air. this is not something you want to mess with.

Actually, ionisation smoke detectors are on their way out due to the increasing difficulties in exporting radio-active materials around the world.

Almost all smoke detectors uses either an LED or laser diode and a photo-sensor. LED based detectors are normally used for most home based smoke detectors and "normal" sensitivity commercial point and aspirated detectors - "normal being in the order of magnitude of being able to detect >0.1% obscuration per meter. Laser based systems are normally used for medium and high sensitivity detectors (up to around 0.005% obscuration per meter).

They genreally operate in either of two ways (1) measuring the light absorbed/blocked by the smoke particles, or (2) measuring the light scattered by the smoke particles. In (1), the detector is in line of sight of the light source. In (2) the detector is kept out of line of sight of the light source & can only see the light which is scattered by the smoke particles.

The simplest approach is to use the light absorption method, since scattering methods require some consideration of specific angles, forward vs back scattering, etc. All you need to do then is to measure the change in signal from the photodiode as a result of smoke passing through the beam. This will be very small and "noisy" relative to the light source.

If you are doing this for more than an experiment, you also need to deal with the difficulties of discriminating between smoke & dust. Nominally, smoke particles are much smaller (nominally 1 -> 10um) than dust particles and there are ways to provide some discrimination but there is no practical perfect solution.

Hope this helps in some way.

Cheers.

ordinary smoker detector use in hotel and deparment is isotope element "americium" in a small ionization cabin. your amplifier has a high resistance input to amplify signal changed.

another way is infrared detector, but you have to get a high sensitivity amplifier.

transistor amplifer will be difficult to implement theamplify for you to do it if you not familiar with the circuit.

try to use IC, so implement it. ask ni for sample.

Yes this is a home project, i need to build a simple smoke detector circuit on a bread board and explain it to my instructor how it works, the basics thats it no fancy lights flashing and screaming ppl....thnx for all the replies and suggestions...

I have just looked up a home brew design it uses two LDR's (light dependent resisters) ORP12 type one in a sealed end of a black tube the other on the end with a slot cut in to it and an LED (red) place exactly between them (Centered) this is important ant then these are connected to a comparator so as to set up bridge circuit you can put in small adustable resisters to get it to ballance. LM311 is a suyitable comparator. You will have to look it up to get the pin out and any application details. You then need some small 1/4 watt resistors a couple of gen purpose transistors a couple of capacitors. Good hunting If you find the 311 is not enough you need a dual device go for the LM393N. NE555N will give you an indicator circuit.

photo sensitive resistor is a good idea as well.

I must make a mistake that infrared is impossible for your smoke detector. its anohter use in smolke detection.

you can use a photo diode, reverse link to power to sense light change. although it has many limit and constraint., but as a simple principle demonstrate, it can be.

use a FET as preamplifier of current netive feedback to match the reverse diode.

hoeever, I still suggest you use Ic.

Yes strip a remote control sensor from an old tv or Hi Fi unit. You can get the led from the hand controller. Just be careful and don't try the one in the living room.