Kidde KN-COEG-3 carbon monoxide/gas alarm teardown

In continuation of Carbon Monoxide teardowns, we now have a Kidde KN-COEG-3 detector designed to handle both carbon monoxide and natural gas in residential use. Unlike the previous target (First Alert FCD2BT), this one never worked that well, causing frequent “GAS” false panics and piercing alarms, so a teardown is a natural progression for this model!

Display error messages

In this design, a separate wall pack is used to deal with AC mains, supplying 9V AC to the unit. The supply can either stay in the case or be used remotely.

9V AC transformer

Taking one screw out, we get the case to open:

One screw in the center holds the halves together

This huge (2"x0.5") cylinder is a Kidde branded CO sensor

Next to it is a small SMT board holding the natural gas  sensor from Figaro:

Gas sensor board from the back. Not a very clean assembly

Figaro TGS2611 gas sensor and a few passives

RNetworks underneath LCD

Neat raised display

Now on to the main board details:

Microchip PIC16, RE46C101 and an unknown chip under the label

Taking the label off, reveals yet another Microchip part:

Unknown chip turns out to be PIC16F628A

MCP602 opamp is processing sensor output.

5V voltage regulator for the logic

Not much is on the back of the board, just somewhat messy TH joints done with noclean solder as a post-operation.

Average solder quality here

Looks like a guard trace around sensitive nodes to capture any leakage

And a bt of a crappy soldering on the AC input pins

Very nice soldering job here!

And finally a block diagram of my limited reverse engineering effort together with a brief description of how this thing works:

High level block diagram

The power supply is very straightforward  an AC wallpack feeding a rectifier and a 5V regulator for the logic. Battery voltage is diode OR-ed in. The logic section is split into two microcontrollers. The older PIC16CR54C serves as a decoder for the 7-segment LED display. It received data serially over two lines (clk and data at about 58Khz) from the main micro, this time PIC16F628A. There is the usual R46C101 piezo horn driver, with enable line connected to the main micro. Its LED drive facilities are not used.

Sensing is done via two separate sensors. Natural gas detection is handled by Figaro TGS2611 sensor. The output is fed into an LM339 comparator and then into the main microcontroller. Carbon Monoxide is sensed by a Kidde branded sensor that appears to be electrochemical in nature, with only two pins coming out of it. The signal is converted from current to voltage by a low bias current Microchip MCP602 opamp and fed into the micro. Of interest is a MOSFET across the sensor, it is intended to keep the sensor shorted when no measurement is performed. That prevents sensor from damaging itself when not being used. I’ve not been able to locate the datasheet for the exact sensor used, but this Figaro model seems very similar. Note the 1-2nA/ppm output current, necessitating guard traces around opamp input node.The Figaro appnote is here 

A nice discussion thread for these teardowns is located over at EEVBlog. Thanks to the folks there for the feedback!



2 thoughts on “Kidde KN-COEG-3 carbon monoxide/gas alarm teardown

  1. Greeting,
    I have your Kidde Co2 and explosive model alarm model KN-COEO-3. We recently had a power outage and my alarm keeps beeping every 15 to twenty seconds. I replaced the battery and plugged it back in and it still beeps. Is my alarm bad and will I have to get a new one? Or do I have to try another battery? I bought this unit in 2012. Is there a special battery I have to use? Please advise. Thanks.

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