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I want to drive a 5 metre strip of 300 5050 SMD LEDs. It comes with a 5A power supply but I don't know what the rating are for these LEDs.

The thing is I want to be able to change the voltage using the Arduino from 1v <> 12volt for dimming effect.

What kind of driver should I build for that? And would anybody have a schematic or something that's already available on eBay or similar :)

I am bit of a newb in the prototyping area-but have good skills; and all I need is some guidance in the correct way.

Thanks!

EDIT Thanks for the great answers- I did find this though.

http://cgi.ebay.co.uk/LED-Bulb-Dimmer-Adjustable-Brightness-Controller-DC-12V-/250828609755?pt=UK_Light_Fittings&hash=item3a668cd8db#ht_1869wt_1140

Guess whats going to be taken apart before it even turned on.. Is this a PWN dimmer? or just a simple Voltage adjuster?

Is it possible to control/replace or build on top of the analogue potentiometer using the arduino? Like for if i turn it on it fades to 100% over a period of 1second for example..

Piotr Kula
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2 Answers2

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I would wire up a Power N-FET, that could handle up to the full 5A and 12V across it, wire it in series between the LEDs' common-cathode and the GND, and pulse-width-modulate its base using the Arduino to control the brightness. That will give you the illusion of dimming, as brightness is a function of the average current experienced by the LED, and the physiology of the human eye will take care of the rest :).

vicatcu
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    Just don't try to sell it - there are some patents out there on PWM-based brightness control for LEDs... – Adam Lawrence Jun 08 '11 at 02:34
  • wow, patents on PWM brightness control? Anyhow, isn't PWM less-than-optimal, since the flickering can be noticeable at low effective brightness levels? – Dave Jun 08 '11 at 07:14
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    @Dave - You won't have noticable flicker if the PWM frequency is high enough (like a few hundred Hz), even with low duty cycles. – stevenvh Jun 08 '11 at 07:37
  • Yup, Philips bought Color Kinetics which has [patents on PWM](http://www.colorkinetics.com/technologies/ipinfo/patents/) for [dimming LEDs used for illumination purposes](http://www.ledsmagazine.com/features/1/12/1). The preferred method is [frequency modulation](http://www.artisticlicence.com/WebSiteMaster/App%20Notes/appnote009.pdf) – Martin Jun 08 '11 at 09:09
  • Yowzers, this all seems like a massive mission and has so many issues. How about I forget the arduino and PWN. Maybe i will just buy one of those DC-Train set voltage controllers instead? Just normal analogue DC control? That should not damage the LEDs in any way right? (not going over the maximum rated voltage that is) Also this is one long pre manufactured strip I cant re-wire anything easily. – Piotr Kula Jun 08 '11 at 09:43
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    wow, I wonder if I can patent the stimulation of a piezoelectric material affixed to a diaphram with a sinusoidal electrical signal to produce a tone... – vicatcu Jun 08 '11 at 12:35
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Here's a way to get the LED ratings, if you have appropriate equipment:

First step would be to measure the voltage drop of a single diode. If you can, try to get one diode a low value resistor and a variable voltage power supply. Do note that by proceeding this way, you can destroy the diode, so don't get too close.

So the idea is that while the diode voltage drop does change with voltage and current, the change is probably going to be in the hundreds of milivolts range. Start with a low voltage on the power supply, say 2 V. Then slowly start increasing it. After each voltage increase, measure the voltage across the resistor and across the diode. While the input voltage is below the diode voltage drop, the voltage across the diode will be greater than the voltage across the resistor and as you increase the input voltage, it will remain higher than the resistor voltage. As the both voltages increase, at one point you'll notice that the voltage across the resistor will start increasing at a much greater rate compared to the voltage across the diode. At this point, you've reached the diode voltage drop. You could then go up and down in order to better note the point. After that, just to be safe, reduce the drop by say 5% to 10%.

Unfortunately, I don't have a good way to determine diode current. The problem is that the diode brightness will continue to increase even past its rated current. You could try looking for similar diodes on the Internet and noting their currents and hope that yours will be similar enough. If you're unsure, 20 mA used to be standard LED current for quite some time, so it should be safe, unless you got some low current diodes.

AndrejaKo
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  • Yeah, I always try to keep unknown LEDs below 30mA just to be safe, and below 100mA if I'm PWMing. – Dave Jun 08 '11 at 07:15