The simple domestic smoke detector/alarm is based on the ionising effects of radioactivity. It consists of a battery, a speaker and an electronic switching mechanism, part of which is a chamber containing a sample of the radioactive material Americium-241. When the circuit is ON, a steady current flows and the speaker is kept silent by the switch. When the size of the current flowing is reduced the switch operates the speaker, which emits a high pitched warning sound.
There are two ways of reducing the current flowing in the circuit. Firstly a TEST switch simply opens the circuit and switches off the current. This causes the warning sound to be set off and tests that the battery is in good condition and that the device is in working order. Secondly an automatic switch will operate in the case of smoke being generated in a fire. This second switch involves the radioactive material reducing the current flow to activitate the speaker. The smoke enters the chamber containing Americium-241, which is an a-particle emitter. In the absence of smoke, the a-particles ionise the air in the chamber completing the pathway for current flow around the circuit, see Diagram 1.
Diagram 1: Normal air ionised by a-particles
When smoke enters the chamber,
the sooty particles cling to the ionised air particles slowing them down.
The air particles can no longer reach the electrodes (electrical contacts
between chamber and external circuit) to complete the circuit as quickly
as before, so the current flow is reduced. This operates the electronic
switch to set off the alarm. See Diagram 2.
Diagram 2: Sooty air entering smoke detector
A simplified version of the circuit is given in Diagram 3.
Diagram 3: Simplified circuit diagram