Ultra Low Current LED Project with Photo Sensor

Discussion in 'The Projects Forum' started by Lukeanddaisy, Dec 2, 2014.

  1. Lukeanddaisy

    Thread Starter New Member

    Dec 2, 2014
    7
    0
    Hi all,

    I'm new to this forum and after spending a bunch of time lurking and researching, I am no closer to finding a solution. I'm working on a pet project and I'm trying to create an illuminated panel in a cabinet. The panel is about 4'' by 4''. I want the panel to be completely illuminated but to only come on light to come on when it gets dark; basically just be illuminated at night. The problem is I only want the panel to be illuminated with two or four AAs and I'd like to get at least 30 days, if not more, out of them. When doing the calculations and taking into account battery capacity and looking at the draw of bright LEDs (at least 25 mA) I can't figure out a way to do this. Does anyone have any ideas? Thanks for the help.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,148
    3,058
    Define "night". Do you want a couple hours at dusk or all night long? The latter will be tough with even 4 AAs.
     
  3. Lukeanddaisy

    Thread Starter New Member

    Dec 2, 2014
    7
    0
    When I use "night" I'm referring to approximately 10-12 hours per day. I could go up to 4 C or even D cells which would give me more capacity but still not the amount of time I'm shooting for.
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,292
    1,255
    Not all LEDs are equal, nor are batteries. What color are you looking for? What do you have so far.
    I'm thinking super bright and Lipo.
     
    Last edited: Dec 2, 2014
  5. wayneh

    Expert

    Sep 9, 2010
    12,148
    3,058
    A single superbright LED at 5mA could do a pretty good job of lighting that up, at least to see it but not to provide light. To be seen, but not to see, as they say for mobile lights.

    Does that sound about right?

    Thirty days at 12 hours and 5mA is 1,800 mA-hrs. That's very doable with 3 or 4 AAs in series. My rechargeable AAs are rated ~2,400 mA-hr.

    You could just hack a solar landscape light to make this an easy build. It'll control the on-off for you.
     
  6. Lukeanddaisy

    Thread Starter New Member

    Dec 2, 2014
    7
    0
    White LEDs. Trying to illuminate a transparent plastic cube about 4" wide and 4" long and about 1/2" thick.
     
  7. Lukeanddaisy

    Thread Starter New Member

    Dec 2, 2014
    7
    0
    By the way, the idea is simply aesthetic rather than serving to provide illumination for anything.
     
  8. Lukeanddaisy

    Thread Starter New Member

    Dec 2, 2014
    7
    0
  9. wayneh

    Expert

    Sep 9, 2010
    12,148
    3,058
    Maybe, but it makes me nervous that they don't even mention dispersion angle or brightness measurements. Those are important!

    Something like this might be better. Note though that the high brightness rating comes from focusing the beam to a narrow angle. You may need to look for wide dispersion unless you can use a diffuser of some kind. Maybe this.

    Are you thinking of using a CdS sensor? It should go where it can see ambient lighting.
     
    Last edited: Dec 2, 2014
    pilko likes this.
  10. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,292
    1,255
  11. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,292
    1,255
    wayneh, can't get your link to work, but I like the idea of the diffusor. Maybe just some fine sandpaper on the surface of the plastic.
     
  12. wayneh

    Expert

    Sep 9, 2010
    12,148
    3,058
    I think both links are fixed now.
     
  13. #12

    Expert

    Nov 30, 2010
    16,337
    6,820
    Not even thinking about the energy budget.
    Edge light the plastic and damage one surface (with sandpaper?) to get an evenly lit square?
    I mean, perfectly clear plastic doesn't present as a panel of light. It looks more like clear plastic with bright edges.

    Well, ronv beat me to this one. :(
     
  14. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,292
    1,255
    Those are hot and the price is right. Almost to good to be true. :cool:
     
  15. Lukeanddaisy

    Thread Starter New Member

    Dec 2, 2014
    7
    0

    Ronv,

    Would I need that many to light something 4" by 4" and about 1/2" thick? What I'm going for is the appearance of something like the apple on the back screen of a macbook, just a bit larger. The key is an evenly dispersed glow without the spotlighting. I'm looking to use plastic like this:

    http://www.tapplastics.com/product/plastics/cut_to_size_plastic/acrylic_sheets_color/341

    Also, I noticed that the draw on the LEDs you suggested is 20mA but that amount would scuttle my goal for 30 days (or more) of battery life. I know that a resistor can cut down the current going to a LED but isn't the power still being drained from the batteries at 20mA?
     
  16. Lukeanddaisy

    Thread Starter New Member

    Dec 2, 2014
    7
    0

    I'm open to any type of sensor that is cheap and effective and is simple to put together. My problem is that I want to put the battery pack, LEDs and the sensor all in the same small enclosure. The problem being, how do I put an ambient light detection sensor next to the light source. Won't the sensor turn off the lights as soon as the lights turn on?
     
  17. DickCappels

    Moderator

    Aug 21, 2008
    2,659
    632
  18. #12

    Expert

    Nov 30, 2010
    16,337
    6,820
    Yes, you can make an oscillator like that, but hiding the sensor from a couple of LEDs shouldn't be all that big of a problem.
     
  19. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,292
    1,255
    Ahh, just a soft glow. My suggestion would be to buy the sample squares in 20% and 40% and a couple of LEDs with 120 degree dispersion angle. We can then see how much current is needed to get the light you want. My guess is it may be as low as 3 or 4 ma. Or you may find that you can make the enclosure just a little deeper and cut the number in half with the larger spot - or both.
    If we use a higher voltage battery and put say 3 LEDs in series only 1 resistor is needed for 3 LEDs so less current is wasted. We could also build an inexpensive circuit to eliminate the resistor, but it may be adding complexity for very little gain. If we use LiPo batteries they can be recharged - does that help?
    Don't worry about turning it on and off that should be pretty easy and less than $5.
     
Loading...