# LED series/parallel array wizard

Hello Everyone,

http://led.linear1.org/led.wiz

I found this LED series/parallel array wizard recently. According to it, I can power 70 LEDs with 9 volts. Is this legit?

If 9 volts can power 70 LEDs does this mean that an arduino board, powered by a nine volt battery, can power a series of 70 LEDs?

The wizard allows you to use the color of your LED to determine the diode forward current. Is this a safe bet? I have three packages of LEDS I purchased long ago-one red, one green and one yellow and I don't have the specs for them. The packages just say 3RD, 3GD and 3DY.

Be well,

JLK

I don't think using the color to indicat the forward voltage is a very accurate way to do it.

Also running 70 LED's from a 9 volt battery, if it works at all it will not work for very long, 9v batteries don't hold much juice.

You can definitly not run 70 LED's directly from An Arduino board.

Any Arduino pin can source a absolute max of 40mA, but the voltage regulator on the board have an overall max of about 200mA without a heatsink. Even with a heatsink 70 LED's will draw too much current.

So i think an external powersupply of some kind is required.

Thanks MikMo,
Based on your post I have other questions.

These three packages of LEDs I have - I need to determine their forward voltage more accurately. Can you suggest a way to do this or an online resource that would help?

Is there an online resource that could help me determine how much power I need? There's likely a formula I should have memorized by now, right?

I've programmed the arduino to make the lights blink to a customized pattern. Because I want the arduino to turn the lights on and off on cue, I'm unclear about how to integrate an additional power source that likely will always be on/high. Any suggestions?

Thank you!

In the absence of a data sheet the only way to get the forward voltage is to measure it at a set current. This link shows you how to calculate resistor values and the thinking behind the formulas.
see:-
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

I'm unclear about how to integrate an additional power source that likely will always be on/high

Just add it on and power the LEDs from it. See:-
http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

"In the absence of a data sheet the only way to get the forward voltage is to measure it at a set current."

I'm sorry, I don't know how to do this. Could you explain?

I have one a low-end voltage meter like this one: http://www.radioshack.com/product/index.jsp?productId=2103174

Thank you for "The Box" - it's a great resource so I tweeted about it.

JLK

Hi again,
After reviewing the schematic for using an additional power supply here:
http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

1. I have a 12 volt DC battery, so the relay I need is a 12volt DC relay. Correct? Like the one here http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=2094135

2. In all three schematics, I don't know how to read the thick black bar that is in between the resistor and the diode. Does it represent a component or is it just symbolic of part of the diode?

Thanks so much everyone!

JLK

I have one a low-end voltage meter like this one

Ugh, I have that one too, and I HATE it!

In all three schematics, I don't know how to read the thick black bar that is in between the resistor and the diode. Does it represent a component or is it just symbolic of part of the diode?

Ha ha ha I hate it, too!
It never comes in handy. At one point, I had two of them and it was double the disappointment.

That said, can I use the one still have to determine what kind of LEDs I have?

Thanks for the clarification about the transistor symbol.

Cheers,
JLK

Is this an appropriate transistor for an led arduino project with an additional 12 volt battery supply?

TRANSISTOR,2N3906,PNP

http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=2094135

Thanks again!

That transistor will hold a couple hundred ma, so it depends on where your going to use it

Also it is PnP type, which (basically) means it will only turn on if the base (middle lead) is at 0 volts, kinda backwards

Finally if this is for the power supply, it is the wrong type, see the little arrow on the transistor schematic icon? if its pointing out that's a NpN type (if its pointing in its a PnP)

OK. Looks like I'll need to do more research on transistors. Thanks for the tips and head start.

Moving forward it's crucial that I determine the specs of the LEDs I have. Earlier in this thread i was told this:

"In the absence of a data sheet the only way to get the forward voltage is to measure it at a set current."

I'm unsure of how to do this. Do I use a multimeter? I'm hoping that someone out there could tell me how to measure the forward voltage of these LEDS. I bought them a few years ago - can't recall from where.

Thank you!

I'm unsure of how to do this. Do I use a multimeter?

I'm pretty much guessing here, but this is what I would suppose:

Connect the + on power supply (after using appropriate resistors for safe use with the LED) to the + on the DMM (set on Ammeter mode). Then connect - of DMM to the + of the LED, and then the - of the LED to Ground (- of the battery).

Again, that could be completely wrong, but it's my guess :P. As long as you use appropriate resistors, I don't think there's any chance of hurting the LED.

hmmm. I don't know how to determine what an appropriate resistor would be. What do you think would be an appropriate resistor?

There is a unlimited number of LED you can power with a 9vdc source. However they must be wired in parallel and each have it's own current limiting series resistor and of course the voltage source must have a current capacity equal to or greater the the combined load current of all the individual LEDs.

For series connected LEDs, you are limited to the sum of the number of the individual forward voltage drops of each LED, as to how many you could power from a 9vdc voltage source. For standard red LEDs I would think 6 or less would be the limit. You still need a current limiting resistor, but just one for the whole series string.

Lefty

I don't know how to determine what an appropriate resistor would be

Having said that use a 220R resistor and measure the voltage across the LED.