Carbon Monoxide detector teardown - Page 1
Forum section: electronics / general

author: reagle
author_rank: Unknown
timestamp: October 24, 2012, 05:23:55 am
content: Just did a quick teardown of an old First Alert FCD2BT Carbon Monoxide detectorhttp://kuzyatech.com/first-alert-carbon-monoxide-alarm-teardownComments are welcome!

author: amyk
author_rank: Super Contributor
timestamp: October 24, 2012, 07:23:29 am
content: R2 is probably the fusible type, and the board under it is probably cut out for safety reasons.Found this interesting line in the appnote at the very end...Quote

author: tom66
author_rank: Super Contributor
timestamp: October 24, 2012, 07:58:59 am
content: For that device, I guess the have to qualify it themselves for this function. I would assume the manufacturer is trying to avoid being sued if it fails to function somehow.I was kind of surprised to see a capacitive power supply. In a widget which has user-accessible battery terminals. Though it does look like you need to remove it to change the battery.Also, Lelon capacitor. Ugh. Hate them... seen too many die.

author: SeanB
author_rank: Super Contributor
timestamp: October 24, 2012, 11:31:21 am
content: Interesting to see what detectors you can get. Note that this sensor is also a little sensitive to alcohol and propanr/natural gas, so not recommended to rely on it alone, but also to have a gas leak detector to give warning as well in a kitchen.

author: madires
author_rank: Super Contributor
timestamp: October 24, 2012, 12:26:24 pm
content: Long time ago I built some gas detectors based on the TGS 822. That sensor is quite sensitive but drains a lot of power.

author: reagle
author_rank: Unknown
timestamp: October 24, 2012, 01:08:00 pm
content: Thinking of various failure modes, the way they connected the input MOV (RV1), is basically right across the AC terminals, with one thin trace on the line side. If the part fails shorted, it will take out that trace and essentially leave the device unprotected without any indication. Would'n't it be smarter to put it behind the fusible resistor?As far as the sensor- the one used in here is listed as having a 200mA heater current, though for a short period of time.And Lelon caps- I was under the impression they are not as bad as the usual suspects (Samwha and Su'scon), considering Mouser sells them these days?

author: reagle
author_rank: Unknown
timestamp: October 24, 2012, 05:06:10 pm
content: Speaking of gas sensors- does anybody know how a two terminal one works? Doing my next teardown of a combination CO and natural gas detector, and it uses a huge metal can (2" x0.5") with two terminals and an opening covered with Tyvek. I presume no heater in that oneThe part is coded with detector manufacturer part number, so either they make the sensor or it's branded for them. It's a Kidde 900-0437Other numbers on it are 09020901532 and 06487989Teardown coming up once I figure out how this last bit works!

author: SeanB
author_rank: Super Contributor
timestamp: October 24, 2012, 05:10:37 pm
content: It has a heater, you get a current change across a resistor on the output. It needs a stable input voltage.

author: reagle
author_rank: Unknown
timestamp: October 24, 2012, 05:14:51 pm
content: Quote from: SeanB on October 24, 2012, 05:10:37 pmIn a two terminal configuration? So you'd have to detect very small resistance change while supplying hundreds of milliamps through the same pins?

author: free_electron
author_rank: Super Contributor
timestamp: October 24, 2012, 05:16:49 pm
content: the new ones are electrochemical. they are basically a fuel cell.a chemical compound in the cell reacts with the incoming co gas and produces a current (NOT a VOLTAGE !!! ).send this current through a resistor and you can measure it as a voltage.these cells need to be shorted when not in use or they destroy their own electrodes ( a layer grows on the electrodes inhibiting further current flow and the cell is dead at that point ).out of the manufacturer they come delivered with a shorting clip installed. this clip is cut after soldering on the board.when measuring ytou need to use a good opamp as it is not allowed to send current into the cell as well. you need an opamp with low input bias current and current going IN to the opamp. do not use opamps that have bias current coming out of the input pins !i used a TGS5041 from figaro a couple of months ago in a project of mine ( multigas analyzer to test the quality of breathing mixture. one of the 'pollutants' tested for is CO.

author: reagle
author_rank: Unknown
timestamp: October 24, 2012, 05:21:20 pm
content: Ah, that one seems more like the one I have. Couldn't find 5041, but the datasheet forhttp://www.figarosensor.com/products/5042pdf.pdflooks very similar to the part in question. They feed it to MCP602 with 1pA bias current

author: tom66
author_rank: Super Contributor
timestamp: October 24, 2012, 05:58:19 pm
content: Quote from: reagle on October 24, 2012, 01:08:00 pmAt badcaps.net we have seen a few of their general purpose caps fail... mainly drying out (low capacitance, ESR usually only slightly higher.) I have pulled a few bulged ones from monitors. Note that Farnell also sell some Samwha caps. Whether or not a company stocks them has no meaning as to their quality.

author: reagle
author_rank: Unknown
timestamp: October 24, 2012, 07:36:02 pm
content: Quote from: tom66 on October 24, 2012, 05:58:19 pmHmm, I think I was looking at Mouser as some sort of curator of electronics parts. As in they would not sell crappy stuff

author: reagle
author_rank: Unknown
timestamp: October 25, 2012, 12:19:25 am
content: And here is the combo CO/gas detector teardown as promisedhttp://kuzyatech.com/kidde-kn-coeg-3-carbon-monoxidegas-alarm-teardownWhat's up with all Microchip designs in both of these things?

author: free_electron
author_rank: Super Contributor
timestamp: October 25, 2012, 01:42:42 am
content: You got it bass-ackwards...That big metal can is the fuel-cell monoxide sensorThe four wire element with heater is the gas sensor.

author: reagle
author_rank: Unknown
timestamp: October 25, 2012, 01:50:12 am
content: Quote from: free_electron on October 25, 2012, 01:42:42 amIt's hard to say without desoldering the Figaro sensor to look at the part number, and they do make electrochemical gas sensors. But you may be right- that would explain the lack of pulsing on the heater pinsEdit: yep, you would be correct. Figaro part is TGS2611- a methane sensor. Thanks for catching it!

author: T4P
author_rank: Super Contributor
timestamp: October 25, 2012, 02:41:07 pm
content: Quote from: reagle on October 24, 2012, 01:08:00 pmAll bad.See this? They were all taken from bad equipment and replacing them with Rubycon saved them from the binSome of them are Lelon but it's not so obvious