Smoke Detection Sensor

[thakidd]

Hey EEVBlog!

Looking for a good smoke detection sensor. Wondered if anyone might have a good place to purchase. I am looking for something cheap but quite functional. Not really sure how they work. I would prefer a single wire or two wire sensor. Nothing SPI as I am already using it for other things.

Thx in advance.

[Psi]

There are two types,  ionization detectors and optical ones.
The easiest and cheaper way to get one maybe to buy a smoke alarm and pull it apart for the sensor.

[Simon]

There are two types,  ionization detectors and optical ones.
The easiest and cheaper way to get one maybe to buy a smoke alarm and pull it apart for the sensor.

yes they are so cheap these days your probably only really paying for the sensor, Just don't take the sensor apart as it contains low radioactive stuff in it

[tyblu]

There are two types,  ionization detectors and optical ones.
The easiest and cheaper way to get one maybe to buy a smoke alarm and pull it apart for the sensor.

yes they are so cheap these days your probably only really paying for the sensor, Just don't take the sensor apart as it contains low radioactive stuff in it

Only the ionization type do. I didn't think there was enough in there to do anything -- how much radioactivity does it emit?

[Simon]

I only know it is very low but the sensor does have shielding, I'm just saying just don't take the actual sensor apart or you might be asking for trouble, particularly if you spent prolonged periods of time exposed to it

[cybergibbons]

Modern ionisation smoke detectors a very small amount of americium-241 in them, in the form of the americium dioxide. Supposedly, according to the authorities, you can swallow the contents of a detector and not suffer harm as it isn't water soluble, so will pass through the system with minimal exposure to the alpha and gamma particles emitted. Also, don't burn it or use a soldering iron too close - inhaling it is very dangerous.

[Uncle Vernon]

It shouldn't be too hard to find a smoke detector you can hot-wire without dis-assembly. There is a LED that lights upon alarm, a battery connector for supply from a UPS and a test button which could allow a remote test connection. Silicon chip magazine did a design for a multi sensor alarm panel a few years ago it had a few shortfalls bit it could give a few ideas for your project.

[ColinA]

I would suggest going with an ion device.

Photoelectric smoke detectors (mostly used in commercial applications) use light pulses scattered in a chamber. then amplify the signal and use comparators to compare the signal to a reference level. Without the chamber outside light sources can affect it. Typicaly photoelectics pulse every 5-10 seconds to extend the life ofthe device versus ion that use oscilating frequencies which are active all the time.

[RayJones]

Trace amounts of radioactive material always cause hysteria and fear.

We have constant ongoing issues at work sending Transmit/Receive Cells used in our radars.
For sure there are radioactive stickers slapped on the item, but the unit is so robust even an unpackaged unit is most unlikely to be damaged by a typical airline baggage handler.

In terms of radioactivity it is well below ARPANSA's safe levels.

The same applies to smoke detectors.
Do you avoid walking down the electrical aisle at the hardware store where hundreds may be stacked together - and no radioactive sign or drenches can be found?

But yes, do not attempt to dismantle the sensor or device beyond it's sealed condition, that just foolish behaviour.

BTW, does anybody remember the old glow in the dark watches?

[DJPhil]

BTW, does anybody remember the old glow in the dark watches?

Yes! I dimly remember reading about the workers that painted the dials having lots of health problems, but I don't recall anyone being the worse for owning such a watch.

My first dealings with the radioactive luminous paint were the dials on button compasses. They were included in survival kits that flew with USAF pilots for decades, and were even marked as radioactive on the back. They were designed to be easily . . . uh . . . hidden, if you get my drift. I wore one on a cord around my neck for several years when I was a kid. Nowadays they don't use the radioactive paint, but otherwise are roughly the same. I've got a few buried somewhere, I'll take photos if I can find them.

The Americium in a smoke detector emits a small amount of gamma radiation, less than background in most places, and alpha particles, which perform the ionization. Alpha particles are essentially a stripped helium nucleus, and are so massive and slow (5%C roughly) they can be blocked almost completely by a piece of paper. The detector modules are quite safe to work with, and even should you break one open the hazard is so minor as to be overshadowed by the use of leaded solder.

[FreeThinker]

Trace amounts of radioactive material always cause hysteria and fear.<Snip>
.......
<snip>

BTW, does anybody remember the old glow in the dark watches?

Brought to mind a bizarre situation I once found myself in as a service engineer.
I was on a call to the yet to be commissioned THORP site at Sellafield in the uk.As a nuclear waste reprocessing site safety was a major issue and entry was a real PITA.You had to change all of you clothes and put on disposable overalls leaving all your personnel items in a locker.On exit you had to go through a body scanner thing (Called a Borer or something like) to check you were not contaminated.This is were I fell foul of the system, on entering the scanner all hell let loose and I was in lock down, people in space suits rushed to usher me to a decontamination chamber and poked and probed me for 10 mins.Then I realized I was still wearing my watch!, with its nice glowing face (nearly as bright as mine).They took my watch off me for disposal as toxic waste, its probably buried at Drigg ( the underground storage facility) ,a severe letter was sent to my company and I had the p**s taken on a grand scale by my mates. Silly thing is as I have said it was not yet commissioned so no nuclear material was on site therefore I could not be contaminated, but try to explain that to the security people! I can laugh about it now but at the time it was very unfunny.As a measure of how insane the fear is on these sites I was told later that it was Legal to buy mineral water from the local shop but if it was brought on to site it was ILLEGAL to remove it as it was above the safe limits of radioactive discharge...Makes you think  

[JohnS_AZ]

This just came across Twitter this morning ...

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2018&mcparam=en552396

[Chasm]

Alpha radiation is very easily blocked. Yet it is also the most dangerous type because the helium nucleus is so huge and carries lots of energy.
Incorporating an alpha source will have /much/ more dire effect on your health than a beta or even gamma source of the same radiation level. Simply because your body will absorb all radiation the alpha source emits.

So don't eat or smoke and also better wash your fingers.

[Time]

They are super easy to rip out of a smoke detector.  I have done it before with great ease.  The cheaper the detector the easier it is to get to the smoke detecting elements.

The only type of radiation that will work in smoke detection is alpha radiation.  Alpha particles are bulky, like previously stated, which causes them to be highly collisional in nature.  They produce electron ion pairs in quite an abundance.  So if you radiate a gap of air between two electrodes with alpha particles you can measure a current between the electrodes since the electron ion pairs will move in the field provided by the voltage.  When smoke enters the gap it causes irratic perturbations in this otherwise steady current.  These perturbations in the current are what triggers a detection in the case of ionization detectors.

[uk_kev]

Microchip just announced some today...
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2018&mcparam=en552396

[Zero999]

The only type of radiation that will work in smoke detection is alpha radiation.  Alpha particles are bulky, like previously stated, which causes them to be highly collisional in nature.  They produce electron ion pairs in quite an abundance.  So if you radiate a gap of air between two electrodes with alpha particles you can measure a current between the electrodes since the electron ion pairs will move in the field provided by the voltage.  When smoke enters the gap it causes irratic perturbations in this otherwise steady current.  These perturbations in the current are what triggers a detection in the case of ionization detectors.

Will high energy electromagnetic radiation work too? I've heard that VUV can ionise the air. Maybe in the future a high intensity UV LED or discharge lamp pulsed at a low duty cycle will replace the isotope in ionisation detectors?

[Time]

Yes, VUV can ionize the air but the problem is that it does not penetrate more than a 1 mm or so in the case of the highly ionizing wavelengths (into normal atmospheric pressure).  It takes expensive/special materials to be transmissive in this wavelength (i.e. magnesium flouride) so whatever the discharge was housed in would need to be made from the right stuff.  Alpha particles from Am241 ( 5 MeV or so) go a few centimeters at sea level and create dense paths of electron ion pairs.

It is speculated that typical discharges occuring in dry air (N2 and O2 basically) produce VUV and its effects actually promote the discharge itself once it has begun.  It has only been shown in theoretical simulations but never proved experimentally due to VUVs poor transmission capabilities.  Its difficult to obtain spectral data on something that wants to be absorbed in everything its incident on.

[RayJones]

From an OH&S course we learnt that electromagnetic radiation below visible light wavelengths is considered non-ionising.
Above visible light is considered ionising.

So according to those rules yes, A UV source is capable of ionising, but I would suspect nowhere near as effective as the radioactive elements.
Ionising of course referring to the ability to strip or add electrons electrons from the outer shell of the atom.

[Zero999]

Of course, I forgot how opaque the air is to VUV, hence the name.

What about using slightly longer wavelengths then, such as far UVC? Or don't they have enough energy?

[Time]

They have the potential to ionize, yes. Its just that ionization happens infrequently when dealing with photon energies that are below the ionization potential of the target. It might not be able to create a realistic measurable current.  Ionization is a statistical process which is a function of photon energy and incident particle.  When photon energy is low the probability of ionization is very low (its highly improbable that ionization will occur below the ionization potential of the target but it can happen, its called quantum tunneling) but as the photon energy nears the ionization potential of the atom or molecule the probability increases.  The general shape for photoionization curves is a sharp rise that peaks around ionization potential and than a long tail that trails off into the shorter wavelengths.