Photoelectric type, ceiling-mounted
A plastic dome that spends years doing almost nothing, very carefully. Inside, a small dark chamber is checked every few seconds with a flash of infrared light; if smoke drifts in and scatters that light, a brass disc shrieks. It twists off its ceiling plate, the cover unclips, and a few small screws hold the rest.
The face you see on the ceiling. Its ring of slots lets smoke in from every side while keeping fingers and most dust out; a window lets the status light shine through.
Made of Flame-retardant ABS or polycarbonate, moulded in one piece.
The plastic of the cover is flame-retardant, as the standards require, so a fault in the electronics cannot set the alarm itself alight.
Est. cost to the maker $0.2–0.6
A black lid over the optical chamber and a ring of fine mesh round it. Smoke drifts through the mesh; insects and fluff are kept out.
Made of Black polycarbonate or ABS lid; stainless-steel or nylon mesh with holes a fraction of a millimetre across.
Even a small insect walking about inside the chamber can scatter enough light to set off a false alarm, which is the reason for the mesh.
Est. cost to the maker $0.05–0.2
The sensor itself: a little black maze with an infrared LED and a light sensor that cannot see each other. Clean air keeps the sensor in the dark. Smoke particles scatter a little of the LED's light onto it, and the chip counts that as smoke.
Made of Black, light-absorbing plastic labyrinth; an infrared LED (around 880 to 940 nm) and a silicon photodiode, each in its own hooded pocket.
The maze of black vanes is the trick: air and smoke can wander in, but straight lines of room light cannot, so the sensor only ever sees light bounced off particles.
Est. cost to the maker $0.3–1
The siren. A thin brass disc with a ceramic layer bonded to it bends back and forth when a voltage is applied, thousands of times a second. The small separate patch on the ceramic feeds a signal back, so the circuit can keep the disc at its loudest pitch.
Made of Brass disc, piezoelectric ceramic (lead zirconate titanate) with silver electrodes, a plastic holder, three soldered wires.
Standards such as UL 217 ask for at least 85 decibels at about 3 metres. A piezo disc does that on very little current by working near its resonant pitch, usually around 3 kHz.
Est. cost to the maker $0.2–0.6
Powers the alarm for a year or so. The snap clip on its end is the same one used on 9-volt batteries for decades.
Made of Six small 1.5-volt cells stacked inside a steel jacket (zinc, manganese dioxide, potassium hydroxide in an alkaline battery); a plastic snap clip and copper wires.
The chirp of a dying battery is deliberate: the alarm keeps measuring the battery and, when it sags, chirps briefly about once a minute rather than sounding the full alarm.
Est. cost to the maker $0.5–1.5
The alarm's logic: one dedicated chip that wakes up every few seconds, flashes the LED, reads the light sensor, checks the battery and, when needed, drives the horn. Everything else is a handful of resistors and capacitors around it.
Made of Paper-phenolic or glass-epoxy board, a dedicated smoke-detector chip, through-hole resistors, capacitors, transistors, a push switch and a red LED.
Ionisation alarms put a tiny source of americium-241 here, typically about one microcurie (some 37 kilobecquerels); its alpha particles make the air in a small chamber conduct, and smoke cuts that current. This photoelectric alarm has no radioactive source at all.
Est. cost to the maker $1–3
The back half of the case, seen from inside. Bosses hold the board's screws, walls make a pocket for the battery, and lugs round the rim lock into the ceiling plate.
Made of Flame-retardant ABS, the date of manufacture moulded or printed on it.
Most makers and fire services advise replacing the whole alarm about ten years after the date on its base, because the sensor slowly ages and gathers dust.
Est. cost to the maker $0.15–0.5
Screwed to the ceiling once and left there. The alarm hooks onto its three ramps with a twist, so it can come down for a new battery without anyone unscrewing anything.
Made of Moulded ABS or polypropylene.
The long curved slots let the plate be screwed to an old electrical box or to bare plaster, whatever the spacing of the holes.
Est. cost to the maker $0.05–0.2
Prices are rough ranges of what the manufacturer pays for each part, pieced together from published teardown and bill-of-materials estimates. They vary a lot between models, years and order sizes, and they leave out assembly, software, research, shipping and profit. The drawing shows a generic device, not any particular model.