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A lot of them these days are photoelectric. They have a “light sealed” chamber (basically vented in such a way that ambient light doesn’t have a path into the chamber but “smoke” does (well smoke, dust and other airborne particulate that is carried in the air)). Instead in the chamber you have a light source and detector. Light is emitted from the source, reflects of the side of the detector and onto the detector. (Actually, thinking about it, I’m sure that they can also work “the other way round”, so instead of particulate blocking the light, lowering the detection levels, they can configured so the light source isn’t pointing at the detector and use the reflectivity of “smoke” to relect light onto the detector. But you get the idea. It’s been far too long since I’ve taken one apart, time to order another for fun. Anyways back to the post)

Dedicated ICs for this purpose can detect the slight lowering of light detected over time by the receiver as dust accumulates instead the chamber and adjust its trigger levels accordingly.

Removes the need for need for the scary radiation source warning label, removes the issue with the radiation source decaying over time and I’m sure comes with that lovely side effect of lowering the BOM cost.

Even though LEDs also decay, as it’s duty cycle is no where need 100% and is completely controlled by the IC I would expect the components in a photoelectric smoke detector to last much much longer than 10, but mine still came with (iirc) a 10 year expiry date on them. I would say because it’s a safety device, an abundance of caution is applied so other things ageing out have to be considered too (like the power supply caps)

Big Clive has taken these apart many a time. If this one doesn’t cover the operation of them, just search for smoke alarm on his channel and I’m sure it won’t take long before find one where he “tears down” the smoke detection chamber of an photoelectric version. https://youtu.be/uzKAZCKjpU8

No idea off the top of my head on how heat detectors (best near your kitchen so your toast doesn’t trigger them) work as personally I’ve never taken one of them apart (yet).

Cheap smoke detectors are pretty much all optical these days, and even the unbranded £2 Chinese ones have pretty sophisticated self-tests: https://www.youtube.com/watch?v=uzKAZCKjpU8

To be honest, I'm not sure whether there's even any point having expiration dates on modern photoelectric detectors, except as a way to sell more smoke alarms.

⬐ AstralStorm

There is an expiration date on the battery which should be listed.