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I am working on a project right now, and it is the first time that I have worked with wiring anything. I have 70 LED lights. 10 red, 10 orange, 10 yellow, 10 green, 10 blue, 10 purple, and 10 pink. I know that the red, orange and yellow have a 2v drop, the green, blue and purple have ~3.7v drop, and the pink has a 3.3v drop. I want to wire all of these LEDs together into a total of 14 parallel circuits coming off of one series circuit. Each parallel circuit will have 5 LEDs of the same color. So there will be 2 parallel circuits of 5 red LEDs, 2 parrallel circuits of Orange LEDs, etc.

I would like to power the LEDs with 9 volt batteries. I am not 100% sure how many I need to get an appropriate voltage to power all of the LEDs. That is the first thing I could use a hand on.

I also have a handful of 200 ohm resistors. I would like to place resitors at the beginning of each parallel circuit so that each led in the circuit does not need its own resitor (I know that this is not the best way to do it if an led fails, but I am limited on resources and need this done semi quickly) I need to know how to calculate just how many 200 ohm resistors each parrallel circuit needs. The part that gets me confused is the different voltages for the different colors. I am not sure how to factor that in.

Thank you in advance! If any more information is needed, please let me know.

Edit: I could probably get away with 3.3 volts drop on the green, blue, purple, and pink so that they all have the same.

Each led takes ~25 ma

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  • \$\begingroup\$ Without knowing how much current each of your different LEDs needs, no-one could really answer this properly. \$\endgroup\$ – brhans Apr 21 '15 at 18:57
  • \$\begingroup\$ @brhans 25 ma per led. I will add it to the op. Sorry. \$\endgroup\$ – CKlinck Apr 21 '15 at 19:04
  • \$\begingroup\$ Does each LED take 25 mA as normal operating current, or is 25 mA the maximum allowed? \$\endgroup\$ – WhatRoughBeast Apr 22 '15 at 0:19
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I think you mean that you will put LEDs in series (ie daisy chain strings) not parallel. if you use 3 times 9 volt batteries in series that gives you 21 volts . a serial string of 5 green, purple or pink should work well with a 200 ohm in series . for the other colours (2v lEDs) a string of 10 LEDs with a 100 ohm resistor can be connected to the 21 volts . you can make a 100 ohm by wiring two 200 ohms in parallel.

If you must have 5 LEDS in a string so that you can switch in groups of 5 you can run 5 (2v lEDs) in series with a 600 ohm resistor . 3 x 200 ohms in series = 600. The group of 10 is better because it wastes less power.

The above should work . If you need to accurately set the brightness . you would need to use a larger value resistor to make a string dimmer or a smaller resistor to make a string brighter.

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You can find online calculators that will choose appropriate resistors for a given supply voltage, LED voltage drop, and current.

Note that you cannot in general run LEDs in parallel. LEDs do not have perfect matching in terms of voltage drop, so what happens is that one LED has a slightly lower voltage drop and therefore runs at a higher current than the other one. This can lead to different brightness in a single color and short lifetime.

You will likely find that the different colors have very different luminosity at a given current. A purple might need 20 mA to appear to be as bright as a yellow at 1 mA. I use a variable resistor to get the luminosity that I want and then measure the resistance with my multimeter to figure out what resistor to use.

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