led circuit help

Discussion in 'General Electronics Chat' started by qeddy84, Apr 14, 2009.

  1. qeddy84

    Thread Starter New Member

    Apr 14, 2009
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    hey guys great site . Im new to electronics and dont no much about them bt im getting married nxt year and am desighning a cake with my fianc'ee and thers lights on it i was woundering cud anybody supply me with a diagram for a multipal led circuit ther cud be a minimum of 8 leds on one circuit and a maximum of 20 in another there will be up to 6 circuits in allid need to no every thing from switchs to resisters. oh and i want to power it from 9v's if that will work any info would be great fellas :cool:
     
  2. electronictech

    Active Member

    Apr 1, 2009
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    Since you are new to electronics, I would suggest you use off the shelf items that require minimal work and only a small investment of money. There is an endless supply of options to choose from on ebay, that is where I would start. Just search for LED kit, or blinking LED(s).

    The key to LED's is to learn the following: Ohm's Law and the forward voltage of an LED....with that knowledge you can do anything with LED's.

    Good luck and have fun!
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    OK, first, please read this, with special attention to Figures 1 and 2...

    LEDs, 555s, Flashers, and Light Chasers

    OK, I'm still unclear as to what you want to do. Figure out a pattern with a rough sketch (on a cake?) and the colors, then we'll work it out.

    If you are wanting special effects such as flashing it can be done, but my advice is to start simple and work your way up.

    The reference to Figures 1 and 2 in the above article is because they explain how resistors are used with LEDs, resistors are a required part of the circuit.

    If you don't understand something, please ask, and we'll do our best to make it clear. This site is about nothing else if not learning.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    White LEDs use about 3.5V each. Two can be connected in series and in series with a current-limiting resistor to make one string. Each string uses 20mA. One 9V alkaline battery can power 4 strings for one hour with the LEDs dimming as the battery runs down.

    40 strings of LEDs will use ten 9V batteries.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    Sent as PM:

    At this point the ball is still in your court. As I said before, I'm still unclear as to what you want to do. Figure out a pattern with a rough sketch (on a cake?) and the colors, then we'll work it out.

    I would not put the electonics in food. Old fashioned solder has lead, and there are other chemicals I'm sure I wouldn't want to eat. Perhaps in the base, or the pillers?

    Some other ideas, I have designed some low voltage long duration LED flashers, you can find them using my blog.
     
  6. qeddy84

    Thread Starter New Member

    Apr 14, 2009
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    bassicly i want a about 80 or so leds so that will mean 5 or more circuits they will be string type circuits like u would see on a chrismas tree the wedding cakes are dummies so wont actualy be eatin if u follow me the circuits will be made up off 9v block batteries a table lamp switch and a resister on each led i need help with the tecnical stuff regards joe
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    OK, you're not going to be able to get that many LEDs per chain, not off a 9V battery. 9V batteries have a very short lifespan, among other things. Using red LEDs you might get 3 per chain, less for blue or white.

    I'll help you, but not through PM, use PM to let me know you've made a post. :)

    How would you feel about something like 12 "C" cells, wired to make 18 volts?

    What parts stores are you going to use? Radio Shack is pretty reasonable on battery holders, but anyone can beat them on LEDs.

    The color used makes a difference, since each color drops a different voltage (but it always drops that voltage), LEDs are current devices.

    I take it you want them on continuously, no special effects.

    You can build some practice chains one you decide on the power source. That and you have to decide on the color scheme, since different colors are different electrically, and we have to plan them.
     
    Last edited: Apr 22, 2009
  8. qeddy84

    Thread Starter New Member

    Apr 14, 2009
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    ok bill so will the 18vs be used to power all the circuits? i have leds all i no about them is
    Luminous Intensity-MCD: Min:12000cd Max: 15000 mcd
    Reverse Voltage:5.0 V
    DC Forward Voltage: Typical: 3.4 V Max: 3.8V
    DC Forward Current:25mA
    Viewing Angle:20±10 degree
    Lead Soldering Temp:260°C for 5 seconds
    Intensely Bright

    i hope there ok and as im from ireland i get my stuff from maplins they usually have most things so were do we go from here can u make me a sketch up
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    OK, like I said, you have to come up with your color mix. The specs of the LED depend on color, you can not use generic numbers. Rereading your post they sound like blue or white units.

    Yes, I'm figuring everthing off one power supply, unless something is so remote as to require its own. "AA" batteries would work OK for this too, figure 9V batteries have 6 cells about the same size as watch batteries to make their voltage.

    Have you tried to read the LEDs chapter of this article?

    LEDs, 555s, Flashers, and Light Chasers


    How comfortable are you with Ohm's Law?
     
    Last edited: Apr 23, 2009
  10. qeddy84

    Thread Starter New Member

    Apr 14, 2009
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    i wouldent say im comfortale with the omhs law. Its a little confusing to be honest but i get the jist of it .As for led colour il be useing warm white i think the coin batteries should work fine will the leds that i sent u a spec of work?
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    Any LED will work, you design around it. I was not saying anywhere to use coin cells, not with 80 LEDs.

    OK, the LEDs are 3.4V typical, so every LED in chain is another 3.4 volts. I'm going to assume 3.6V, since the max is 3.8V. If you were using just 9V you could only have two LEDs per chain, and since I have it on good authority that 9V batteries drop to 7.5V rather quickly even the two will dim out, so I'm also going to assume you'll follow my suggestion and use qty 12 "AA" batteries for 24V.

    To light up white LEDs you'll need a circuit something like this...

    [​IMG]

    Ohm's Law comes into effect in how I select the resistors. I take the voltage across the resistor (bottom row is 24V-21.6), and figure 20ma (0.02A). The resistor is calculated by Vr/A (3.4V/.02A), which is 120Ω. You need to check the voltage at the top of the chain of LEDs to verify this voltage (21.6V) and get back with me. Since LEDs vary a lot even in the same batch every chain needs measured.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    Who makes lead-acid 2V AA cells??:confused:
    The last time I looked an AA alkaline cell is only 1.5V so 12 of them makes only 18V.
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    OK, OK, so it needs to be Qty 18 of them. You got me. :rolleyes:

    I'm trying very hard to talk the OP out of 9V batteries.

    As is this circuit is going to pull a bit over ¼A.
     
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