A special project has me stumped.

[Luff]

Backstory:

I've inherited a project from a local foundation and need some help. The project involves adding lighting effects to a reasonably large shadowbox holding a memorial to friend who died a few years ago. The shadowbox will be a gift to the friend's family.

The first part of the project called for using 5mm LED's to backlight three pictures. 36 Wide-Angle Superflux LED's (12 per image) did the job. This was the easy part and it's done.

The power supply I have to work with is a brick, 120v AC in, 12v 3.5A out. The 36 LED's draw about 700mA total, which leaves about 2.8A excess. We have about $50 we could use to buy a different power supply if that's the better way to go but it would be better if we could use the one power supply.

The challenge:

The shadowbox will have three Fenix LOP flashlight heads aligned on the top front edge (hidden by moulding) pointing down onto the main object in the shadowbox. I need to provide power to those LOP heads. Since the LOP runs on a AAA battery, the voltage to the LOP needs to be between 1.2 - 1.5v ... and I don't know the best way to make this work.

I used a resistance calculator to see what resistor value would be required, and it suggests a 22 Ohm, 6 Watt resistor for each LOP (not that I know where to get those). It generated that number because I was thinking of wiring the LOP heads in parallel.

Those of you in the know ... is this the right way to do what I need done? Do you have better recommendations? We are somewhat strapped for cash and I have about 2 weeks to pull this off. Any help will be appreciated, and thanks in advance.

 

[matrixshaman]

Finding a 1.2 to 1.5 volt power supply may be a challenge. I personally wouldn't use a resistor as there is not enough control over other factors and good luck finding a 6 watt resistor - that would be a challenge in itself or anything close to that high short of some mail order specialty electronic suppliers and who knows how long that might take. So my first thought is that if you wired them in series and have a wall wart that is labeled about 3 volts or 3.6 volts - which should be easy to find as most cell phone chargers will be 3.6 volt. Then you check the voltage to make sure it's not much over 4.8 volts open (which is about what 3 fresh Alkalines would be) It might be a bit low on current for all three but I think it would work. Or possibly a Li-Ion battery charger - you would have about the right voltage as well as current control. Beware some chargers do put out more than 4.2 volts but check the output as above. Note cell phone chargers or AC chargers are typically DC out but check that and also note on them that the limiting charger is usually setup inside the phone. The wall warts are just a transformer with a rectifier and probably filter - which may be what's best as long as you aren't drawing too much from it such that it overheats the unit. Older chargers from the early 90's would probably work great. That's just some thoughts but you need to do some work on details to make sure you are not overdriving or overworking anything too much.

 

[Luff]

matrixshaman:

Thanks. Sometimes, I miss the obvious! Although I'd rather use a single power supply, using an appropriate 3.6v or so wall wart is probably the best bet and surely easier to put into play than other options. Finding one that'll pump out nearly 3A may be a challenge, but I didn't expect this to be especially easy. I'll definitely put the DVM to good use before plugging anything in.

Thanks for your quick response.

 

[Mr Happy]

Finding one that'll pump out nearly 3A may be a challenge

I doubt you'd need 3 amps. Can an AAA cell provide that much current when the LOP is installed in the flashlight?

 

[TorchBoy]

I have a spare cellphone charger but it puts out 18V unloaded, so yes, do be careful there.

I don't think it makes much sense to drop 12V down to 1.2V so you can bump it up to 3.6V again. I don't know what would be involved in taking the L0P heads apart, but how about bypassing the step-up boards and driving the LEDs in series with a suitable resistor? Is your power supply a regulated one or really big and very heavy (ie, with a transformer inside it)? Either way, a resistor will work well - the setup won't be changing and you don't have to worry about efficiency.

I find http://ledcalc.com/ quite handy.

 

[LumenHound]

If you do decide to go with another supply here's an inexpensive one that gives a regulated 3.3 volts at up to 4 amps. Driving those heads at 1.1 volts each would likely mean you don't have to worry about them overheating.

 

[J!m]

I know this idea kind of 'guts' your entire set-up, but maybe not...

How about finding some of those solar-powered sidewalk lights and taking them apart? Mount the solar panels on the top of the box, and the lights inside, and you have a self-contained maintenance free box!

I have no idea what those things run at, but it opens up many mounting options, and eliminates reliance on an A/C line to the box as well...

 

[TorchBoy]

Nice suggestion LumenHound. That looks like the least hassle way of doing it, better than taking the torch heads apart.

 

[Luff]

Mr. Happy - You're right. 3 amps is high. I seem to remember that the LOP can gobble down about 700 mA. I have to go find those specs and verify that number before I go plugging them in to anything other than a battery.

TorchBoy - The day will come when the heads have to be replaced. I expect a 1.5v equivalent flashlight head will still be around when the LOP isn't. While I'd be comfortable breaking down the heads to bypass the circuitry, I'm concerned that whoever follows in my footsteps most likely won't be ... so, simplicity rules.

LumenHound - that power supply looks perfect. I have a bit of scrounging to do to see if I (or local friends) have something comparable before I order, but I'll get that done tomorrow evening. Otherwise, that supply should work especially well.

J!m - what a great, creative idea. However, the shadowbox will be in a low-ambient-light room, which is why the lighting will be so dramatic.

I can't thank you enough for your advice and recommendations.

 

[waiwak]

If I had to do this, I'd go to RadioShack and buy two 10:1 transformers, such as these: http://www.radioshack.com/product/i...roductId=2102700&support=support&tab=features You feed 120VAC into the first transformer, and get 12VAC out. Feed that 12VAC into the second transformer, and get 1.2VAC out. BTW, ignore the center taps on this transformer. Then feed that 1.2VAC into a full-wave bridge rectifier, such as one of these: http://www.radioshack.com/product/i...&cp=2032058.2032230.2032269&parentPage=family Feed that into a capacitor to smooth things out, such as one of these: http://www.radioshack.com/product/i...ectrolytic+Capacitors&fbc=1&parentPage=family Done! Well, not done, you still have to wire it up, put it in a project box, add a power cord, output wiring, strain reliefs, etc... But still, with a few parts, you get from 120VAC to around 1.5VDC unregulated with only a handful of parts.

 

[Brlux]

Or skip the whole fenix head idea and find something else like 3 power led's of your choice on star boards with optic/reflectors, wire the 3 in series and using a small value resistor to set the current hook it up to your current 12V power supply. Probably a lot cheaper and easier. Depending on your drive current you may want to add a heat sink of some sort to the star boards.

 

[TorchBoy]

Where did you get those three L0P heads from? Was it kinda expensive?

 

[mdocod]

skip the LOP head, use a D26 style drop-in module with a buck style regulator that can handle the voltage you have available, or can make available for cheap.

ideally speaking, if you had the time, a set of 3 Cree stars, and the step down board from KD that can do up to 18V input would work perfect, (wire the crees in series) mount em to a piece of aluminum (like old CPU heat-sinks) and mount a small reflector over them. (cheap small reflectors also available from KD).... the problem with this advice, is you are probably talking 3 weeks just for shipping.

 

[TorchBoy]

... the step down board from KD that can do up to 18V input would work perfect ... the problem with this advice, is you are probably talking 3 weeks just for shipping.

And, very annoyingly, that board is out of stock, and has been for a while (certainly longer than the four weeks until shipping mentioned). It's a very nice board tho.

 

[Luff]

If I knew I'd be around to do maintenance on this, I'd build from scratch, use a router to create a channel in the frame, embed bare emitters mounted on aluminum bar stock as heat sinks and attach reflectors. Power options in that scenario are plentiful.

However, I'm an old fart and may well not be around WHEN the shadowbox lighting needs maintenance ... and finding someone with our 'ahem' particular skills to work on it some years down the road will be a challenge for the folks receiving this memorial. Basing the 'spotlights' on a flashlight that someone with less technical ability could find a replacement for seems the best option for now. The rate of development being what it is, my best guess today will likely be fodder by then, but what the heck.

TorchBoy - the LOP flashlights came from BatteryJunction.com and cost $30 a pop. I'm also donating mine. Battery Junction had some in inventory back in January and had the lowest price for new units I could find. They may still have a some. Money became tighter after buying the flashlights, but once the new owners saw my Draco, they thought that'd be a great way to go. :shakehead The search for a less-expensive alternative began once I mentioned it's cost.

I picked the LOP because its older tech, costs less than current Fenix models and is limited to a single level, which is a plus in this case. I'm sure there are other viable options out there, but I'm confident this one will work.

You just keep proving why I've been around this place so long ... gracious suggestions all. And yes, "that board" that's out of stock looks perfect for another little item I'm working on. Thanks for the head's up.