Ultra Low Current LED Project with Photo Sensor

Thread Starter

Lukeanddaisy

Joined Dec 2, 2014
8
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.
 

Thread Starter

Lukeanddaisy

Joined Dec 2, 2014
8
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.
 

ronv

Joined Nov 12, 2008
3,770
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.
 
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wayneh

Joined Sep 9, 2010
17,496
I'm trying to create an illuminated panel in a cabinet. The panel is about 4'' by 4''.
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.
 

wayneh

Joined Sep 9, 2010
17,496
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.
 
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ronv

Joined Nov 12, 2008
3,770
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

Joined Nov 30, 2010
18,224
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. :(
 

Thread Starter

Lukeanddaisy

Joined Dec 2, 2014
8
Maybe something like this would work. If you have 120 degree LED you won't need so many to illuminate the whole surface. These are pretty bright for the current. You might make 15 work, 3 in series to each battery (with proper resistor).
Watcha think?
http://www.mouser.com/ds/2/311/LB T773, LT T773 - TOPLED-334997.pdf

http://www.ebay.com/itm/BQY-11-1V-2...Control_Parts_Accessories&hash=item51c78d4e72

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?
 

Thread Starter

Lukeanddaisy

Joined Dec 2, 2014
8
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.

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?
 

#12

Joined Nov 30, 2010
18,224
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?
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.
 

ronv

Joined Nov 12, 2008
3,770
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.
 
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