Puzzled with the question of ‘what components are in a smoke detector?’ you should rest assured that after going through this guide, you’ll have a fair knowledge of what the smoke detectors comprise of. Also, knowing what the smoke detectors are made of will help you see smoke detectors in a better light. So don’t resist that urge to read through and find out for yourself.
A smoke detector is a device that senses the presence of smoke in a building and warns the occupants, enabling them to escape a fire before succumbing to smoke inhalation or burns. Equipping a home with at least one smoke detector cuts in half the chances that the residents will die in a fire.
In 1992 the readers of R&D Magazine selected home smoke alarms as one of the “30 Products that Changed Our Lives.” Smoke detectors became widely available and affordable in the early 1970s. Prior to that date, fatalities from fires in the home averaged 10,000 per year, but by the early 1990s, the figure dropped to fewer than 6,000 per year.
Two basic types of smoke detectors are currently manufactured for residential use. The photoelectric smoke detector uses an optical beam to search for a smoke. When smoke particles cloud the beam, a photoelectric cell senses the decrease in light intensity and triggers an alarm. This type of detector reacts most quickly to smoldering fires that release relatively large amounts of smoke.
The second type of smoke detector, known as an ionization chamber smoke detector (ICSD), is quicker at sensing flaming fires that produce little smoke.
It employs a radioactive material to ionize the air in a sensing chamber; the presence of smoke affects the flow of the ions between a pair of electrodes, which triggers the alarm. Between 80 and 90% of the smoke detectors in American homes are of this type.
ionization smoke detectors are the most sensitive to invisible particles found in highly dangerous, fast-burning, smokeless fires. The low manufacturing cost of this type of sensor also makes them highly affordable.
Although most residential models are self-contained units that operate on a 9-volt battery, construction codes in some parts of the country now require installations in new homes to be connected to the house wiring, with a battery backup in case of a power failure.
The typical ICSD radiation source emits alpha particles that strip electrons from the air molecules, creating positive oxygen and nitrogen ions. In the process, the electrons attach themselves to other air molecules, forming negative oxygen and nitrogen ions.
Two oppositely charged electrodes within the sensing chamber attract the positive and negative ions, setting up a small flow of current in the air space between the electrodes.
The electrodes do not conduct anymore. When smoke particles enter the chamber, they attract some of the ions, disrupting the current flow. Sometimes, this depends on the type of sensor and manufacturer, the conductivity conditions may change but the idea remains the same.
Based on the output of the smoke detector, an alarm system can be implemented.
The sensor used in most smoke detectors is the MQ-2 Gas/Smoke sensor. It is sensitive to LPG, Hydrogen, Smoke, Methane, Propane, Alcohol, Butane, and other industrial combustible gases.
It has two electrodes made of Aluminum Oxide (Al2O3) and a heating element made of Tin dioxide (SnO¬2) which acts as the main sensing layer.
A similar reference chamber is constructed so that no smoke particles can enter. The smoke detector constantly compares the current flow in the sensing chamber to the flow in the reference chamber; if a significant difference develops, an alarm is triggered.
An ICSD smoke detector is composed of a housing made of polyvinylchloride or polystyrene plastic, a small electronic alarm horn, a printed circuit board with an assortment of electronic components, and a sensing chamber and reference chamber, each containing a pair of electrodes and the radioactive source material.
Americium 241 (Am-241), a radioactive isotope, has been the preferred source material for ICSDs since the late 1970s. It is very stable and has a half-life of 458 years. It is usually processed with gold and sealed within gold and silver foils.
- MQ-2 Sensor
- 10KΩ POT
Smoke Detectors are amazing devices as they are small, cheap yet very useful. There is an implementation of a simple Smoke Detector Circuit with adjustable sensitivity.
Smoke Sensor MQ-2 is used as the main sensory device. The working of the circuit is simple and is explained below.
LM358 acts as a comparator in this circuit. The inverting terminal of LM358 is connected to POT so that the sensitivity of the circuit can be adjusted.
The output of LM358 is given to an LED as an indicator although a buzzer can be used as an alarm. The non-inverting terminal of LM358 is connected with the output of the smoke sensor.
Initially, when the air is clean, the conductivity between the electrodes is less, as the resistance is in the order of 50KΩ. The inverting terminal input of the comparator is higher than the non-inverting terminal input. The indicator LED is OFF.
In the event of a fire, when the sensor is filled with smoke, the resistance of the sensor falls to 5KΩ and the conductivity between the electrodes increases.
This provides a higher input at the non-inverting terminal of the comparator than the inverting terminal and the output of the comparator is high. The alarming LED is turned ON as an indication of the presence of smoke.
The production of a smoke detector consists of two major steps. One is the fabrication of the Am-241 into a form (typically a foil) that can be installed into the sensing and reference chambers. The other is an assembly of the entire ICSD, beginning either with all of the individual components or with prefabricated sensing and reference chambers obtained from the manufacturer of the radioactive source material. The following description covers all steps, even though some may be done by different manufacturers. Tests and inspections at several stages of the assembly process ensure a reliable product.
- Radioactive source
1) The process begins with the compound AmO 2, an oxide of Am-241. This substance is thoroughly mixed with gold, shaped into a briquette, and fused by pressure and heat at over 1470°F (800°C). A backing of silver and a front covering of gold or gold alloy are applied to the briquette and sealed by hot forging.
The briquette is then processed through several stages of cold rolling to achieve the desired thickness and levels of radiation emission. The final thickness is about 0.008 inches (0.2 mm), with the gold cover representing about one percent of the thickness. The resulting foil strip, which is about 0.8 inches (20 mm) wide, is cut into sections 39 inches (1 meter) long.
2) Circular ICSD source elements are punched out of the foil strip. Each disc, which is about 0.2 inches (5 mm) in diameter, is mounted in a metal holder. A thin metal rim on the holder is rolled over to completely seal the cut edge around the disc.
- The sensing and reference chambers
3) One disc of the source material is mounted in the sensing chamber and another is mounted in the adjacent reference chamber. The electrodes are installed in both chambers and connected to external leads which project out of the bottoms of the chambers.
- The circuit board
4) Printed circuit boards are prepared from design schematics by punching holes for the component leads and by laying a copper trace on the back to form the paths for electric currents. On the assembly line, the various electronic components (diodes, capacitors, resistors, etc.) are inserted into the proper holes on the board. Leads extending out the back of the board are trimmed.
5) The sensing chamber, reference chamber, and alarm horn are installed on the printed circuit board.
6) The board then passes over a wave solder machine, which solders the electronic components into place.
7) The plastic housing consists of a mounting base and a cover. Both are made by an injection molding process in which powdered plastic and molding pigments are mixed, heated, forced into a mold under pressure, then cooled to form the final pieces.
8) The circuit board is seated on the plastic mounting base. A test button is installed so the device can be tested periodically after installation in the home. A mounting bracket is added to the base, and the cover is added to complete the assembly.
9) The smoke detector is packaged in a cardboard box, along with a battery and an owner’s manual.
- The heating element in the Smoke Sensor must be preheated before it can sense any smoke or gas.
- The sensor gets hot because of the heating coil and it is advised not to touch the sensor while it is switched on.
- The sensitivity of the circuit to different concentrations of smoke can be adjusted by using the POT.
- The output LED can be replaced with a loud buzzer for effective alarm.
Nowadays, smoke detectors and smoke alarms are very cheap as its usage is increasing and cost of manufacturing is decreasing. With the above review, you should have an idea of what is used in manufacturing smoke detectors.