Made in 2012 this Garrison smoke detector is not very old. Broken and lying around so why not tear it down. This is a 9V Garrison brand photoelectric smoke detector model # 46-0085-0. Manufactured on 28th May 2012 according to the sticker.
It was tricky to get into (Its not made to be serviced.) I had to pry apart three stiff tabs which lock the two pieces together. With one piece housing the board and detector cell the other half holding the piezoelectric alarm buzzer.
The front of the board with the detector, LED and cheap switches.
The detector cell is 15mm tall and 40mm diameter, the top can be pried off to reveal the structure.
The design is interesting, first the sides have continuous vents to allow air (And smoke laden air) to flow in, but are shaped and staggered so as to block out light. The LED (top) and detector (Bottom) are offset from each other. In addition there are baffles a such that normally no light hits the detector. Smoke particles will cause the light to scatter and some of it will reach the detector. Ripples on The top and bottom surfaces reduce reflections. The LED has a purple lens and I think it is an infra-red LED. (The data sheet of the chip also indicates this is for infra-red LED.)
LED and Detector
However the detector (photodiode) is pretty large at 3 mm on a side. I guess it needs to be very sensitive be able to detect a tiny amount of light scattered off a few smoke particles. I am not sure if this is a silicon photodiode, or depending on wavelength uses a compound semiconductor (Such as AlGaAs). Given the large size and the need to make this very low cost I’d guess it is a silicon photodiode.
PCB and Package
And there is a chip here! Scraping the wax off I can see a Microchip logo and RE46C144 part number (Together with week 13 2011 date code). A search quickly turns up the datasheet for this chip. The datasheet describes the RE46C144 as providing all the features of a photoelectric smoke detector with minimal external components. These include driving the alarm horn, and indicator LED as well as the detector LED and detection circuitry and amplification.
Decapping the part reveals an interesting and attractive small die (Measuring 2.76 mm x 1.76 mm)Looking over the die it is made on a fairly large geometry two metal CMOS process (For mid 2000’s) I think 1.25μm gates.
The die marks indicate the part was designed in 2003 and they needed 11 versions! That is a lot of design and fab churn. Additionally the fact the V10 mark is still there (In M1 with a stripe of M2) and V11 is in M2 tells us that the the last revision only required a change in the top metal. (Thus the V10 mark could not be removed)
With the functional block diagram and the pin out in the datasheet I am able to make out some of the functionality
Starting with Vss (Ground) and then Vdd I was able to translate the pin out to the die photo
On the left side of the die you can see the large driver transistors for the piezoelectric buzzer – as well as the LED driver transistor.
On the top right you can see the detect pin (Output from the photodiode input to an amplifier whose gain is adjustable by external capacitors on pins C1 and C2. Two rows of contacted ‘ground’ shields surround the amplifier.
Although this is a CMOS process but they have used Bipolar devices, in this case almost certainly lateral PNP devices, which you can make using the normal CMOS process.
The four transistors is a classic layout for a bandage reference circuit. This voltage reference circuit is used in all sorts of analog circuits ADC, DAC, voltage regulators and in this case a comparator to compare the photo amplifier output to an internal reference voltage.