This second picture was taken with the LED's way down in intensity so the PCB could be seen. The top photo was taken with a smart phone and
gives a relative intensity. The object was to make this flag fully visable in day lithe with the sun shining.

This United States LED flag is on of my LED creations. The PCB is double sided and was fabricated by ExpressPCB. I used their free software to generate the files. It uses 5mm (T1-3/4) LEDs. It uses 397 LED's. The led's are connected in strings of led's in series. It uses 170 white ones, 34 strings. 91 blue led's consisting of 13 strings of 7 in series. It uses 136 red led's, with 17 strings of 8 led's per string.

The PCB is 9.25" long and 6" wide. The Power for the board is 24VDC. Each of the different colors has a LM317T adjustable voltage to allow adjustment of the intensity. There are 4 adjustment circuits. ! for the red stripes, 1 for the white stripes, 1 for the white stars, and 1 for the blue star field.

 

Very cool! Just two small suggestions if you ever make another one: There are LEDs with wider viewing angles. The ones in the photo, like many you find online, appear to be highly focused to a narrow beam. This makes them seem crazy bright when your eye is in the beam, but not nearly so much when you're off at an angle. Depending on the effect you want, wider angle LEDs might be nice. Second, controlling LED brightness with constant voltage instead of constant current is a dicey approach. LEDs will draw more current (and get brighter) at a constant voltage if they heat up. Since more current means more heat, they can go into thermal runaway. Your series resistors probably help prevent that, but you could also use the LM317 in a constant-current arrangement and then eliminate those resistors.

Again, very cool. Was it ready for flag day?

 

Very cool! Just two small suggestions if you ever make another one: There are LEDs with wider viewing angles. The ones in the photo, like many you find online, appear to be highly focused to a narrow beam. This makes them seem crazy bright when your eye is in the beam, but not nearly so much when you're off at an angle. Depending on the effect you want, wider angle LEDs might be nice. Second, controlling LED brightness with constant voltage instead of constant current is a dicey approach. LEDs will draw more current (and get brighter) at a constant voltage if they heat up. Since more current means more heat, they can go into thermal runaway. Your series resistors probably help prevent that, but you could also use the LM317 in a constant-current arrangement and then eliminate those resistors.

Again, very cool. Was it ready for flag day?

Thank you for your comments. Actually this project was completed a little over a year ago. I am aware different viewing area's available with led's. If I were to build another one I would shop for some different led's for the new flag. I thought of different ways to change the intensity and
decided on the present voltage adjust. The current drawn by each string is less than 2mA. I just checked the total current draw and was surprised to find it to be close to 150mA with the 24V input.
Ned

 

The current drawn by each string is less than 2mA. I just checked the total current draw and was surprised to find it to be close to 150mA with the 24V input.

Ummm, something doesn't add up! But your low current setup, assuming it's really only ~2mA per string, if why you can get away with voltage control. If you were trying to drive your LEDs at, say 18mA, and they are rated to 20mA, then you'd be far more likely to have a problem.

I think the regulators need 5-10mA minimum current in order to regulate properly. That doesn't explain your discrepancy.

 

If I were to build another one I would shop for some different led's for the new flag.

One trick that works is to just abrade or grind down the "lens" at the tip of the LED so that light diffuses from there. Low tech but effective. I suppose you could place a flat sheet of light diffuser over the whole thing but I don't think that would look quite as good.

I'm just going to take a second to say that the flag matters to me. The flag that draped my grandfather's coffin is in a case above my mantle. One hundred years ago, he was a young man of 25 sitting in the bottom of a muddy trench on the western front of WWI. He lived through what is regarded by many as the muddiest, most godawful experience man has ever wrought.

Yeah, it matters.

 

I've done similar LED display projects over the years. If I were going to do one currently, I would use diffuse surface mount LEDs, pack them tightly and try to scale the graphics as accurately as possible to get a more realistic image.

 

You need to align the top stripe with the top of the blue field - likewise, align the 4th red stripe with the bottom of the blue. Calculate the other line spacings from those two.

Good job, though. I'm waiting for a 4,000 LED flag with much more detail!

 

Ummm, something doesn't add up! But your low current setup, assuming it's really only ~2mA per string, if why you can get away with voltage control. If you were trying to drive your LEDs at, say 18mA, and they are rated to 20mA, then you'd be far more likely to have a problem.

I think the regulators need 5-10mA minimum current in order to regulate properly. That doesn't explain your discrepancy.

If you check the schematic, you will see a 120 ohm resistor between the output and the adjust terminal of the 317's. This means minimum
current of 10mA. I learned a long time ago that just because a led has a maximum current I don't need to operate it at that current.
I will typically use a 1.5k resistor for a single indicator LED with a 12vdc power supply.

 

You need to align the top stripe with the top of the blue field - likewise, align the 4th red stripe with the bottom of the blue. Calculate the other line spacings from those two.

Good job, though. I'm waiting for a 4,000 LED flag with much more detail!

Thanks for mentioning the alignment of the red stripes with the blue field. That is one thing I didn't consider when I did the PCB layout.