How smoke detectors are made

The first home smoke detector, sold in 1971, cost about $125.

stations: 8
things upstream: 80
raw sources: 25
steps from the ground: 10

The short version

An ionization alarm is a circuit board carrying two small chambers, each holding a speck of americium sealed in gold and silver foil. A specialist supplier may make those chambers. On the line, parts and horn go on the board, a wave of molten solder fixes them, and the board is seated in a molded plastic base. Photoelectric alarms use a light beam instead.

Roll the source foil

At a specialist maker, americium oxide is blended with gold and fused into a briquette, then sealed under gold with a silver backing. Repeated cold rolling turns it into a thin foil strip.

Goes in
Gold Silver
Punch and seal discs

Tiny round discs are punched from the strip and each goes in a metal holder. A thin rim on the holder is then folded over to close off the cut edge all the way around.
Build the two chambers

One disc goes in the sensing chamber and one in a reference chamber next door that smoke cannot enter. Electrodes are fitted to both, with leads poking out the bottom. Many alarm makers buy these ready-made.
Stuff the circuit board

Diodes, capacitors and resistors go onto the board, through punched holes with trimmed leads on older designs and by surface mounting on many current ones. The two chambers and the alarm horn are added.

Goes in
Printed circuit board
Wave-solder the board

The loaded board passes over a wave solder machine, which fixes every component in one pass.

Goes in
Solder
Mold base and cover

Powdered plastic and pigment are heated, pushed under pressure into a mold and cooled, giving a mounting base and a cover.

Goes in
Plastic Polystyrene PVC
Close the housing

The board is seated on the base and a test button is fitted so owners can check the alarm at home. A mounting bracket and then the cover finish it, with inspections along the way.
Box with battery

The alarm is boxed in cardboard along with a battery and printed instructions.

Goes in
Cardboard box

End of the lineSmoke detector

Worth repeating

Ionization smoke detectors have been on sale in the United States since 1951, at first only for factories, warehouses and public buildings.
The first single-station alarm dates from 1970, ran on one 9-volt battery and cost about $125.
The photoelectric smoke detector was patented in 1972, and the 10-year lithium battery alarm arrived in 1995.