Dumpster Dive Teardown #1 -- Aspirating Smoke Detector - Page 1
Forum section: electronics / general

author: TheAmmoniacal
author_rank: Unknown
timestamp: October 25, 2014, 04:41:19 pm
content: As a poor student, one of the highlights of the day is when a new item arrive in the E-waste disposal house, another opportunity for a new fascinating teardown. This morning I grabbed what I - at first glance - assumed to be a big power inverter. But which turned out to be an aspirating smoke detector, I had actually never heard of those before, and a lot of this was new to me. I'll post pictures for those that care, and questions for those that know more about this.Full aluminium chassis, the front tells you the meaning of LED light combinations.At the top there are two tubes for air (inputs I assume), power cable and some communication cables for communicating with a central.As these are made to be checked and maintained quite a lot, the front unsurprisingly just opens.The black panel is just a filter lifts up. On top you can see potted transformer, a PCB and a battery.A closer look at the PCB.There are actually two potted transformers here, the one on the PCB is 15V/1.66A while there's also one beneath it that's 15V/1.75A but which is wired in series with a 200 mA fuse. This other transformer is just wired to something inside the air tubes.On the other side of the unit is the air output and gas sensor, that you can see on the PCB to the left.The smoke sensor circuitThe components on the PCB. The microcontroller with the sticker is a Microchip PIC16C55A-4/P.The PCB has a light-depdendent resistor on it, which I can only assume is used to detect when the unit is opened/serviced (or breached).To the more mysterious part, the air inlet tubes both had a metal rod (or filament) which is wired to the 15V transformer through the device in in the tube (no idea what that is). Anyone else know what these rods are? Could they be just heating elements? (at 200 mA? Then why those thick wires?)That's all.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 25, 2014, 07:13:12 pm
content: Interesting. It looks like the sensor board can also be used as a standalone unit, there is place for a buzzer on the pcb.It looks like the thing inside the tubes is some kind of heating element, but I can't think of a reason why it is there.The device on the side of the tube looks to be some kind of overheating protection, it should have a temperature indication on it.

author: Vgkid
author_rank: Super Contributor
timestamp: October 25, 2014, 09:24:11 pm
content: I was wondering if they were using the heating element as part of a mass air flow measurement system, but I see no thermocouple.

author: BradC
author_rank: Super Contributor
timestamp: October 26, 2014, 06:10:55 am
content: Quote from: Vgkid on October 25, 2014, 09:24:11 pmNo thermocouple required if the heater resistance varies with the temperature. Just like a hot wire MAF sensor in a car.This looks like a cheap attempt to clone a VESDA system. The heater probably serves two purposes. One as a general air flow indication and the other to prevent condensation from interfering with the optical detection. These are often used for smoke detection in HVAC ducts. If it was used in a heating duct there is a good chance that hot moist air from the duct would cause condensation when pulled into equipment sitting in conditioned space.

author: SeanB
author_rank: Super Contributor
timestamp: October 26, 2014, 02:34:48 pm
content: From what I see it is a standard ( likely UL registered) optical smoke detector, on the round board. The fan blows air from the filter past it, then the air exhausts via one tube.The resistance wire in the tube is likely a fan failure or clogged filter detector, simply heating the resistor with enough current ( 200mA is enough to get it to work) to raise it a few degrees above ambient then measuring the resistance of the sensor as it heats up. Airflow will cool this, and it likely does a calibration cycle every so often by turning the fan off and allowing the sensor to cool for a few minutes while measuring till it reaches a stable resistance and then starts to heat and sees if the resistance rise, then turns on the fan and sees if it starts to cool. The clixon sensor on the tube likely detects that the tube is getting too hot ( likely a 70C switch if it is held in by epoxy) and cuts the power to the heater, and likely also trips an alarm as well of unit failure.Seeing the unit has comms to a control panel it also monitors the connection and will do a local comms failure warning, and it also has a pressure sensor to detect there is no blocked outlet tubing seeing there is a differential pressure switch ( or an actual differential pressure silicon sensor, easy to tell from the bottom if it has 2 connections or 4 or more) to detect if there is a pressure differential between ambient and the case inside. That is why there is a light sensor, to disable this detection when the case is opened for servicing.The battery is there as smoke detectors have to have a local power reserve, that operates even with no power for 12 hours to provide full functionality. Mains input looks good, with proper fusing and decent overvoltage protection. Only thing I do not see is a local alarm transducer.

author: atferrari
author_rank: Frequent Contributor
timestamp: October 26, 2014, 11:03:12 pm
content: Quote from: SeanB on October 26, 2014, 02:34:48 pmTo make aware who of what? No experience on these things at all.

author: SeanB
author_rank: Super Contributor
timestamp: October 27, 2014, 08:08:51 pm
content: If there is a smouldering fire that it detects you probably would like a local warning that you were in imminent danger of being the bread slice in the giant toaster. A distant alarm might not get your attention immediately.

author: BradC
author_rank: Super Contributor
timestamp: October 28, 2014, 07:20:18 am
content: Quote from: SeanB on October 27, 2014, 08:08:51 pmI've never seen a remote smoke detector mounted in the same space as people, so it'd be screaming to an empty equipment room. That's why we have fire indicator panels. That's like suggesting that every smoke detector on a building network have a local annunciator.

author: atferrari
author_rank: Frequent Contributor
timestamp: October 28, 2014, 10:55:42 am
content: Quote from: SeanB on October 27, 2014, 08:08:51 pmIt makes sense. Gracias.In other words: charity begins at home.

author: PointyOintment
author_rank: Frequent Contributor
timestamp: November 09, 2014, 11:13:02 am
content: The device attached to the tube in the last photo is a thermal switch.Quote from: TheAmmoniacal on October 25, 2014, 04:41:19 pm