Reverse engineering a big old American smoke detector

[klugesmith]

Just noticed the thread about big old Soviet smoke detector, which makes this one seem as safe as baby ducklings.  Told this story yesterday, in almost the same words, on another forum.

Here is an old stock Pyrotronics F5B smoke detector head, similar to the F3/5 head. They are still available for people maintaining old installations, but typically cost well over $100 on ebay.

Ionization-type smoke detectors seem to have started in the 1960's. Preventing fires in industrial & institutional settings easily justified the proliferation of 80 microcurie Am-241 alpha particle sources. Those produce enough air ionization that a cold cathode trigger tube can switch the alarm circuit.

I got the pictured device to turn on a neon lamp, within a few seconds of being exposed to smoke from a smoldering popsicle stick. My circuit is shown below, on the right. The original application uses Base Units (sockets) with resistors and a neon indicator lamp, and if any reader has one I'd love to know the circuit details.

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These old smoke detectors, just like modern ones with less than 1 uCi of radioisotope, use a reference ionization chamber as the high impedance on top of voltage divider. Cheaper than a many-gigaohm resistor, and it automatically compensates for pressure and temperature and decay of the alpha particle source.

I found out that the internal construction is similar to this unfinished drawing.  The ion chambers in schematic above were rendered accordingly.


It came as a surprise that the larger ionization source, which irradiates the smoky chamber, is directly visible through the window-screened ports. Close enough that energetic alpha particles, not just ions, could be escaping from the enclosure. Let's check with some alpha-viewing fluorescent film, like what's sold for making spinthariscopes.


Set shutter time to 30 s, aperture f/1.4, ISO 12800. Lights off, not completely. Go...


This device could be disassembled without any screwdriving, wrenching, or cutting. 
Not sure if that's a good idea, or a thing to talk about.
Even if it were reassembled and tested as a working smoke detector, good as new, ready to protect lives.

[John123]

Looks quite big in the photo compared to the ones inside modern detectors.

How come they were designed as replaceable parts, do they wear out and become unreliable over time?

[johnf]

I think the part has been made much like tubes were made
in fact that looks like a standard octal tube base with a few pins missing.
this would fit well with early electronics manufacture allowing all to be tested before the detector head was put in

[TheMaxii2007]

Hello My old smoke detector Am-241 (72 microcurie ) 

[klugesmith]

Welcome to the forum, and thank you for the interesting report.

The sectional drawing you shared is much better than anything I've seen before.
It sure beats measuring parts and drawing things from scratch.
Does the document it came from include details of the circuit in base unit?

Is there a model number and/or date code on your device? 
How come the nominal activity is 72 uCi, vs 80 uCi in my specimens and in the F3/5 model we can read about?

Might as well share a picture I made with alpha-viewing film, to get an idea of relative activity in the outer and inner chamber irradiators, next to a modern sub-1-uCi source.

[TheMaxii2007]

very nice !!!   klugesmith  thank you friend 

https://www.nrc.gov/docs/ML1129/ML112990360.pdf

https://www.tvfr.com/DocumentCenter/View/111/Disposal-of-Smoke-Alarms?bidId=