Accutire battery change - caution

[mathog]

I have an Accutire tire pressure meter with a "permanent battery". It is many years old and it began showing "L" instead of a reading. That means low battery. The package said to return it to the maker if the battery ever failed. Since the manufacturer appears to be defunct (although similar meters are still sold), and the thing was only $10 when new, I figured I would take it apart and see what this mysterious permanent battery was. The unit isn't hard to disassemble, and inside there was a bog standard CR2032. So I replaced that. That eliminated the "L". Now instead it briefly flashes a 188.8 display (typical sort of LED warm up display) and then shows "01.0". No matter what the pressure. A bit of googling turned up several other people who had had the same experience. This one posted a nice set of pictures, and circuit description, so I'm not going to bother posting my pictures too, as they look exactly the same.

http://jf64.wordpress.com/2010/10/24/pressure/

I don't think the "01.0" is a result of either the electronics having been blown up or an air leak. There is a little rubber seal, and while it might leak a little, the whole thing is quite tightly held together when the screws are in. It might leak a hair, but certainly not as much as the air that leaks out around the front when trying to make a seal to the tire stem. Rather I think there must be some sort of calibration step, or reset, not the "zeroing" the minimal instructions describe, to get the thing to work when a new battery is installed.

Anybody have a guess what it might be? I tried holding it onto an inflated tire for a long time (20 seconds) and for a short time (.3 seconds?) but neither did it.

[lti]

Re: Accutire battery change - caution

Maybe a portion of the program code is stored in RAM or the chip with the transparent cover is sensitive to light and commits suicide when exposed to light.

[mathog]

Re: Accutire battery change - caution

Maybe a portion of the program code is stored in RAM or the chip with the transparent cover is sensitive to light and commits suicide when exposed to light.

I don't think so. The clear plastic cover goes over the LCD display, and that part looks passive - it has a line of conductors that are driven from the main board. There may be a few on board transistors, but I doubt it has much logic beyond that to speak of.

In the link above search for "The main IC (U1) is entirely covered in the black stuff". To the left is the relevant image. It doesn't look like a UV erasable part, or at least, if it was, it would have been very silly to cover it in black junk.

There's no way to change the battery without completely disassembling the device. (And I would happily take the bet that they were not originally assembled in the dark.) So my best guess is that there is still some sort of initialization process they apply to these things to calibrate them / get them to work. Kind of hard to guess what that might be though, given total lack of documentation!

[Krankshaft]

Re: Accutire battery change - caution

I had the exact same gauge did the exact same thing and got the same result.

I just ending up chucking the piece of crap and got one with an actual replaceable battery.

Like this:

http://www.autozone.com/autozone/acc...entifier=32830

All I can think of is some calibration setting is stored in volatile memory which is a horrible design.

[joshnz]

Re: Accutire battery change - caution

There is only one solution and that is to apply 3v to it with the old battery in it remove old battery with 3v still applied then insert new battery remove 3v supply re assemble.
To late for any one else that has already removed old battery.

[mathog]

Re: Accutire battery change - caution

... is a horrible design.

Probably horrible on purpose. My guess is that, knowing the battery would die eventually, they set the device up with a sort of booby trap, so that the user could not change the battery and thus avoid both of their future revenue streams: S&H charges and replacement sales.

[Krankshaft]

Re: Accutire battery change - caution

I never realized it their battery replacement "service center" is a short drive from me on Route 46 in Parsippany.

Maybe I should have gone down there and "returned" the piece of junk via slingshot through one of their windows .

[lti]

Re: Accutire battery change - caution

I don't think so. The clear plastic cover goes over the LCD display, and that part looks passive - it has a line of conductors that are driven from the main board. There may be a few on board transistors, but I doubt it has much logic beyond that to speak of.

In the link above search for "The main IC (U1) is entirely covered in the black stuff". To the left is the relevant image. It doesn't look like a UV erasable part, or at least, if it was, it would have been very silly to cover it in black junk.

There's no way to change the battery without completely disassembling the device. (And I would happily take the bet that they were not originally assembled in the dark.) So my best guess is that there is still some sort of initialization process they apply to these things to calibrate them / get them to work. Kind of hard to guess what that might be though, given total lack of documentation!

I was talking about U2. It is covered in "transparent goop."

If anyone still has one of these with a dead battery, they could try applying voltage across the battery terminals while replacing the battery. However, everyone that had one of these has probably thrown it away becuse the battery died.

[b700029]

Re: Accutire battery change - caution

Why else would they mount that part under normally dark conditions, and with transparent encapsulation? It is light-sensitive, probably anti-tamper serial SRAM to store calibration data. The microcontroller itself is probably mask-programmed since those are much cheaper for volume production than EEPROM or SRAM and it is not necessary to protect the code in MCU. Also since you say it shows "188.8" before "01.0" that also means MCU code is working.

I have seen high security design where all memory is SRAM, but that is rare and more expensive, with higher power consumption.

After battery is installed and the board assembled, the calibration fixture applies known pressures to the sensor, reads the sensor output, and writes the proper values to the SRAM via the test points.

RSA SecurID is similar example.

If you really want to figure out how to do that, good luck and may your oscilloscope be with you...

[larrymoencurly]

Re: Accutire battery change - caution

Is there some kind of reset that has to be shorted briefly after the battery is changed, as required with some Casio watches?

[guho]

Re: Accutire battery change - caution

I just took mine apart to do the same thing and I also get 188 followed by 01.0. Must be lost calibration data. You can still get these replaced from the successor company if you have the leaflet showing lifetime warranty on the non-user-replaceable battery.

If you go to http://measurement-ltd.com/index.html and send an e-mail explaining your model has the lifetime warranty and the L showing, they can replace it. I just did that with mine and am waiting for the Accutire gauge replacement.