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#23210 - 08/19/12 12:15 PM Third Light Bobcat stuck above 220
Cristian Adamo Offline
junior


Registered: 01/21/03
Posts: 32
Loc: Buenos Aires, Argentina
Hi there,

I have 8 light bobcats (among other modules) in an adnet.
About 3 to 4 months after initial installation, one of those light bobcats suddenly got stuck above 220 (320 un-corrected). Assuming the bobcat went bad, I just replaced it with a new one.

Another 4 to 5 months later, the same behavior with the new light bobcat I had replaced. I assumed it was just plain bad luck and replaced it again with a new light bobcat, but this time I installed it in a new room, on a different location along the Adnet chain.

Now, 5 months later, the same happens again.
All other modules work fine, including the other light bobcats. It is always the same bobcat (Adnet address 22). This can't be coincidence, but given that I tried 3 different light bobcats at two different location and places, I can't think of the reason.

I just replaced the wrong bobcat again and the new one works fine. I installed the bad bobcat in a temporary adnet with just once ocelot and it is still stuck. I checked the resistance between the sensor cables on broken bobcat and it does change with light, so I am assuming that the sensor is not the problem.

See link for a graph that shows behavior exactly when the bobcat started getting stuck around 220:

https://dl.dropbox.com/u/1872121/Untitled-1.jpg

Any ideas as to what might be causing this?

Thank you,

Cristian.-

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#23211 - 08/19/12 04:54 PM Re: Third Light Bobcat stuck above 220 [Re: Cristian Adamo]
Guy Lavoie Administrator Offline
Beyond All Hope
*****

Registered: 12/21/02
Posts: 6548
Loc: Montreal, QC, Canada
My impression is that the sensor (tip) itslef is going bad for some reason. You might get an idea if that`s the case by comparing the resistance between the wires of a good bobcat.

The part number of the sensor is a Perkin Elmer VT93N1.
_________________________
"If you don't know what you're doing, do it neatly..."

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#23212 - 08/19/12 08:31 PM Re: Third Light Bobcat stuck above 220 [Re: Guy Lavoie]
Cristian Adamo Offline
junior


Registered: 01/21/03
Posts: 32
Loc: Buenos Aires, Argentina
Guy,

I did the following meassures by placing the tips right where the sensor cables meet the PCB:

- On the "good" Bobcat, resistance goes from 50 Ohm (light) to 18.3K Ohm (dark).
- On the "bad" Bobcat, resistance goes from 50Ohm (light) to 10-12K Ohm (dark).

Is the difference from 10K to 18K supposed to make all that difference from 220 to 0?

Cristian.-

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#23213 - 08/20/12 03:57 PM Re: Third Light Bobcat stuck above 220 [Re: Cristian Adamo]
Guy Lavoie Administrator Offline
Beyond All Hope
*****

Registered: 12/21/02
Posts: 6548
Loc: Montreal, QC, Canada
It could, especially if the response of the light sensor is non-linear. I don't know enough about that part to say for sure.
_________________________
"If you don't know what you're doing, do it neatly..."

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#23215 - 08/21/12 05:57 PM Re: Third Light Bobcat stuck above 220 [Re: Guy Lavoie]
Cristian Adamo Offline
junior


Registered: 01/21/03
Posts: 32
Loc: Buenos Aires, Argentina
Guy,

I cut the sensor cable right in the middle and replaced the sensor with a 22K resistor. I now get a constant reading of 181 in CMAX (81 corrected). If I remove the resistor and leave the cables untouched, I get a constant 101 (1 corrected).

Two questions at this point, assuming this proved the problem is the sensor:
1) I don't know much about electronics. Where can I get a replacement part equal or similar to the Perkin Elmer? It doesn't seem to be readily available like other parts.

2) In your opinion, what can be the best way to "debug" what is going on with the sensors? The sensor is being placed outside through a whole in the wall but it doesn't get wet with rain. Even if that would be the case (which I sincerely doubt), can a couple of rain drops break such a sensor sensor to the point it doesn't work anymore? Exterior humidity is usually about 80-98% but I have another exterior sensor which has been running flawlessly for more than two years in a close by location.
I just can't understand what is going on and i refuse to just replace the sensor just to have it break again in a couple of months.

thanks for the constant help and support!

Cristian.-

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#23217 - 08/21/12 06:34 PM Re: Third Light Bobcat stuck above 220 [Re: Cristian Adamo]
Guy Lavoie Administrator Offline
Beyond All Hope
*****

Registered: 12/21/02
Posts: 6548
Loc: Montreal, QC, Canada
Your test does seem to indicate that the problem is likely the sensor itself. I cannot say what would cause it to go bad. The datasheet for it will normally give you the environmental conditions (temp range, humidity, etc) that is is meant to work in. Just google for "Perkin Elmer VT93N1 datasheet".

As to where to buy them, try various online sources. Finding one that ships to Argentina might take a bit more searching.
_________________________
"If you don't know what you're doing, do it neatly..."

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