Electronic circuits & projects forum - Need help with electronic circuits & projects?

[Swanky]

Hi,
I tried to do this very simply using the batteries and LEDs in the online design for LED Throwies, but these LEDs are so dim, a firefly would outshine them by a mile!

We are hanging paper lanterns in the trees at our wedding and I want to use LEDs in them so we don't have to try to run wires all over the yard. I figure we can put them up and they should run all night without a problem.

I know zero about this sort of thing. My plan is to look through sites that sell LEDs for the one with the brightest bulbs and the widest angle, and then I need to figure out what batteries to use and use the formula to figure out a capacitor to go in between. I am just not sure about any of that exactly.

I'd appreciate any simple ideas. I will solder what is needed and get to the electronics store for parts like battery holders and anything else needed. I just have to get this figured out in the next month or so.

Thanks!

[lem787]

Isn't this the best man's job?

One thing to ensure is that the battery voltage is greater than the voltage drop of the LED. This drop depends largely on colour and efficiency. Red, for example, typically drops about 1.7V so 2xAA batteries giving 3V would suffice. However blue and (more relevently) bright whites can drop around 3.4V, and in the absence of a convenient 3.4V battery you would have to go up to 3xAA. Obviously there are other batteries besides AA but I can't think of anything more cost-effective.

You mentioned a capacitor; 'tis actually a resistor you require.

Vs-Vf/If = R (Battery voltage - Voltage drop / Current = Resistor value)

[Swanky]

Yes, resisitor is what I meant.

I think I can get a AA batterie holder easy enough that holds 3 batteries. I want to use a white LED.

[lem787]

I would advise you to try a couple of resistor values. It's easy to assume with an LED that brightest (maximum forward current) is best, but modern 50,000mcd LEDs can put out surprisingly huge amounts of light and may rather spoil the lantern effect, turning it into more of a "photon cannon". Perhaps try 15mA and 30mA.

4.5-Vf/0.015=R dim
4.5-Vf/0.030=R bright

Assuming the drop is no greater than 3.5V, the resistor should be between 30 and 100 ohms. A 220R pot in series with a 22R resistor would let you fine-tune.

[Swanky]

Okay, speak s-l-o-w-l-y. I'm afraid the terms you are using I don't know.

Yes, I don't want to light up the woods, just give a nice glow to the paper lanterns. Something close to a 4 watt night light. in brightness. And knowing that LEDs are pretty directional. I may have to rig some sort of diffuser on it with tissue paper or something.

So, please break it down to me in terms of what sort of materials to get for the job.

[lem787]

The problem is this: LEDs vary so wildly that I cannot give you any "fixed" terms (minus the technicalities) without knowing what LEDs you are planning to use. I could suggest some, but I don't know if you have access to the same ones.

As a starting point; where are you from and do you have an ebay account?

[Swanky]

I'm in East Tennessee, USA. Ebay yes.

I can get materials at the local electronics store, and really, they may be super friendly and help me with this.

I bought LEDs using this site as a guide and got the wrong ones. I got the 105PWD which was way, way too dim. Is the lv value the brightness? Maybe if I got the non-diffused ones, they would do the job. They are about 8 times brighter.

[Swanky]

Okay, so, if I get these LEDs which have a 70 degree angle and 8-13k lumens, do you think that would do the job? And what would I need to power this?

[lem787]

The top two on that site will be fairly bright but are quite inefficient.
The "Iv@100mA" refers to brightness at a specific current. 100mA is alot of current for that brightness and will drain 3xAA batteries in a few hours.
The brightness itself is measured in millicandela, or mcd.

The reason I asked about ebay is because of these...

http://cgi.ebay.com/100pcs-5mm-20000mcd-LED-Lamp-Ultra-Bright-White-DIY_W0QQitemZ350172834733QQcmdZViewItemQQptZCycling_Parts_Accessories?hash=item350172834733&_trksid=p3286.c0.m14&_trkparms=72%3A1205%7C66%3A2%7C65%3A12%7C39%3A1%7C240%3A1318%7C301%3A1%7C293%3A1%7C294%3A50

Four times brighter and more efficient.

[Swanky]

What about these? This was suggested to me for the project. It has a 360 degree light, is bright and runs off 2 AA batteries.

[Swanky]

Sorry. Here is the right link with specs.

[lem787]

According to the datasheet those LEDs are <500mcd, which is pretty dim. Also 360'deg is quite deceptive, since you do not get the same intensity from all angles; at some angles there is barely any intensity at all.

Since this is for your wedding, it's all to do with the look and feel of the light and I don't think that can be interpreted with datasheets (certainly not by me). You don't want it to look naff and I think the only way you can achieve what you want is to get a small selection of different types and see what best suits your needs. If you need some help with resistor values at that point then I will be happy to lend some assistance but I can't accept the weight of responsibility for choosing the right lights for your wedding!

My apologies (and good luck)

[liveinlove2112]