Reverse engineering a big old smoke detector
Posted: Sat Mar 14, 2020 6:05 pm
This is to show more detail about a thing closely related to Richard's "interesting electrometer device".
viewtopic.php?f=18&t=13261
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. .
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.
The F5B can be taken apart without tools, but I'm not sure we should do that or even talk about it. I found out that the internal construction is similar to this unfinished drawing, previously posted in Richard's electrometer thread. 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 from "geoelectronics" on ebay (see Low Light Photography thread). Set shutter time to 30 s, aperture f/1.4, ISO 12800. Lights off, not completely. Go... Would it be against the law to disassemble a device like this, for closer inspection? Does it matter if no screwdriving, wrenching, or cutting is required? Can anyone substantiate rumors that internal parts could be shedding active material?
viewtopic.php?f=18&t=13261
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. .
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.
The F5B can be taken apart without tools, but I'm not sure we should do that or even talk about it. I found out that the internal construction is similar to this unfinished drawing, previously posted in Richard's electrometer thread. 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 from "geoelectronics" on ebay (see Low Light Photography thread). Set shutter time to 30 s, aperture f/1.4, ISO 12800. Lights off, not completely. Go... Would it be against the law to disassemble a device like this, for closer inspection? Does it matter if no screwdriving, wrenching, or cutting is required? Can anyone substantiate rumors that internal parts could be shedding active material?