IR hidden picture advice

[theyallhateme]

Hi! I'm fixing to build this:

http://makeprojects.com/Project/Take-a-Picture/625/1

I'm trying to simplify it a bit (no masonite, no circuit boards). I'm a little confused on the math, and hoped you guys could help me understand. As you can see, the tutorial kinda glosses over it.

The design I have uses 49 LEDs. I'd like to power it via battery if possible, to avoid ugly hanging wires. I don't need it on all of the time, just when showing it off, so there will be a switch too. (plus I think it's kinda funny to turn something "on" and nothing happens lol)

When I enter the specs into Circuit Playground, I get this:



What does "low power" mean? Is 9v not enough juice? Or do I not even need resistors?

Plugging in the specs at http://led.linear1.org/led.wiz gives me a couple different options (including 1/2W resistors?!). I'd like to get your opinions on the best way to go about this. Thanks!

[franklin97355]

in series 49 x 1.6 = 78.4v

[Stephanie]

I wouldn't run them in series; if one dies they'll all go out. And as Franklin has pointed out, when you add up the forward voltage drop of all 49 of them you're looking at voltage that gets into dangerous levels.

If I were you, I'd look to wire them parallel. Then your forward voltage does not have to be so high. With two AA batteries you could run the whole thing for roughly 2 hours. Or if there's space, two C or even four D batteries would run it a lot longer.

[philba]

As stephanie pointed out, parallel, not series. Don't forget to use a resistor per LED - i.e. don't try to use one for all 49.

You seem to want use a 9V battery. This is a bad idea because 9V batteries suck it big time. Very little capacity. Definitely take steph's advice on AA batteries if you want it to last longer. I would also consider using 4 AAAs and then use 16 parallel sets of 3 LEDs + 1 resistor in series. Add the 49th LED separately. The reason for this is 49 LEDs in series will draw 49 * 20mA - almost 1 Amp for about 2 hours of life. Where as 16 triples + 1 single will draw a third of an Amp for 3X run time (6 hrs). Plus, you only need 17 resistors instead of 49. The link you pointed to comes up with this plan when you plug in 6V, 49 LEDs, 20 mA, 1.6Vf.

[theyallhateme]

Thanks for the quick replies! I thought that tutorial was a little sketchy (speaker wire?!).

I'm not married to 9v, my original concerns were space & weight. The frame I have is a little shallow, so AAA's might be a better choice. Weight isn't a concern anymore, because I found these awesome super-strong velcro mounting strips yesterday.

The LEDs I have are marked "1.3-1.6", should I adjust my calculations? I want to get the right resistors, and since I have to get a AAA holder now... (I always seem 1 component away from a completed project )

As long as I stick to the parallel plan, this shouldn't get hot, right? I wanted to try a new mounting idea I saw http://hackaday.com/2012/03/06/interesting-substrate-used-to-position-leds-of-this-word-clock/ Now I'm chickening out and thinking about going with perfboard...

Whelp, looks like no bread-boarding today, unless I go to RatShack.

[philba]

Actually, AAA was a typo, I meant AA. Not much bigger in size and a lot more capacity. I very strongly urge you to consider scheme I suggested. Less wiring and much better use of battery capacity.

[theyallhateme]

Ok, gotcha. Just waiting for parts now...