smoke alarm

[Thunderchild]

why is the sensor in a smoke alarm marked as radioactive ?

[pug100]

That's because it uses ionisation to detect smoke and a very small amount on radioactive isotope americium-241 is part of the technique. More info here

http://en.wikipedia.org/wiki/Smoke_alarm#Ioniz ation_detect
or

and here

http://home.howstuffworks.com/smoke4.htm

[Thunderchild]

ok thanks so theres no other method of detection other than the old fashioned infrared LED TX/TX system that goes off if something obscures the gap between TX and RX

[Panic Button]

Eh?

[weeal]

lol @ panic button

all sounds very technical eh

[Thunderchild]

simpler smoke alarms used an infrared light bem just like intruder alarms but the transmitter and receiver were on the same board about 1 cm apart, if smoke came between the two the beam was disrupted and set off the alarm same principle as bugular alarms but put all on the same board

[Hymie]

Optical smoke detectors do not detect smoke by the smoke particles blocking the transmission of light from source to detector.

The light path within the sensor cell is not pointed directly towards the detector. The detector is activated by light, scattered by the smoke particles, entering the detector.

Just thought you�d like to know.

[Thunderchild]

well when I lived in Italy (up to a short while ago) I read the explanation of a kit project that worked exactly as I described and to my mind it is crazy but there you go italy is a bit crazy