How do smoke detectors work?

And why does burnt toast set them off?
12 March 2019


White smoke alarm



What’s the science behind smoke detectors? How do they actually work?


We've all been there, you pop on some dinner, forget about it and then your smoke detectors bursts to life... But how? We received this question from listener Alistair. Chris Smith was joined by technology journalist Tim Revell who explained the science behind these noisy devices.  

Tim - Smoke detectors are actually one of those things most people ignore usually. You stick them up on your ceiling and then you never think about them again. But actually inside those fairly plain, white boxes there some quite cool science inside them. So there’s two main ways that they work: the first is called an optical smoke detector and these have an LED light that fires out a small light beam, and there is also a solar panel which is similar to what you might have on your roof. Now if there’s no smoke this light never actually touches the solar panel, but as soon as smoke enters it the light scatters hitting a little bit of the solar panel, creating a little bit of electricity and triggering the alarm which is, if you’ve ever heard one, incredibly annoying and enough to make you get up and check if there’s a fire.

The other type of detector is called an ionisation detector and these are also pretty cool. There are two metal plates and there is also a small radioactive source that produces particles called alpha particles. These what’s called ionise the air in between the plates which essentially means you end with an electrical current between two plates. Know if everything is functioning normally this electrical current just keeps happening and is not interrupted, but as soon as smoke enters the gap you no longer have the electrical current flowing, it triggers the alarm and off it goes and the same thing happens as with the other type of detector.

Chris - Indeed. I think it’s amorisian is the alpha source in your average smoke detector?

Tim - Yeah.

Chris - It’s quite radioactive isn’t it? It’s quite amazing that everyone’s could this radioactive source in their roof so don’t play with it I think is the bottom line, isn’t it?

Tim - Yeah. It’s quite radioactive but these alpha particles don’t travel very far. You don’t need to worry about it. It’s not like they’re producing gamma rays or something that could actually harm you from the ceiling so they never actually escape the little boxes that you keep them in.


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