​ ​ ​ ​ Oversized 7 Segment Display
Page 1 of 3 1 2 3 LastLast
Results 1 to 10 of 28

Thread: Oversized 7 Segment Display

  1. #1
    Senior Member
    Join Date
    Nov 2009
    Location
    UK
    Posts
    1,066

    Default Oversized 7 Segment Display

    Despite the fact that I still haven't quite finished the last clock I was asked be build (just waiting on one last thing to arrive), I have been asked to build another.
    This time its a digital one with oversized digits. The display should be white in colour with each number being about 3 inches tall. should ideally run from AA batteries. This clock will be for some one who is partially sighted.

    At this point I am thinking 10 LEDs per segment, there is no need for decimal place, but there is for a colon separator between hours and minutes, second do not need displaying.
    It would be ideal if it was able to pick up the MSF (or similar) clock signal as well as keep it own time if needs be.

    The first hurdle I think is getting the seven segment displays to work as a normal seven segment. I am thinking that a transistor (maybe a mosfet will be better?) from each segment and a logic level mosfet to act as the common cathode would make the display work with most display drivers if I were to choose to go that route. It would be simplest I think to have a display driver, I am open to suggestion on components and circuit suggestions at this point.

    I quick glance through some white LED specs show they are around the 3v vf, so wired in pairs with no resistor for 6 AA batteries should give fairly uniform results across the hundreds of LEDs.

    I have no idea on how to deal with radio clock thing, it on the research list, but I suspect there are some here who have already dabbled in that.

    any thoughts on this, am I going about it wrong?
    Attached Images Attached Images

  2. #2
    Senior Member
    Join Date
    Feb 2009
    Location
    Cheshire, England
    Posts
    2,324
    Blog Entries
    4

    Default

    Hi oracacle, ( or should that be ClockMaker ),

    Do a few calculations.

    How many segments lit at once ?

    What current per segment ?.

    What capacity does an AA battery have ?

    I think you will find your customer will be spending a lot on batteries !.

    Cheers,

    Buzby

    Regarding LEDs, Check out some thing like this : http://uk.rs-online.com/web/p/products/2359382/

  3. #3
    Senior Member
    Join Date
    Nov 2009
    Location
    UK
    Posts
    1,066

    Default

    yeah I know, its an ideal. if its 23:59 20 segments will be lit, 10 LEDs per segment, that's the better part of 4 amps presuming 0.02A for each LED if they are all lit at once.
    If the displays are multiplexed so only a single display is lit at once a max of 70 LEDs will be lit which drops things to 0.14A plus drive circuitry, which is a wee bit more manageable.

  4. #4
    Senior Member
    Join Date
    Feb 2009
    Location
    Cheshire, England
    Posts
    2,324
    Blog Entries
    4

    Default

    Even at 0.14A ( 140mA ), an AA battery of say 2400mAH will last less than one day.

    The only technologies that can provide continuous displays at low currents are LCD and e-ink, but neither are good for high contrast.

    Maybe you could make the display triggered on demand, a bit like an LED watch, with a button or a sound detector.

    Your challenge here is not how to drive the LEDs, but how to reduce current consumption to an acceptable level.

  5. #5
    Technical Support
    Join Date
    Jan 1970
    Location
    UK
    Posts
    23,402

    Default

    AA and AAA batteries are rather impractical for powering higher current circuits for any continuous period of time. Assuming that 2400mAh rating, your battery life would be -

    1 day @ 100mA
    7 days @ 15mA
    1 month @ 3mA

    In terms of other aspects; is it really necessary to have 10 LED's per segment ? Fewer and a diffusing cover might actually work better.

    For people with poor eyesight, higher brightness isn't necessarily the best solution for making things easier to read.

    Two other alternatives are electromagnetic 7-segment displays like outside petrol stations displaying prices, or possibly a car odometer type display with digits on cylinders which can be rotated.

    There are also mains alarm clocks which have large 7-segment digit displays which could perhaps be modified to be brighter.

    Or, a 7" LCD display or monitor which can show a clock time. It might even be possible to use a digital picture frame and use a PICAXE to cycle through the images.

  6. #6
    Senior Member
    Join Date
    Jan 1970
    Location
    Perth, Western Australia
    Posts
    4,290

    Default

    So much wasted energy! So much computing power not being used!

    Perhaps you need a sensor to detect when someone is looking at the clock and only illuminate it when someone is .

  7. #7
    Senior Member
    Join Date
    Nov 2009
    Location
    UK
    Posts
    1,066

    Default

    He has wet macular degeneration, so big and bright and high contrast is exactly what he needs - its where the requirement for white LEDs came from

    as an idea he cans struggle to watch sport a 50" TV from a 2 meters and be able to read the score.
    I will speak with his daughter and see if I can wangle a wall wart in there instead batteries.

    I may produce a couple of prototype displays with different LED numbers, so 5 per segment and 10, maybe a an 8.
    I have some strip board that I could nock up some segments on for testing.

  8. #8
    Senior Member
    Join Date
    Feb 2012
    Location
    London
    Posts
    2,515

    Default

    Hi,

    Quote Originally Posted by inglewoodpete View Post
    Perhaps you need a sensor to detect when someone is looking at the clock and only illuminate it when someone is .
    A PIR might be a possibility. Or "face recognition" is quite advanced in modern digital cameras.

    Another way to improve power consumption (and clarity/contrast) is to flash the LEDs on and off, anything between 10 Hz and 0.1Hz (i.e. flashing on once every 10 seconds) might be appropriate. Also, I think you're being pessimistic about the current required for a "directly viewed" high efficiency white LED.

    However, I wouldn't normally consider battery operation for a clock like this, except perhaps one using "dayglo" mechanical digits/segments as suggested by hippy, which of course still needs an adeqaute ambient light source.

    If battery operation is essential, perhaps for safety (e.g in a bathroom or to avoid trailing cables), then I'd consider a "cassette/cartridge" system for the battery. For example two dedicated "battery packs" which are swapped between clock and mains charger every few days. The charger and clock would both be designed to NOT fully charge/discharge the cells, thus extending their lifetime to perhaps thousands of cycles.

    Cheers, Alan.
    Last edited by AllyCat; 27-11-2016 at 18:42. Reason: typo

  9. #9
    Senior Member
    Join Date
    Sep 2011
    Location
    Montpellier (FRANCE)
    Posts
    2,646

    Default

    Quote Originally Posted by oracacle View Post
    He has wet macular degeneration, so big and bright and high contrast is exactly what he needs - its where the requirement for white LEDs came from

    as an idea he cans struggle to watch sport a 50" TV from a 2 meters and be able to read the score.
    I will speak with his daughter and see if I can wangle a wall wart in there instead batteries..
    So you can use 144 LED/m strip
    1m-5m-APA102
    With 10 LED/segment, each segment will be 7cm long, and a full number 14 cm tall, and very bright !
    Power need : 3W per segment ==> 84 W for 28 segments ==> 5V 17 A...


    You also can use 5w-cob-900lm-3000k-warm-white 4.8 cm/segment

    Dimming via PWM seems necessary...
    Last edited by BESQUEUT; 27-11-2016 at 18:35.
    There are 10 types of people in the world: those who understand binary, and those who don't.

  10. #10
    Senior Member
    Join Date
    Oct 2012
    Location
    USA
    Posts
    442

    Default

    This should do it, runs on one AA too

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •