1. Reverse Engineering Project The Smoke Detector
David Lutz
Mohamed Al-Maaz
Mary Savalle
Afnan Abdulazeez
Be 1100
03/27/02

2. Smoke Detector How and why the smoke detector was created
10 Step Design Process
Different types of smoke detectors
Detailed description of ionization detector
Alternative Solutions
Conclusion

3. Identify A Need
A safety device

4. Defining the Problem
A safety device that provides a warning sign to fire.

5. Search For Existing Solutions
Before the smoke detector was created, there was no other form of detection of a fire besides the instincts of the person in the household. However, forms to put out a fire were established before the smoke detector was founded. For instance a form of the fire extinguisher had been created, as well of the knowledge that water could diminish a fire. Considering that the creation of the smoke detector was an accident, no form of warning device for a fire was known of.

6. Criteria
Size
Cosmetic Appearance
Affordability
Ease of Operation

7. Constraints To make the device safe
To accommodate the needs of every type of person.
Hearing impaired
Blind
Etc.

8. Two Types Of Smoke Detectors
Photoelectric Detector-uses a beam of light emitted, unimpeded from one end of the chamber to the other. Smoke disrupting this beam will set the alarm off.
Ionization Detector-uses an ionization chamber and a source of ionizing radiation to detect smoke. It is the most common type of smoke detector because it is inexpensive and is better at detecting smaller amounts of smoke produced by flaming fires.

9. Photoelectric Detector
                                         
Light, under normal conditions, is emitted from pt. A straight across, unimpeded. There is a Photo-Detector (pt. B) at 90o that, when smoke enters, scatters the light. It hits the sensor and triggers the alarm.
These are a bit more expensive then Ionization detectors and are better at detecting larger particles of smoke like in a smoldering fire.

10. Ionization Detector Analysis
The housing is usually a plastic shell, housing the main components of a smoke detector. It has several slots around the outside to allow air to flow inside and into the detecting ionization chamber.
Also has button to test the alarm in order to make sure the batteries and the detector itself are functioning properly.

11. Inside the Detector
Inside the Ionization Detector is a small amount (about 1/5000th of a gram) of a the radioactive substance: americium-241 which is a good source of alpha particles.
From this picture, the ionization chamber in the upper right, the bell in the lower right, and the circuit board are visible.

12. Ionization Chamber
Outside
The cover on the chamber has several uses. First, since the chamber is sensitive to vibrations which could inadvertently set the alarm off, the cover acts as a shield to from these vibrations. Secondly, since there is a radioactive element in the chamber, it acts as a simple barrier between the chamber and the rest of the smoke detector. Finally, there are slotted holes in the cover to allow the air to flow through it, allowing for detection of smoke.
Inside

13. How the Ionization Chamber Works
There are two plates: one positively charged, one negative. The battery gives the plates these charges
The alpha particles that the americium-241 generates, ionizes the air molecules in the chamber. The following reaction takes place:
O2 + He(+2) --> O2(+1) + e(-1) + He(+2)
                                       

14. How the Ionization Chamber Works cont.
The ionized air molecules (pink and blue spheres) carry a small current between the two plates. The free electron is attracted to the positive plate while the positively charged atom is attracted to the negative plate.
When smoke (brown spheres) enters the chamber, it attaches itself to the ions therefore reducing the current flow which sets off the alarm.
                         
                         
Americium-241

15. The Horn
Makes a loud audible pitch when triggered by the ionization chamber.

16. The Light
Provides a source of light, in case the electricity is lost in the fire, so occupants can see a way out.
Acts as a detection for the hearing impaired because the bell would not be heard.

17. The Batteries Batteries are what power the smoke detector.
There are two 9-volt batteries on our particular smoke detector. One powers only the light because more current is required by the light source. Also, if the battery that powers the light fails, the rest of the detector would remain working.
The second battery powers the rest of the detector.

18. The Circuit Board
The printed circuit board is the brains behind the operation. It connects the power source, ionization chamber and horn. It allows for “communication” between the components of the detector. For instance, if the ionization chamber senses a drop in voltage the circuit board is what carries the signal from the ionization chamber to the horn and light for it to sound.

19. Alternative Solutions
The smoke detector could be connected to the fire department to alert them in case of a fire.
The smoke detector could contain a heat sensor that would allow an earlier warning of the fire.

20. Decision of Best Solution
The choice of the best alternative solution is dependant on where the detector is placed. The ionization detector is good for rooms that contain highly combustible materials. Such rooms could possibly include materials like flammable liquids, newspapers, paint, cleaning solutions, etc. An ionization detector would be good in a kitchen. Photoelectric detectors are better for rooms such as the living room and dining room because it would require larger amounts of smoke to set it off. Because the smoke would be flowing though the room at different paces, it would not set off the alarm as quickly.

21. Conclusion
Based on our research we feel that the smoke detector is an essential to everyone’s household. The smoke detector that we have explained in detail is the most commonly bought and used. The alternative smoke detectors are useful as well, but the ionization detector is the cheapest and best with alerting people of the danger of a fire. Since the smoke detector was created many forms have been derived to accommodate all types of people.