Old smoke alarm more sensitive

[edy]

Just a question about smoke alarms. I have an old one that is due for replacement. It operates using Americium 241. My understanding is that the radioactive source ionizes atmospheric particulates and that when more particles (smoke) is in the air, greater charge flow occurs between 2 plates which is detected and starts the alarm.

The detector is wired into 9V supply from the mains. I would expect an old alarm to have less sensitivity as the radioactive source decays to produce less ions... unless there's a compensation circuit to deal with it and the issue becomes one of signal to noise ratio. Too few ions and change between normal background and smoke signal is influenced by random fluctuations.

Any thoughts?

[johansen]

half life is 432 years.

usually the electronics fail.

[edavid]

Just a question about smoke alarms. I have an old one that is due for replacement. It operates using Americium 241. My understanding is that the radioactive source ionizes atmospheric particulates and that when more particles (smoke) is in the air, greater charge flow occurs between 2 plates which is detected and starts the alarm.

That's backward - smoke reduces the chamber current.

The detector is wired into 9V supply from the mains. I would expect an old alarm to have less sensitivity as the radioactive source decays to produce less ions... unless there's a compensation circuit to deal with it and the issue becomes one of signal to noise ratio. Too few ions and change between normal background and smoke signal is influenced by random fluctuations.

Also backward - old alarms become more sensitive due to a buildup of dust.  Eventually they start giving false alarms.

[cvanc]

- old alarms become more sensitive due to a buildup of dust.  Eventually they start giving false alarms.

^^^This.  I've also seen sensitivity spike when a spider took up residence in the chamber.  Blow the dust out, or (much better idea) just replace it. 

[Brumby]

We've recently had the NSW Fire Brigade recommend replacing all ionizing smoke alarms with photoelectric ones.

Reason: The photoelectric ones are more sensitive.  By the time the ionizing ones detect smoke, you are at a higher risk of being overcome.

[Brumby]

Are the smoke detectors involved the ionizing type or photoelectric?

[SeanB]

Had issues with 50 year old ionisation switches ( which contain a speck of Americium 241 as an ionisation promoter) getting slow as they aged. Solution till we ran out of headroom and ordered new switches ( at a price) was to simply adjust up the trigger voltage to around 300V from 250V IIRC.

In a smoke detector it will generally be the electrolytic capacitors on the board failing that limits lifetime, the actual source is way over specification when new to compensate for a 10 year decay to a minimum activity ( not that it will not work below minimum, just the specification for it will require a certain activity as a minimum), and they typically will last 25-30 years for those made in the 1970's and 1980's on old CMOS process. The chip itself likely will last 200 years or more, no stored charge in memory cells, large gate sizes and very low leakage levels. The most common failure will be environmental, either corrosion or build up of contaminants on the board or the chamber itself.

[edy]

Thanks, I had it backwards. I have attached some photos, while I cleaned it with some compressed air.

I see only caps, resistors, diodes, Zener and contact go to 9V and piezoelectric alarm speaker (flat disc). No transistors or chips of any kind. When it goes off it also activates a CO alarm upstairs (they are linked). Somewhere there's a power supply to both, obviously it is not in this one.

I was looking online for a simple schematic but can't find any without IC's or transistors. As far as I can tell, my detector and alarm has none. The alarm sound I assume is generated by what? Built into the piezo or an RC oscillator?

[edy]

Wait...me thinks I see pins on the back of the PCB under the Radioactivity can that is a chip. I think it is hidden under there!

[SeanB]

RCA chipset, and the input side has a standard capacitive dropper to deliver the 9V the chip runs off. When you disconnect power the unit will chirp away for around 5 minutes just from the stored charge in the capacitor.

Almost guaranteed to be this chip.

http://www.nxp.com/files/analog/doc/data_sheet/MC145017.pdf

Or the older RCA version, as this was amongst the first ionisation smoke detector IC that was UL listed.

If it was an 8 pin package then this more modern Microchip is the chip used.

http://ww1.microchip.com/downloads/en/DeviceDoc/22162A.pdf