# LEDs on AC - Project Diagram - Help! -Isolated power supply question

#### TylerJ

Joined Apr 2, 2015
3
It's been way too long since I've done any circuitry, and I need some help on a project. I'm planning to make a custom sports sign that is lit with LEDs. I'm planning to drill holes in a board, push the LEDs through the holes and solder them together on the back. I want it to run on 110 V AC (plugging it into a socket). I know I will need an AC to DC converter to get me to 12V, but I don't know how many LEDs I will be using yet. Probably around 100. Is that even possible or will they be too dim? I also realize I will need resistors, but I'm guessing it's impossible to calculate the resistance without knowing how many LEDs I will use? And lastly, I want to implement a switch, and probably a fuse. Can anyone help me with a diagram for this?

#### #12

Joined Nov 30, 2010
18,217
The diagram is child's play...right after we know the specifications of the LEDs.
Start at the beginning. Define the conditions where the LEDs need to be seen.

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#### TylerJ

Joined Apr 2, 2015
3
I would like the LEDs to be bright enough for people driving by my house to see them lit at night. The LEDs I ordered are white, green, and blue. 20mA, 3.4V. I ordered 300 of them, but I don't think I will use nearly that many.

#### #12

Joined Nov 30, 2010
18,217
Not true. Different colors require different voltages...significantly different.

I suppose a white LED might be encased in green plastic, but that seems unreasonable because it is more expensive than making a green LED.

#### #12

Joined Nov 30, 2010
18,217
I seem to have shot myself in the foot. There ARE green LEDs and white LEDs within 1/10th of a volt of each other.

(3) LEDs in series will use up 10.2 volts. The remaining 1.8 volts will be used up with a 91 ohm resistor. Probably 1/8th of a watt for the resistor, but larger wattage will work the same.

The third drawing has a good section on the bottom, right. Three LEDs in series, with a resistor for each set of 3.

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#### elec_mech

Joined Nov 12, 2008
1,500
Welcome to AAC.

1. Can you post an image (or link to an image) of what it is you are trying to create?
2. How far away do you want the sign to be seen?
3. Will it only be on at night (in the dark) or do need it to be seen during the day in (full) sunlight?
4. Please post a link(s) to the LED's you've purchased so we can see their specs.
5. What are the overall dimensions of the sign?
6. Roughly how far apart are you planning to put the LED's from one another?
7. We assume your neighbors, HOA, and community in general will be okay with this?
Diffused LED's can be seen pretty well at night, but never in sunlight. Clear, high-brightness LED's work well in sunlight, but they can blind you at night if they aren't dimmed enough.

Depending on the size of the sign, you could use 1/8" diameter pegboard then drill out slightly larger holes assuming you're using 5mm LED's. Even if you want them spaced closer, this would save you a lot of measuring and drilling. I've done this with a few large 7-segment displays.

Depending on how far away you want this to be seen, you may consider using colored translucent acrylic cut into the image and placing the LED's behind to light the acrylic.

#### TylerJ

Joined Apr 2, 2015
3
1. I've attached a photo of my idea, and keep in mind, this doesn't have to be professional grade.
2. I'm not too worried about the distance, but ideally, about 50 feet or less.
3. I think it would only need to be lit at night. It will have paint behind it to show up in sunlight.
4. I ordered from 3 different suppliers on ebay, and didn't realize they could be so different. The white ones are "super bright" and the specs are here: http://www.ebay.com/itm/120644259405?_trksid=p2060353.m2749.l2649&ssPageName=STRK:MEBIDX:IT
The blue LEDs are diffused and the specs are here:
http://www.ebay.com/itm/171270124062?_trksid=p2060353.m2749.l2649&ssPageName=STRK:MEBIDX:IT
The green LEDs didn't have any specs listed on the purchase page, but the title says "100Pcs LED DIFFUSED F5 5MM GREEN COLOR GREEN LIGHT Super Bright Bulb Lamp L3"
5. The dimensions of the sign are 24" x 48".
6. The LEDs probably won't be more than an inch apart. I'm going to draw the design on the board and then drill along the lines as I see fit.
7. I think people here are going to love it.

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#### elec_mech

Joined Nov 12, 2008
1,500
The blue LED's should be perfect. The green LED's are going to be too weak I think. I have some from the same eBay seller and they are not bright - I went so far as to purchase several diffused green LED's from Digikey, but even the brightest diffused LED I could get was very bright. For green, you'll need a high-brightness LED. The caveat with ordering high-brightness LED's is they are often made with narrow viewing angles. A diffused LED, for example, can be seen from nearly all directions. Many high-brightness LED's have a 20-25 degree viewing angle which means you won't see much beyond looking at them dead on. So look for a green LED say with a viewing angle of 90 degrees or so (anything beyond 25 degrees would be great) and something rated 3500mcd or more. You shouldn't need anything over 10,000mcd. Beyond that you'll be looking into a spotlight effectively.

With all the above in mind, I think the white LED's you've selected will be too narrow and bright. They'd be prefer for making a flashlight, but as a display, I'm afraid they'll blind you. I'd suggest using diffused white LED's, something like this. These should be plenty bright for the distance.

A half inch apart or so should be good. A #10 machinist's drill bit should be perfect for 5mm LED's.

You could use a 12VDC supply, but you'll be limited to putting three LED's in series and with the number of LED's you'll need, your current requirement will add up quickly. If you could use 24VDC instead, you could put six LED's in series and reduce both your current load and the number of resistors you'll need to solder.

Once you figure out the total number of LED's you'll need in each color and what voltage you'd prefer to go with, we can help you select the appropriate resistors and power supply.