You probably have a couple of smoke detectors in your house. In this edition of How Stuff Works we will take one apart and see what's inside! See also the article on smoke detectors to understand how smoke detectors work. Here is the smoke detector we will be exploring today:

When you take off the cover you find that a smoke detector is pretty simple. This one consists of a printed circuit board (see the article Inside a Remote Control for an explanation of printed circuit boards), an ionization chamber (the silver cylinder toward the top right in the following picture) and an electronic horn (the brass cylinder toward the bottom right in the following picture):

Here is a close-up of the board:

and the underside of the board:

Ionization chamber

The ionization chamber is an aluminum can containing the ionization source. You can see that the can has slots in it to allow air flow. The can itself acts as the negative plate of the ionization chamber.

Underneath the can is a ceramic holder that contains the positive plate of the ionization chamber. Under that plate is the ionization source, WHICH YOU WANT TO BE SURE TO NOT DISTURB.