Faux window: LED brightness dependant on outside sunlight levels.

[CanadaGuy]

Hi all,

I am renovating our basement for an in-law suite, but with only 2 small windows in an 800sqft area, I am looking for a way to get more "natural light" inside. Or more accurately, the FEELING of natural light.

My idea is to install frosted glass panes, backlit by LED's, on certain walls to mimic windows. This would be simple with a 120V switch to have them either on or off, but the plan ( possible or not) is to have them truly represent the light level outside.... ie: come up in the morning, dim at dusk, off overnight etc...

The idea is to have a 12V panel on the roof supplying power to some 12V LEDs. My noobie-sense says that wiring the (dimmable) LEDs directly to the panel, should result in power coming up and down relative to the sun?
But my brain says thats gotta be far too simple to work. Controllers,inverters,batteries must be involved to screw up my simple plan, eh?

How would YOU do this?

I'm not sure if I explained it very well, but I Googled a picture here. Imagine this was a basement, and you wanted that frosted glass above the vanity to look like a real window:

bathroom-interior-design-ideas.jpg

[Mike90250]

Fiber optic light pipe, or a "sun tunnel" skylight plumbed into the "window". The LED's will be hard pressed to give a good "daylight" light quality as the sky brightness varys.

[CanadaGuy]

Fiber optic light pipe, or a "sun tunnel" skylight plumbed into the "window". The LED's will be hard pressed to give a good "daylight" light quality as the sky brightness varys.

Thanks Mike,
But a "sun tunnel" skylight will not work in the basement of a two storey house. Same with Fiber Optic option.

But the second part of your answer is exactly what I am trying to do. I WANT it to vary with the sky brightness.

I guess my root question is:
If I had a panel on the roof connected to a lightbulb, would that bulb slowly brighten as the sun came up? Burn full power with sun, dim with clouds, then run down as dusk turned to night?

EX: a 20W 12V panel wired directly to one 20W 12V LED light strip (dimmable).

[daz]

If I had a panel on the roof connected to a lightbulb, would that bulb slowly brighten as the sun came up? Burn full power with sun, dim with clouds, then run down as dusk turned to night?

LEDs dim in a different way to conventional incandescent lighting. Incandescent lights will dim according to the voltage applied, and could work for what you are trying to do.
However, LEDs dim in a far different method! LEDs require a constant voltage, as the LED needs to be forward biased to switch on. They are dimmed by changing the current that is fed to them. Most LEDs are dimmed using PWM, as it is a cheap easy way to do it.

EX: a 20W 12V panel wired directly to one 20W 12V LED light strip (dimmable).

As mentioned above, LEDs require a constant voltage, and a variable current to dim (the varying current is what causes the LED to dim). This principle applies to both dimmable and non-dimmable LED lighting (as a point of interest, the only difference between a dimmable LED light and a non-dimmable LED light is the driver. The actual LED is the same in both versions!).
So connecting the LEDs straight up to the solar panel will not work. What will end up happening is; during dawn, the LEDs will stay off until the voltage has risen enough to tun on (has gone above it's on threshold). They will stay on till dusk, when the voltage starts going down. As soon as the voltage falls below the LEDs threshold, the LED will turn off. So....the LEDs will either be on...or off...no dimming.

The only way to make this work, if you still want to use LEDs, is to get involved in electronics and build a dimmer that will have a comparator to compare light levels, and adjust the LEDs current accordingly (using a PWM signal).

However, you could still use an incandescent light bulb to do this, as these bulbs will be dimmed/brightened by the changing voltage level from the solar panel. The disadvantage to incandescents is that they are very inefficient at producing light, which means you are going to need a bigger solar panel

[inetdog]

So connecting the LEDs straight up to the solar panel will not work. What will end up happening is; during dawn, the LEDs will stay off until the voltage has risen enough to tun on (has gone above it's on threshold). They will stay on till dusk, when the voltage starts going down. As soon as the voltage falls below the LEDs threshold, the LED will turn off. So....the LEDs will either be on...or off...no dimming.

The solar panels will produce an almost constant Voc independent of light level, with only the current available changing, so that part will be OK.
The problem you will have is that most LED controllers/drivers are built to provide a constant current independent of voltage, with the option of making that current variable via Pulse Width Modulation when the driver is fed from AC. If you have that kind of driver, you will have the problem described by daz.

What you could do is omit the sophisticated driver circuit and run the panels to the LEDs directly with a series resistor for current control (for the time that the PV is producing full output if that is more than the LEDs can handle.) Then as the current available from the PV drops, it will just drop the panel output voltage to the point where the LEDs are drawing some fraction of the available panel current.
Slightly wasteful because of the energy lost in the resistor, but straightforward to design. You will have to put the individual LEDs in series to get close to the panel Vmp voltage for best results. LEDs or strings of LEDs in parallel without each having its own ballast resistor will not be good as the current will not divide evenly and can burn out the LEDs one string at at time.
Depending on how accurate you want your simulation to be, you may want to experiment with using mixture of LED colors to get a red sunset effect. That would require active control or timer circuitry.

[Shockah]

How about this:

As in most Solar Light Combos, you would need:
Solar panel
Charge Controller
Battery

And instead of a night light switch, you could run (3) 12 volt timers...
Timer 1 = 6am to 6pm turns on 1 set LEDs
Timer 2 = 8am to 4pm turns on 2nd set LEDs
Timer 3 = 10am to 2pm turns on 3rd set LEDs

Also, there are dawn to dusk switches that could substitute for Timer 1.

You'll need to do simple math to calculate how big of a panel you'll need... and how big of a battery you'll need for consistent operation even on cloudy days.

I like the idea, CanadaGuy.