# Multiple LED project. Absolute beginner.

Discussion in 'The Projects Forum' started by xstackx, May 27, 2009.

1. ### xstackx Thread Starter New Member

May 27, 2009
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Hello, I'd appreciate any help on this at all. Apart from a few flashbacks to high school, my knowledge of electronics is pretty limited. Here's what I'm looking to do:

Remotely power 20 5V LEDs, (10 red, 6 yellow and 4 green (I read in another forum that colour was a factor???)) and also power a small 15W light bulb (ideally on the same circuit but that's not a dealbreaker). I'll need to control each light individually with switches.

This is for a one off project that will most likely be broken up or binned after one use, so it doesn't need to be technically wicked, I saw in another post about having one switch which with every push would light another LED. That would be cool, but probably is more than I need if it's too complicated.

Further silly question, if I use these http://www.maplin.co.uk/module.aspx?moduleno=35737 that would mean that I don't need to buy resistors right, even with thirty of them?

SO if this: http://gallery.me.com/chrisstack#100052/Circuit&bgcolor=black is my circuit diagram...
what sort of power supply should I be looking at for 30 5V LEDs and a 15W bulb?
Is it better to have the bulb on a separate circuit?
What resistors (if any) should I be using.
Is there a simpler way of doing what I'm trying to do?
If I'm attaching metre long wires between the LEDs and the switches do I need to take this into account and what wire should I be using?

Again, any help on any of this would be greatly appreciated. Thanks.

Apr 5, 2008
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Hello,

At the page you gave us the current of the 5 Volts leds is given as 13 mA.
So 20 leds will use 20 * 13 mA = 260 mA.
Is the lighbulb you are talking about also for 5 Volts?
15 Watts will take 3 A at 5 Volts.

The total current will be 3 + 0.26 = 3.26 Amp.
A powersupply of 4 or 5 Amp will be fine.

I think the thing with the smal circle is the light bulb.
I circled it red. This wire will create a short, so remove it.

Greetings,
Bertus

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3. ### PRS Well-Known Member

Aug 24, 2008
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No resistors are needed. I'd use telephone wire. You can get a cable of 4 solid copper wires insulated in red, black, yellow, and green plastic, and buy it by the foot, at radio shack. Incidently this wire fits in breadboard holes.

If 3 Amps from the bulb bugs you (it would me) then use a seperate supply for it.

Good luck!

Mar 24, 2008
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5. ### xstackx Thread Starter New Member

May 27, 2009
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Brilliant, thanks everyone for your help. Just one more question, if I put the 15W bulb on a separate circuit, what sort of powersupply should I use for it, and what sort for the LED circuit. Cheers.

6. ### Wendy Moderator

Mar 24, 2008
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The light bulb can be whatever you like. If you have AC handy, use it. This would make the diode end much easier, a wall wart (you can even get them regulated if you look around) can handle the rest. The LEDs draw almost nothing.

If you need help regulating 5V just ask, lots of ways to do this too.

7. ### ELECTRONERD AAC Fanatic!

May 26, 2009
1,146
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I assume you understand the concept of parallel and series circuitry, but I'll go ahead and explain them anyway.

Lets say you want to hook up four LED's in series and each LED takes 5V. With each additional LED you add, you'll have to add 5V. So for four LED's you need 20V (5V X 4 LED's). But if you were to do parallel (which I would recommend) and each LED took 20mA, you would need 80mA. If you were to keep 20mA, each LED would consume only 5mA (so they wouldn't be very bright...).

I also would recommend looking at the specs of your LED's if you have them. On the specs, you'll find the "Forward Voltage." This is typically 3.2V or 3.6V with 20mA. Since the LED's contain internal resistance, with 20mA you get a 3.2V or 3.6V drop across the LED. This means that you want to apply your power source voltage higher than that voltage drop. So for example, if we had an LED with a forward voltage drop of 3.2V with 20mA, and I have #3,1.5V batteries (which is 4.5V), 4.5V-3.2 = 1.3V. Then according to Ohms Law, 1.3V / 20mA = 65Ω. So I need a 65Ω resistor to regulate 20mA of current. But that is just for one LED. If you had 20 LED's (20mA X 20 LED's = 400mA), you would need 400mA of current so that each LED is as bright as they can be. So what would the resistor value be? Well, 1.3V / 400mA = 3.25Ω

If you have any more questions I'd be glad to try and answer them.

Apr 5, 2008
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Hello,

@ ELECTRONERD, Putting "normal" leds parallel with only one resistor is a bad idea.
Due to forward voltage differences the leds will get different currents if you place the parallel with one resistor.
The led with the most current will fail, the current in the rest of the leds will rise.
The next led with the most current will fail ........
For "normal" leds each led should have its own series resistor.
The leds the Topic Starter wants to use are made for 5 Volts, they will have a resistor build in.

Greetings,
Bertus

9. ### ELECTRONERD AAC Fanatic!

May 26, 2009
1,146
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Bertus,

Wouldn't it be fine if you had the exact same LED for all twenty parts? But with your advice, I would have to put a resistor in series with each LED that is parallel, correct?

Thanks!

Apr 5, 2008
19,170
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Hello,

@ ELECTRONERD,
That is just the problem, each led will be different, even from the same batch.
Thats why I advice to use a resistor per led or string of leds when leds are put in series.

Greetings,
Bertus

11. ### ELECTRONERD AAC Fanatic!

May 26, 2009
1,146
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Thanks Bertus, sounds good!

12. ### Wendy Moderator

Mar 24, 2008
21,500
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Note to ElectroNerd:

The OP got around this problem by getting LEDs with build in current limiting resistors. They save on labor, but are a bit more expensive. My personal cost per resistor is 2¢.

You might try reading the same link I recommended to the OP.

LEDs, 555s, Flashers, and Light Chasers

This is covered in the LED chapters.

13. ### Bernard Expert

Aug 7, 2008
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Power requirements: 5V @ 1/2 A, 15W bulb, automotive,13.6V, @1.1A, so use 12V @ 1.5A. Note: no resistor for bulb, or the LED's, carefull with other LED's as they are not likely to have internal current limiting.

14. ### Wendy Moderator

Mar 24, 2008
21,500
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But they glow real pretty just before they blow!

15. ### xstackx Thread Starter New Member

May 27, 2009
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Ha, I'll try that too. Thanks guys.