battery backup for large digital counter

Discussion in 'The Projects Forum' started by skot9000, Mar 21, 2009.

  1. skot9000

    Thread Starter Member

    Jan 28, 2009
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    Hello,

    I am working on a digital counter to go in a display case at my school. I have built the circuit and it is working well. The goal of the circuit is to count down 5 billion years, in tenths of a second. (it is estimated that the sun will swallow the earth in 5 billion years. much more on this on wikipedia) The display is composed of 20 7-segment displays. Only 11 of the digits will change in 300 years, so each of those is wired with a 7447 74190 pair and hooked to a clock source derived from the AC mains. The other 9 digits are hard wired to show the appropriate number. It is possible this could run for 5+ years. I realize when significant figures are taken into account this project is ridiculous. It is meant for illustrative/entertainment purposes.

    My question is this; I would like to implement a battery backup for the current count, incase a janitor decides to unplug the display or the power goes out, etc. It is not a requirement for the counter to increment while on battery backup, only retain the current count. I have a few ideas, and am looking to see what the best route may be.

    - have a small battery and a circuit to write the current count to eeprom when mains are disconnected, then turn off. display would always load current count from eeprom when AC mains are turned on.

    - have a larger battery maintain power to all 11 74190's so they retain their count when AC mains are off. The batteries could also be trickle charged/maintained when the AC is on.

    - have some sort of flash memory always have the current count stored.

    I have some 12V sealed lead-acid motorcycle batteries that could prolly work. However I can't have any funky battery acid leaking all over the display case. Also the display case is not well ventilated.

    Of course the project budget is very limited. We have a electronic parts store room that was very well equipped in the mid 80's and hasn't been touched since. no fancy all-in-one ICs, LOTS of logic ICs, transistors, resistors and capacitors.

    your help is much appreciated!
    thanks,
    Skot
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    When you multiplex the display, only one digit is on at the time and the power consumption is lowered dramatic.

    Greetings,
    Bertus
     
  3. skot9000

    Thread Starter Member

    Jan 28, 2009
    18
    0
    Hi Bertus,

    I don't have any problem with all of the LEDs being off when in battery backup mode. in fact, I think it might be better since it will alert someone that something is wrong.

    my reasoning for going non-multiplexed was the abundance of free 74' logic and the fact that I have 11 digits. If there is a way to multiplex 11+ digits with standard logic, i'd love to know for version 2.

    Multiplexed would be cool since it may be very possible this project sees the ≈100,000 hour lifespan of LEDs otherwise. (long after I have left this school)

    thanks,
    -skot
     
  4. bertus

    Administrator

    Apr 5, 2008
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  5. SgtWookie

    Expert

    Jul 17, 2007
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    How about 4000 series CMOS logic? Are you stocked with those as well?

    The old '74 series TTL is pretty power hungry compared to CMOS.
     
  6. skot9000

    Thread Starter Member

    Jan 28, 2009
    18
    0
    Bertus, that link for the gel cell charger looks interesting.

    SgtWookie, yes, i think there is a pretty good collection of 4000 series chips. I don't suppose there is a counter pin-compatible with the '190 is there?

    -skot
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Not pin-compatible that I know of :(
    The 4029 could be a good bet for your next iteration.

    But if you really wanted to reduce your parts count, you could use a microcontroller. Once you get a programmer set up, the uC's themselves only cost a few dollars.
     
  8. skot9000

    Thread Starter Member

    Jan 28, 2009
    18
    0
    A CPLD would be pretty sweet too. but at least for this revision, the circuit is built. and since it was roughly a billion connections (all wire-wrapped! check out this pic) I'm not going to redo that all before the end of the semester.

    Any thoughts on which battery backup approach would be best? complete battery backup? storage on eeprom? flash memory?

    -skot
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Batteries will eventually need replacing. Sealed lead-acid will need to be replaced about every 3 years. For rechargeables such as NiCD or LithiumIon, it depends on how many cycles.

    If you are only going to store the current count to flash or EEprom, you might consider supercapacitors, charged by the power supply. Monitor the AC input from the transformer; if the AC dissapears, immediately store the contents of the TTL counters to flash/EEprom. On startup, restore values from the EEprom.

    Of course, you'll need quite a few logic ICs to implement all of that, and you'll have to maintain the supply voltage in the range of +4.5 to +5.5 until it's completed, or the TTL IC's will lose the data they contain.

    But you could use a microcontroller to clock some CMOS IC's and crank in the data to store it in flash memory...
     
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