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Newbie wiring 20 red LEDs to a 9v battery in parallel help pls

  1. Jul 20, 2006 #1
    Newbie wiring 20 red LEDs to a 9v battery in parallel...help pls :)


    i am completely new to the LED world and electronics world. I am wanting to make a little box with LED lights as an aniversary gift. Basically what I need to know is how to wire 20 red LEDs in parallel so that they run off of one 9v battery. I also will be putting a magnetic reed switch so that they are turned on when the box is opened and off when closed (but this will come later).

    For now I need to know:

    1. What resistor requirements I have
    2. Where to get the resistors and LEDs (i tried radio shack and local electronics stores but they dont have enough variety to meet my needs). Ive tried using online resistance calculators to determine the ohms that I need but then I cant find those resistors. I dont need these lights to be too bright.

    Any help would be GREATLY appreciated!

    Thank you,

  2. jcsd
  3. Jul 20, 2006 #2


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    I don't know anything about electronics, but I've made a few LED things. Frankly, I've never used any resistors at all. Just pick LED's that are made to handle 9-12 volts.
  4. Jul 20, 2006 #3


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    Staff: Mentor

    That's why they call you Danger, Danger. :biggrin:

    To the OP -- you want to set the current through the LED to whatever it is spec'd at for good brightness. For old jellybean LEDs, that's usually around 20mA. For newer high-brightness LEDs, the current can go down to 5mA or even lower. You can experiment with 5-10-20mA and see what current gives you the best brightness.

    The easy way to set the current is to assume that the LED will have about 2V across it when on. This is a pretty good assumption for most LEDs. You can check the LED datasheet if you need a better number for Von, but 2V is what I use most of the time. So if you hook the 9V battery to a resistor to the LED, the resistor will have about 7V across it (9V minus the 2V of the LED). You choose the resistor value to set the current. Like if I want 10mA, I'll choose the resistor to be close to 7V/10mA = 700 Ohms.

    And you can parallel connect a bunch of these R+LED circuits to the same battery. Each LED needs to have its own series resistor to set the current. You can also buy LEDs with built-in resistors (I think that's what Danger-guy was referring to), but usually you get the plain LEDs and add your own external series resistor to set the current.
  5. Jul 20, 2006 #4
    berkeman: thank you very much for your response. when I was at radio shack yesterday i was looking at some red LEDs which were 2.6v and 28mA rating. What I understand from your post is that even though this is 2.6v i can just assume 2v (is 2.6v the max rating?).

    I arrived at two conclusions from your post and those specs:

    1. using precise specs: 6.4v (9-2.6) / 28mA = 228-Ohm resistor
    2. using your guidelines: i can use a 700-Ohm resistor on these.

    Like I said in my original post, radio shack did not have very much variety. Can you recommend somewhere online that I can purchase these? (im looking for miniature glass ones in the USA)

    Thank you!
    Last edited: Jul 20, 2006
  6. Jul 20, 2006 #5


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    No, I'd use the specs if you have them. So if your LEDs say to run them at 28mA and their forward voltage drop at 28mA is 2.6V, I'd use those numbers to calculate the series resistor value. The best thing is to start with those numbers and see what the LED looks like. If it's too bright or too dim, adjust the resistor.

    The best online places to buy electronics stuff are Digikey and Mouser:



    I think they both have something like a $25 minimum order, but I'm not sure.
  7. Jul 20, 2006 #6
    thanks! ended up talking to digikey and figuring it out for the most part...i'll see if things dont blow up and i'll post back. here is my transcript:

    info: Your request is being processed. A representative will be with you momentarily.
    info: Your Digi-Key representative for this Live-Help session is Quinn Ext 2289.
    Quinn Ext 2289: Welcome to Digi-Key Live-Help. How may I assist you?
    Rody: Hi, i was just speking to Andy and my browser crashed, could you transfer me back to him please?
    Quinn Ext 2289: Yes
    info: Please hold a moment while I transfer your chat.
    info: Your Digi-Key representative for this Live-Help session is Andy ext. 1851.
    Andy ext. 1851: Welcome to Digi-Key Live-Help. How may I assist you?
    Andy ext. 1851: Andy ext. 1851: You would want normally closed. This way when you close the box, the magnet opens the contacts, and disconnects the light
    Andy ext. 1851: Andy ext. 1851: This way the when the contacts are closed (when the box is open) the contacts are closed which complete the circuit
    Andy ext. 1851:
    Rody: hi andy, i was just speaking to you about the LED circuit
    Andy ext. 1851: yep
    Andy ext. 1851: You would want normally closed. This way when you close the box, the magnet opens the contacts, and disconnects the light
    Andy ext. 1851: This way the when the contacts are closed (when the box is open) the contacts are closed which complete the circuit
    Rody: okay great! so which one is normally closed on your site?
    Andy ext. 1851: These spst types (magnetic reed switches) will usually be normally open. SPDT types may have a normally open and closed style
    Rody: so do you have an NC on your site that you know of?
    Andy ext. 1851: http://dkc3.digikey.com/PDF/T062/1525.pdf
    Andy ext. 1851: this page takes you to some of our reed switches
    Andy ext. 1851: The Meder ones towards the bottom center have a normally closed option. 374-1030-nd
    Rody: oh yea i see it. that one would work with my idea?
    Rody: are these the miniature glass ones?
    Rody: (thank you for all yourhelp!)
    Andy ext. 1851: The switching current is .25amps and the body is around 1" long
    Rody: that sounds good to me
    Andy ext. 1851: http://rocky.digikey.com/WebLib/Meder Electronics/Web Info/MK3.pdf
    Rody: okay, so that takes care of that...now LEDs...
    Rody: i found this on your site: 160-1499-ND which is red like i want...and inexpensive. now i need some help figuring out the reisistors for this
    Rody: agai, i want to wire 20 LEDs in parallel on a 9v battery.
    Rody: from your site i see that its a 2v LED but i cant find the mA on it
    Andy ext. 1851: It is 50mA max
    Rody: so if i want to run this circuit in parallel...with each LED at 2v and 40mA of power...then do i need a 175 Ohm resistor per LED?
    Rody: running off a 9v battery
    Andy ext. 1851: one moment
    Rody: thanks!
    Andy ext. 1851: To get the ohm value needed, you take supply voltage (9v) minus forward voltage of led (2.0v). So that is 7v. The divide that by forward current (50mA). This would leave you with a value of 140. But it is good to go up to the next highest value to compensate for tolerance
    Andy ext. 1851: Basically a 175 ohm would work
    Andy ext. 1851: one moment
    Rody: okay, cool. and that would be one 175 ohm at each LED if wired parallel. do you think that will eat up the battery too quickly? since i think those batteries have 200mAh...and i think i am drawing around 1000mAh, right? thanks!
    Andy ext. 1851: one moment
    Andy ext. 1851: You may actually only need 1 resistor, double checking
    Rody: for parallel or for series?
    Andy ext. 1851: I found a good website. They recommend still using a resistor for each led. This way if one led goes bad, then the rest of the leds will not go bad. Basically it would allow for higher current at each led.
    Andy ext. 1851: Here is that website:
    Andy ext. 1851: http://www.mindspring.com/~jeffpo/ledlite.htm
    Rody: i am concnered about the current draw too... what do you think of what this diagram gave me? http://www.tdelaney.com/Rody/wiring.JPG
    Rody: what my reasoning was is that if i have 20LEDs at 50mA each wired directly in parallel then i would be drawing 1000mA. the 9v batteries have about 200mAh capacity...so that would leave me with about 12 minutes of power...but that other diagram shows that the circuit would draw only 200mA. so i would have an hour of power... does that make sense?
    Rody: (thank you again for all of your help with this!!)
    Andy ext. 1851: Here is a great link for parallel hookups. You may run into a problem because of how many leds you have in circuit. Basically if you have 4leds hooked up in parallel it will drain less off the battery than 20 leds would. Which means battery is constantly being drained and therefore would not last too long.
    Andy ext. 1851: http://www.theledlight.com/ledcircuits.html
    Andy ext. 1851: This website descibes that using a wall transformer, you would not have to worry about drain, but with a battery it is more of a problem. On here they show only using 1 resistor as well. This way the voltage is constant, but the current is distributed evenly through each led
    Andy ext. 1851: this may mean that you would require a higher amp/hour battery, or a larger source
    Rody: but the other guy was recommending indiv resistors for reliability, right?
    Rody: what type of reisitor is best...ceramic or carbon?
    Andy ext. 1851: correct. The problem with using individual resistors for each led with one power source is the drain will be alot more on the battery
    Rody: ah i see.
    Andy ext. 1851: Each resistor is fine for this type of application, as long as the resistor has a high enough wattage
    Andy ext. 1851: Carbon film are common
    Rody: great thank you!
    Rody: okay, so i am ordering these parts... how long does it take to ship out?
    Andy ext. 1851: No problem
    Andy ext. 1851: I would have to get you to our sales department for that information
    Rody: oh... do i need to place these on a circuit board? how do i get all this stuff connected up without anyting buning? lol
    Andy ext. 1851: I can either transfer you there, or you could call in at 1-800-344-4539
    Andy ext. 1851: A circuit board would be a good idea.
    Rody: i am very new to this...which one do you recommend for my applicatoin? these leds will cover an area about 6"x6" and will be in an enclosure
    Andy ext. 1851: http://dkc3.digikey.com/PDF/T062/1627.pdf
    Andy ext. 1851: this is a good page to look at. It is page 1627 in our catalog.
    Andy ext. 1851: Here you will see that we have many different sizes to choose from
    Rody: yea i do see that... i deffinitly need your advice on this! hehe
    Rody: i dont know what type i need
    Rody: i want to lay out the 20LEDs in the shape of a heart
    Andy ext. 1851: It looks like our closest size would be 6.3 x 9.19. Something like part number v1226-nd
    Rody: can these be cut?
    Andy ext. 1851: I have heard of people cutting these yes
    Rody: whoa...expensive...hehe. is there a less expensive alternative?
    Andy ext. 1851: Everything is either to small or larger in size (and more expensive)
    Rody: hmm... well maybe i'll skip the board for now. the only other thing i think i am missing is the 9v battery connector
    Andy ext. 1851: a 9v battery holder?
    Andy ext. 1851: a pc mount type or a holder with leads coming off it?
    Rody: the thing that clips onto the battery leads and has two cables coming out of it
    Andy ext. 1851: 72k-nd
    Rody: could i use the 473-1004-ND to set everything up? i am concerned about the heat and fire and sheetrock buildings
    Andy ext. 1851: These do not have the pre-drilled holes
    Rody: i can make the holes...i just got concerned with heat and flamability
    Andy ext. 1851: ok
    Rody: okay, so i have the batt strap, board, LEDs, reisistors and the mag reed switch...anything else you can think of that i need?
    Andy ext. 1851: These seems like everything
    Rody: great alright thank you very much for your help and time again!
    Andy ext. 1851: No problem, thank you and have a nice day
    Andy ext. 1851: Thank you for visiting. Please feel free to contact us at anytime.
    Rody: could you transfer me to sales please?
    Rody: thanks again!
  8. Jul 20, 2006 #7


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    Gold Member

    Yeah, well... I have no particular aversion to fire.

    :uhh: Actually, I sniped them out of my boss' VCR... but it's nice to know that the 'pre-resisted' type exists. :biggrin:

    Sorry for the bum advice, rody.
  9. Jul 20, 2006 #8


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    Homework Helper

    Ummm a basic 9v battery is only good for about 50ma.
    Unless a very short battery life is acceptable.

    You would probably want to use a series parallel configuration as well.
    Wire three leds in series and multiply the voltage by 3 for the resistor equation.

    Pick a resistor for about 10ma and not the max current.
    Max means just that. Its the max current the led will take without burning up and that assumes you keep it cool.

    Also 2.8v sounds a little high for red leds, but not when passing the max current.
    Does the spec give the minimum turn on voltage?
    That is probably more like 2.2 volts and I would use that voltage for the equation instead of the max voltage.
    The average red led has an intenal resistance of about 22 ohms. Multiplying that by 28ma gives .6v which would make the 2.2v just about right.

    Putting the leds in series saves the power wasted by the resistors.
    You don't really have any power to spare with a 9v.

    10ma per string should give you plenty of brightness.
    If you really need more you might need a bigger battery or consider pulse mode.

    Edit:If you want to have an led pass an exact current you need to subtract its internal resistance from the calculated resistance. When wiring leds in series in this case about 66 ohms that can become significant.
    Last edited: Jul 20, 2006
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