Low power accurate 6 month timer - suggestions?
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  1. #1
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    Default Low power accurate 6 month timer - suggestions?

    Hi folks,

    I'm looking for suggestions on achieving the lowest power, but accurate, timer to last around 6 months.

    It will be at the bottom of the ocean and I have minimal real estate to fit the timer inside an acoustic device so a RTC is out of the question. This device will initially operate for an hour as confirmation on deployment and then sleep (off, powered by the PIC/mosfet) for a period of 180 days. After this the device will be powered up and remain so until its battery supply is exhausted (approx. 2 weeks).

    I've looked at DT's Elapsed Timer but sleep command results in havoc with the interrupts it seems, so I'm thinking an SMD 12LF1840 running from a coin cell counting wake-ups from 10 second sleep events could be an avenue. Some quick and dirty code sees about 10uA average consumption using a 4Mhz crystal but speed isn't an issue, but power, size and accuracy is.

    Does anyone have any suggestions other than above to offer?

    Thanks and regards to all,
    Bill

  2. #2
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    Default Re: Low power accurate 6 month timer - suggestions?

    My first question:

    How accurate is "accuracy" ?

    For example, for 180 days of target, is +/- 20 days acceptable?
    "If the Earth were a single state, Istanbul would be its capital." Napoleon Bonaparte

  3. #3
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    Default Re: Low power accurate 6 month timer - suggestions?

    Hi Sayzer, thanks for the quick response.

    No, +/- 20 days is way off the target - I'd like to think +/- minutes or, worse case. hours would be a goal. What do you have in your bag of tricks?

    Regards,
    Bill

  4. #4
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    Default Re: Low power accurate 6 month timer - suggestions?

    I have Option 1 and Option 2.



    If the accuracy was not critical, Option 1 could work;
    Still, it may provide some accuracy but requires confirmation & tests.

    Option 1:

    Name:  12f683_UWAKEUP.png
Views: 120
Size:  175.1 KB


    As the accuracy is critical, then the following idea should provide good results depending on the ppm of your crystal; Very cheap and very easy way.

    Option 2:

    Name:  12F675_Wakeup.png
Views: 117
Size:  173.1 KB

    Sorry for the big screen shots; it comes from 4K monitor.
    "If the Earth were a single state, Istanbul would be its capital." Napoleon Bonaparte

  5. #5

    Default Re: Low power accurate 6 month timer - suggestions?

    If you have room for coin cell and that pic then you probably have room for

    https://datasheets.maximintegrated.com/en/ds/DS3231.pdf

    it has 2 alarms on it which I think will do what you want, it is also quite accurate.

    I use this on a Pi

    eBay item number:
    273773628089

    Many ebay suppliers have this or similar.

    Just under 15mm square, and 8mm high - just remove the unwanted connector.
    you could remove the battery and use a slightly larger one if necessary or whatever powers you main PIC.

  6. #6
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    Default Re: Low power accurate 6 month timer - suggestions?

    If you need low power, accurate, and small part I'll go with microcrystal RTC.
    3.2x1.5x0.8mm, 40 nA @ 3 V, ±1 ppm @ 25°C, Fout 32768...to...1Hz, alarm with int pin.
    Need more?

  7. #7
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    Default Re: Low power accurate 6 month timer - suggestions?

    Thanks for the suggestions, for best flexibility I think sayzer's second method is the way to go although the first method is one to remember for other projects. That 4k monitor sounds good...

    @ aerostar that part looks a bit hungry if I'm reading the spec sheet correctly.

    @ pedja089 that's a really cool chip, thanks - samples on order!

    Best regards to all,
    Bill

  8. #8
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    Default Re: Low power accurate 6 month timer - suggestions?

    If it is critical I would stay away from ebay suppliers. May get counterfeit or seconds (rejects)

    What about temperature / pressure / humidity inside the vessel at the bottom of the ocean?

    Also might consider a redundant system, again if it is critical and costly to get in place and what cost if it fails??
    Dwight
    These PIC's are like intricate puzzles just waiting for one to discover their secrets and MASTER their capabilities.

  9. #9
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    Default Re: Low power accurate 6 month timer - suggestions?

    Hi Heckler,

    One would like to think the pressure and humidity inside the vessel would remain as they were when it was sealed at ambient levels - anything other than this means there's a leak and the contents kaput... It's a shallow deployment and temperature will vary but previous WT readings in this area have indicated nothing extreme or wild swings from one season to the next.

    It is not a critical item and if it fails, or the timing goes to mud for whatever reason, then it isn't a catastrophe. This is actually a trial that will lead to larger expansion of same.

    Thanks for your input, appreciated.

    Regards,
    Bill

  10. #10
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    Default Re: Low power accurate 6 month timer - suggestions?

    If you use an 16F18326 like I'm using for my remote temperature sensor, and setting the WatchDog timer for the maximum of 18388608 or 256 seconds you can count to 60750 which is still a word size variable and that would be 180 days. I put the Micro into deep sleep mode and only draw about 4uA. total. I have used a 3 volt regulator and a 9 volt battery for the power source. That will allow the battery to be reduced to a level of about 3.3 volts before replacement. I have used an MCP1703A regulator. It is still working after 3 months and the battery voltage is only to about 8.9 volts. Too bad you don't have room for a 9 volt battery.
    Dave Purola,
    N8NTA
    EN82fn

  11. #11
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    Default Re: Low power accurate 6 month timer - suggestions?

    Hi Dave,

    Thanks for sharing. Yes I don't have the luxury of a large battery... I'll look into other PICs but I'm not sure the WDT timing is accurate or reliable enough for any real confidence in long term timing.

    As I mentioned, this is a trial and I'll continue to investigate other methods during the time the instrument is deployed. The other alternative is to use an external battery canister but this means subsea connectors and the problems that they come with (apart from initial high cost).

    Cheers,
    Bill

  12. #12
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    Default Re: Low power accurate 6 month timer - suggestions?

    How you will get data from bottom of sea?

  13. #13
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    Default Re: Low power accurate 6 month timer - suggestions?

    Bill, Here is the connector I used on my TUBSUB project for COHO fishing. I have used it to depths of 250Ft. with NO leaks. I hope this might help for your future builds.
    Attached Images Attached Images   
    Attached Files Attached Files
    Dave Purola,
    N8NTA
    EN82fn

  14. #14
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    Default Re: Low power accurate 6 month timer - suggestions?

    Hi Dave,

    Thanks for the info, that's a nice looking bit of kit you've produced there. I'll look up the manufacturer of those connectors and see if some $$ can be saved on the usual subsea connectors we have been using to date.

    @ pedja089 data will be downloaded when the instrument is recovered at the end of its deployment, this is not a real-time system.

    Regards,
    Bill

  15. #15
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    Default Re: Low power accurate 6 month timer - suggestions?

    Thanks for info. I'm wondering how much devices will be recovered...
    Also please report your experience with microcrystal RTC. I have one dev board on shelf, but newer tried...

  16. #16
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    Default Re: Low power accurate 6 month timer - suggestions?

    No worries, I'll surely report back on the RTC - samples are on the way but unknown delivery date at this time. It won't be used on this deployment test but may be on future improvements to the timer device.

    Not sure what your question is on devices recovery. It is a seabed frame holding the acoustics and other instrumentation deployed by divers for level surface (minimal tilt) and recovered by same after 6 months. The timer is to turn on an acoustic pinger so the frame can be located if it is dragged or moved during deployment - the authorities have decreed the pinger cannot run continuously due potential harm to marine species... this is BS but we have to comply. I hope this clears any confusion.

    BTW I'm running the 12LF1840 with IntOsc at 4Mhz on 3v0 coincell with Sleep and 1hz Int's with 1min updates serially for time confirmation and seeing an average current of 8.4uA (with serial line removed) - this on a breadboard so pcb may be better still. Damn impressive I think.

    Regards,
    Bill

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