# Options for solar rechargeable battery powering LEDs?

As part of my practice I make paintings and drawings with embedded LEDs controlled by an Arduino clone and LED driver. I showed 10 new pieces during an exhibit recently and had great feedback but there were a few of the inevitable comments about "the wire" (i.e. the wire to the wall wart transformer).

I'd love to lose "the wire" and power these through some combination of solar panel and rechargeable battery. I'm no electrical engineer but the bit of research I've done leads me to believe that this is no trivial technical task. Combine that with the fact that most installations of work like this in the home (office, restaurant, etc) would not be in an area getting full sun and that even if it were possible, hanging work in direct sun has a harmful effect on it. And then there is the additional cost, whatever that might be.

So...before I conclude that it is not viable (technically, economically) I thought I would ask for more informed opinions. Spec-wise, in these current versions, there are 64 LEDs (average 2.3-3.3v), an Arduino running at 5v and a Maxim7219 running at 5v. No realtime clock to sleep the piece.

Would it even be possible to charge solar cells/recharge a battery under typical indoor, non-direct sunlight lighting conditions?

• Yes, very slowly. But drop your Arduino (or better yet, bare AVR) to 4V instead. Commented May 7, 2014 at 3:23
• Can you show more pictures? Thats very interesting art indeed :) Commented May 7, 2014 at 3:46

Voltage doesn't matter without information about current.

You have 64 LEDs there, but at same time you probably have 50% of them turned on.

Typical LED maximum current is between 10-20mA. Your LEDs need 2.3-3.3V but your supply is 5V. You should reduce supply voltage to 4V (as Ignacio suggested in comment).

P = 4V * 15mA = 60mW

For 32 diodes:

P = 32 * 60mW = 1920mW = 1.92W

You will need 10W (thats at least 30x30cm size) or even much bigger solar panel to power it in building (no direct sun). Solar panel rated at 10W have maximum power of 10W, and thats rating for direct sun light.

So - technically - thats bad idea.

However you can try to extend time on battery with few small solar panels. They will look good in your art.

If you are really want to use solar power

You can use one of phone solar battery chargers from ebay. Prices start at 15USD, these cheap chargers are probably total garbage, but you can try. Everything is there - rechargeable battery, charging circuits, constant voltage output. Just disassemble it and put its guts into your painting.

You can reduce power requirements by adding some movement sensor to detect people in front of the painting. It may be infrared or microphone (needs less power).

Another idea - you can use photoresistor to measure light and make some adaptive brightness.

And for god sake - remove these wires during exhibition, use any kind of battery!!! :)

• A few well-placed Li-poly cells will go far. Commented May 7, 2014 at 4:30
• @Kamil - thanks. So the short answer is "no", and the long answer is a new hobby chasing elusive solutions. That's what I figured but thought I would ask people more knowledgeable about such things. If these were temporary installations, I could use batteries, sensors, etc. but it's not reasonable to expect someone to have to stand directly in front of a piece in order to experience it or to have another "device" to plug in every few days. So sadly "the wire" will remain. Surprisingly, I wasn't able to find white/light colored wall warts to mitigate the "wire" effect. Commented May 7, 2014 at 12:33
• @skinnyTOD Well, for permanent installations you can try these solar chargers anyway. LEDs will glow probably few hours per week, but without maintenance at all. This is not chasing elusive solutions. You can calculate how much energy you need, you can measure how much you have. No elusion. Just logic. You still can have very sensitive sound sensor and detect people presence in room, not only in front of painting and turn on LEDs for 15 minutes or longer. In my opinion - few hours of glowing per week is much better than wire. Commented May 7, 2014 at 13:34
• @skinnyTOD ... and glowing only sometimes may be interesting too. Let's say I have guest. After week he comes again and see that paiting has lights in it. How awesome is that? :) And one more suggestion - consider selling your paitings with piece of wall. Seriously. Big sheet of plywood, painted with same paint as wall. Or suggest to your customers to make groove in wall hide wire in it, then fix and repaint the wall. Thats not that expensive... Commented May 7, 2014 at 13:45
• @Kamil - ha ha - piece of wall. I have enough trouble selling work now ;-) What actually would be good is if I could find transformers which used flat ribbon cable so that the dreaded "wire" would be easier to conceal. I know I can hack something like that together but I'm not Underwriter's Laboratory approved... Commented May 7, 2014 at 17:01