Night Light with white LED's

Discussion in 'The Projects Forum' started by mkbutan, Aug 12, 2012.

  1. mkbutan

    Thread Starter Senior Member

    Sep 30, 2008
    270
    16
    hi
    I would like to make one night light with 8-12 white LED's for reading the book's at bed time
    which should turn on when power is on and should turn off one by one (as rechargeable battery (1.2V 2100mAh * 4) ) go's weak it should run for about 1 or 1 1/2
    (may be some thing like Battery Tester , which shows the strength of the battery)
    pl help
    Should I use decade counter or the op-amp to design the ckt or something else
    I don't want to use the timer in the ckt.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,060
    Why? It would be an easy way to generate a clock pulse, which you'll need from something.

    Just to be restate your goal - you want 8-12 LEDs to turn on with the power switch, but then have one go out every 5-10 minutes? Reset on power switch off.

    First off, in my experience, you only need 3-4 LEDs for plenty of light. The commercial reading lights are like that. I have a newer one that uses only 1 very bright LED.

    Secondly, I'm wondering how this would work. If you start reading with 10 LEDs, how will it look when only 1 is lit? It'll get harder and harder to read, I think.

    I like the idea of having the impartial timer enforce bedtime lights off.
     
    mkbutan likes this.
  3. mkbutan

    Thread Starter Senior Member

    Sep 30, 2008
    270
    16
    OK 3-5 LED's will also work for me but not that 1 very Bright LED (it hurts)
    the reason i go to sleep while reading the book

    can i use the capacitor as a timer
    as the current in the capacitor discharge the led's go's off

    any help
    sos
    sos
    sos
     
    Last edited by a moderator: Aug 12, 2012
  4. Austin Clark

    Member

    Dec 28, 2011
    409
    44
    Good luck getting a capacitor that will work that way.
    You could maybe use an analog to digital converter to turn off the LEDs as the battery voltage gets too low.
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Use an LM3914.
     
    mkbutan likes this.
  6. mkbutan

    Thread Starter Senior Member

    Sep 30, 2008
    270
    16


    i don't mind charging the battery's daily
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,060
    What Austin means is, a capacitor able to do this job would be quite large, even if you use a circuit to only monitor its voltage as that voltage slowly decays. (It won't work at all to use a capacitor to power your lights). If you have to buy a big capacitor, you won't like the price. If you can find one in an old audio amp, TV or such, you might be able to do it. Without doing the math, I'd guess you'll need at least about 2,000µF to have a chance. More is better.

    To do the math properly, you'll need to know how much current will flow through your voltage detector. The capacitor will have to supply that current throughout the working time of your circuit.

    Another problem with the capacitor voltage decay is that it's logarithmic, not linear. If you trigger of a ∆V of say 0.5V, the first ∆V will be much faster than each succeeding interval. The LM3914, or maybe a similar chip, might be able to compensate for that by looking for successively smaller ∆Vs.
     
    mkbutan likes this.
  8. absf

    Senior Member

    Dec 29, 2010
    1,493
    372
    The LM3914 suggested by Mr Chips would fulfill all of your requirements. Very good choice. Study the datasheet attached.

    Allen
     
  9. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,060
    The similar LM3915 compensates for a logarithmic decay instead of linear, if the OP wants to use a capacitor as a timer.

    The datasheet suggests the input current is no more than 0.1µA. So the capacitor might have to supply as much as 0.1µAh to operate as a timer for an hour. 0.1µAh = 360µAs At 3V average, this is only 120µF. So a modest cap of 1000µF could be used in an RC timer with the IC itself drawing only a fraction.
     
  10. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    A SMPS current regulator will suck a battery dry before the LED dies, and will maintain constant light throughout. Problem is, it is not very good for the battery to be fully depleted.
     
    mkbutan likes this.
  11. mkbutan

    Thread Starter Senior Member

    Sep 30, 2008
    270
    16


    thanks
    but ti data sheet says :-

    The LM3914 is very easy to apply as an analog meter circuit.
    A 1.2V full-scale meter requires only 1 resistor and a single
    3V to 15V supply in addition to the 10 display LEDs. If the 1
    resistor is a pot, it becomes the LED brightness control. The
    simplified block diagram illustrates this extremely simple
    external circuitry

    Question is whether the 1.2*4=4.8 v/2100mA/h battery
    (Kodak Rechargeable Digital Camera Battery Ni-MH AA HR6 2100mAh 1.2V)
    with 5-10 LED's will glow sufficiently for reading the book in dark?
     
  12. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    One LED is enough to read a book in the dark, if it is a book light and next to the book. It sounds to me you are wanting a reading light for a room, basically a desk lamp. Since LEDs tend to be directional, it should be fine. Are you wanting to deplete the battery completely, or put it on a timer to preserve battery life? How portable are the batteries going to be?
     
  13. mkbutan

    Thread Starter Senior Member

    Sep 30, 2008
    270
    16
    yes i want the reading light which should run only for an hour or one and a half (60-90 Min.)
    There are 4 AA battery's
    1.2*4=4.8 v/2100mA/h battery
    (Kodak Rechargeable Digital Camera Battery Ni-MH AA HR6 2100mAh 1.2V)
     
  14. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,060
    Those batteries will provide hours and hours of operation with even 5 LEDs.

    Each LED uses about 15mA at 3v, or about 45mWh for each hour of operation. Each battery delivers 2100mAH at 1.2V or 2,520mWh. There will be losses due to circuitry and/or or current-limiting resistors, but still plenty of power.
     
    mkbutan likes this.
  15. mkbutan

    Thread Starter Senior Member

    Sep 30, 2008
    270
    16
    thanks but i want the ckt. to be on only for 60-90 min. with out timer!!!
     
  16. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,060
    Then you're back to the RC decay as your "clock" I mentioned earlier, paired with the LM3915 (or LM3914).
     
    mkbutan likes this.
  17. absf

    Senior Member

    Dec 29, 2010
    1,493
    372
    I simulated the circuit with 5 LED with proteus. When battery is 4.8V, all 5 are lighted but only 4 are lighted when the battery voltage drops to 4V. The current is almost as expected by wayneh ie 15mA per LED, but the consumption per LED drops when more LED are lighted (see circuit attached).

    I have not yet studied how to add the delay for 1 to 1.5 hours but I think it shouldnt be too hard.

    Allen
     
    mkbutan likes this.
  18. mkbutan

    Thread Starter Senior Member

    Sep 30, 2008
    270
    16
    thanks today i will make thic ckt. And will let u know.
     
  19. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,060
    If you insist on using the battery itself as the clock, I wonder if you couldn't use a single AA with a joule-thief circuit. This would drop the battery voltage much faster.
     
    mkbutan likes this.
  20. #12

    Expert

    Nov 30, 2010
    16,345
    6,829
    If the joule theif lasts too long, you can add a load resistor to make the battery fail sooner or use a AAA battery or maybe even a "button" battery.
     
    mkbutan likes this.
Loading...