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So I'm going to ask my girlfriend to prom and I'm currently in a digital engineering class in high school and I thought to myself hey what if I made a little board with some LEDs that lit up and said "Prom?". I have ordered some Chanzon 5mm 3v and 20ma LED diodes from Amazon, I'm not entirely sure how many LEDs I will need just yet but, I know I will be using more than 60 but no more than 100, I came into this thinking I would be able to just run them all in a series and it would light up. I have now gotten deeper into this project and I am in need of some help, if you can offer any help it would all be greatly appreciated, thank you

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  • \$\begingroup\$ I would highly suggest you get a "strip" of LEDs already arranged with a current limiting resistor, they commonly exist in 12v varieties that can then be powered from a variety of sources & are generally more forgiving than an individual unprotected LED \$\endgroup\$ – user2813274 Feb 25 '17 at 3:36
  • \$\begingroup\$ @user2813274 no reason other than laziness to do that. Op wants to build it, and it's simple enough to do. \$\endgroup\$ – Passerby Feb 25 '17 at 3:40
  • \$\begingroup\$ 60 to 100 is a lot of leds though. Power isn't to big of a problem, but how big of a sign are you thinking? And you definitely want to defuse them (sand them foggy) \$\endgroup\$ – Passerby Feb 25 '17 at 3:53
  • \$\begingroup\$ I was planning on placing a sanded piece of plastic or plexiglass or something of that sort and I'm planning on using 24 bolts a 9 volt and 4 double a \$\endgroup\$ – Jordan Scott Feb 25 '17 at 4:58
  • \$\begingroup\$ And the board I'm thinking of buying is 4.6 x 6.6 \$\endgroup\$ – Jordan Scott Feb 25 '17 at 4:58
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No, you won't be able to wire them all in series, but how you wire them depends a lot on what power source you intend to use. Do you want this to be portable, i.e. battery operated? How long do you intend to let it run? I image a fairly short time?

You could drive them directly without resistors as Matt suggested, but I would be careful about that - you might not get consistent intensity from them, and on the other hand you might also overdrive them, especially if you have them all in parallel at 3V.

You can minimize the number of resistors though with longer chains. Maybe a couple of 9V in series would work. Wire about 4-5 per chain plus a resistor in series. Assuming 3V each with 5 in series, you would have 15V. For nominal 9V batteries you would have about 18V, so 3V to drop 20mA. 150 Ohm resistor. You may not need 20mA though, so you could try resistors like 180, 220, or 330 Ohm. Having more in series should help keep them more consistent in intensity because the forward drops will average out somewhat in each chain. You can experiment along these lines to see what works best for you. And good luck!

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Thank you very much I have been thinking about it still and I ended up using 61 LEDs just for the sake of planning to set a for sure number I think I'm going with a series/parallel with the most being 8 in a column and then a resistor at the end and I found this nifty LED circuit generator led.linear1.org/led.wiz and I'm going to determine the needed resistor by just chasing the amount of LEDs that I'm using but keeping the same voltage and current. \$\endgroup\$ – Jordan Scott Feb 25 '17 at 4:40
  • \$\begingroup\$ I was wanting to do battery pack of 2 9 volts and a pack of 4 double a in the bottom of the box I'm making for it. Does this sound like it will work? If so is there a special switch I would have to buy or would just a regular one from Radioshack work? It wouldn't let me comment the whole thing so this is the second part \$\endgroup\$ – Jordan Scott Feb 25 '17 at 4:40
  • \$\begingroup\$ Cool site! The switch you use will depend on your final requirements. Why are you planning to use 2 different battery packs? Are the batteries going to wired in series? If so you would only need a SPST switch at a minimum. If they will be wired to LEDs separately, you would need a DPST switch. Either way, it would need to be rated the current you are planning. So e.g. if you have 10 chains of 6, with each drawing 20mA, the switch would need to be rated to allow at least 200mA. Most likely a switch you find at Radio Shack would be fine. \$\endgroup\$ – AngeloQ Feb 25 '17 at 12:56
  • \$\begingroup\$ There is an easy circuit editor that you can use to draw what you are planning if you want it checked over. \$\endgroup\$ – AngeloQ Feb 25 '17 at 12:56
  • \$\begingroup\$ I was planning on using two battery packs because wouldn't I need the total voltage of the amount of LEDs in the series, I was planning on doing chains of 8 because it's the size I would like and the letters come out looking great in design, I just figured I would need to do the 8 LEDs at most in the line and then multiply that by 3.4 which I need 27.2 volts or am I doing this wrong? \$\endgroup\$ – Jordan Scott Feb 25 '17 at 17:06

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