Author Topic: LED picture backlighting circuit  (Read 3992 times)

Offline raynerd

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LED picture backlighting circuit
« on: January 10, 2014, 06:38:03 PM »
Evening, my father in law has an interesting project on the go and I'm trying to help him out. The idea and concept is quite unique but difficult to describe without pictures which I haven't got but ultimately there is a suspended 300x 300mm picture floating off the back board by about 20mm. We want to see what one looks like back lit with LEDs but have a few questions.

Just to ignore his particular full picture concept and describe what I'm trying to do in an easy to understand method: Imagine a standard flat picture is being susspended on a bracket 20mm off your wall...so there is a 20mm gap behind the picture. We want to mood light/back light from behind the picture on the wall.

Ideally we would like to run it off batteries so there are no messy cables running up the wall. In an amazingly ideal world we would love to be able to control the brightness. What LEDs would you use (8 - 2 on each  side?) ? How would you arrange the circuit? Could you calculate any run times before the batteries died?

Are there any better ways other than LEDs to backlight the picture directly from behind it half way up a wall?

Any help appreciated. I'll post pictures as this develops.

Chris

Offline Brass_Machine

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Re: LED picture backlighting circuit
« Reply #1 on: January 12, 2014, 05:26:07 PM »
I am sure someone smart out there can calculate how many LEDs needed by lumens etc...  However, if it was me, I would do it by trial and error. Whatever ends up being pleasing to the eye. The amount of LEDs, how far they are from the edge etc...

I think I can see this in my mind.

How do you propose to turn it off and on?
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Offline David Jupp

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Re: LED picture backlighting circuit
« Reply #2 on: January 13, 2014, 02:45:38 AM »
By 'back lit' - do you mean that you want light to show through the picture, spill out around the sides, or a bit of both?

I recently saw a picture simply placed to cover a small window in an exterior wall - it gave quite an unusual effect.

For even illumination (if important) you might lean to more lower powered LEDs rather then fewer high powered ones.

OLED lighting panels could be interesting (if available yet). 

A perspex panel with polished edges and inset LEDs might act as a 'light guide' though I'm not sure how good coupling you'd get between the LED and perspex.

To extend battery life (after testing the basic idea for the lamp first), I think I'd be looking at PWM control of the LEDs - check out high spec LED torches for examples.  You can make the eye perceive almost full power illumination whilst drastically reducing power used.

I'm sure others will be able to help with details.

Offline awemawson

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Re: LED picture backlighting circuit
« Reply #3 on: January 13, 2014, 04:05:23 AM »
OK Chris here is an "Off the Wall" suggestion - pun intended  :lol:

In many parts of the uk you are near enough to a reasonably powered transmitter to draw a few milliamps from a circuit tuned to the right frequency. Just a simple coil and parallel capacitor, diode rectification and you have a micro power source with no wires  :ddb:

It is even possible in urban areas to tune to 50 hz mains. Don't expect to be able to run fan heaters off it, we are talking a few milliamps.

Andrew
Andrew Mawson
East Sussex

Offline PekkaNF

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Re: LED picture backlighting circuit
« Reply #4 on: January 13, 2014, 02:17:39 PM »
Somewhat like this?:
http://www.enide.net/webcms/?page=power-pic-rgb-irda

I would try firs with cheap RGB stripe + remote controller to see how much leds/power you need. If I get this right, you are working with diffused and multicolored light with cheap power source....that is not an exact sience.

I have seen plenty on RGB led stripes with remote controller on local stores, something like this:
http://www.reichelt.com/LED-Stripes/LED-RGB-STRIPE/3/index.html?&ACTION=3&LA=3&ARTICLE=92596&GROUPID=3959

You said you don't like visible cables/fixtures. How about less visible, like using something like this:
http://www.ebay.com/itm/RGB-GU10-LED-Lamp-Light-Bulb-4W-3W-Remote-Control-16-Colors-AC100-245V-Top-US-/360835010638

And project it into back of the picture frame diffuser/reflector.

Light guides look simple, but are hard to get right on any bigger scale.

PekkaNF

Offline John Rudd

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Re: LED picture backlighting circuit
« Reply #5 on: January 13, 2014, 02:25:55 PM »
OK Chris here is an "Off the Wall" suggestion - pun intended  :lol:

In many parts of the uk you are near enough to a reasonably powered transmitter to draw a few milliamps from a circuit tuned to the right frequency. Just a simple coil and parallel capacitor, diode rectification and you have a micro power source.

Andrew

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Offline John Swift

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Re: LED picture backlighting circuit
« Reply #6 on: January 13, 2014, 03:38:34 PM »
Hi John

thats an idea if you live near this :-

http://en.wikipedia.org/wiki/Droitwich_Transmitting_Station

or  you can use this circuit to step up a single cell's output :-

http://en.wikipedia.org/wiki/Joule_thief
or
http://www.madscientisthut.com/pdf/JT_Kit011AOW_assembly_instructions.pdf

    John

Offline John Rudd

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Re: LED picture backlighting circuit
« Reply #7 on: January 13, 2014, 03:46:42 PM »
Ummm.... How about the tuned circuit followed by a charge pump circuit to lift the voltage?
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Offline John Swift

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Re: LED picture backlighting circuit
« Reply #8 on: January 13, 2014, 04:12:02 PM »

yes , but the joule thief will look brighter for the same power

2A for 1% of the time will look brighter than 20 mA for 100%
due to the way the eye responds to the intense pulses of light
I'll have to look closer at the data sheet to check the maximum current and pulse width


  John