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[on:]

Hi there, got some weird problem with my ledstrip. It's probably my very basic electronics knowledge, but I don't get it.

I've made a ledstrip that goes across my wall. This is about 8 meters. At the end of the strip it's got the right colour, for example yellow. At the start of the strip (where the wires are attached) however, it ends greenish.

When I want white, the end is greenish, the start is blueish.
Cyan: end more green, start more blue
Magenta: end more blue, start more red

What can be causing this gradation on the ledstrip?

[Reply #1 on:]

Have you calculated how much current you're drawing and whether your power supply can actually provide that?  Not knowing how you wired these up, it's hard to figure out what's going on, but the last time I had color issues like that it was because I was drawing more current than what's being provided.

[Reply #2 on:]

It sounds like you're using RGB LEDs? How are they wired up? How many LEDs? Is the color shift change gradual across the string or is it abrupt? If you reverse the LED string does the problem stick with the LED ends or the near/far ends?

[Reply #3 on:]

Do you have 8 meters of light strip being powered from just one end?  This would cause an issue with too much voltage drop.  For 8 meters, you should have power from both ends and one in the middle to ensure even power distribution.

But as stated, without knowing more details about your strip and how you have it wired, it is hard to give concrete suggestions.

[Reply #4 on:]

Hi there,

I have a 12V 3A adapter, this feeds the arduino and the ledstrip. The ledstrip got a IRF44Z mosfet for each color. It's wired up like this :http://www.ladyada.net/products/rgbledstrip/

The total current draw is about 1A when the strip is white.

The strip itself is a 30 leds per meter low power consumption one.

@Cranium: It seems you were spot on. Brought a +12 wire to the other end of the strip and the gradation problem is solved. Now I also know why all these strips are 5m at max everywhere :-)

[Reply #5 on:]

Good deal.  How about some pics or a video?

[Reply #6 on:]

A little demo :-) It's also got a function so the ledstrip will work a thermometer :-)

Attached a pic of the web interface

[Reply #7 on:]

Nice.  What are you using to control it with a browser?

I have 4 - 5m white led strips in my office to replace the florescent lights and am waiting on 2 -5m RGB led strips (LPD8806 based) to add some color.  I plan on controlling the color strips wirelessly using xbee.

[Reply #8 on:]

I'm currently using an ethernet shield, going to make it wireless in the future so I don't need a cable running to it anymore.
I created a little webserver on which I can set parameters. Requesting information from the arduino spits out JSON.

[Reply #9 on:]

great job looks great,makin something very similar,but im using rope lights.currently making the system to control 110AC. did u make the led strips yourself?

[Reply #10 on:]

Hi there,

I have a 12V 3A adapter, this feeds the arduino and the ledstrip. The ledstrip got a IRF44Z mosfet for each color. It's wired up like this :http://www.ladyada.net/products/rgbledstrip/

The total current draw is about 1A when the strip is white.

The strip itself is a 30 leds per meter low power consumption one.

@Cranium: It seems you were spot on. Brought a +12 wire to the other end of the strip and the gradation problem is solved. Now I also know why all these strips are 5m at max everywhere :-)

You should be using a logic-level MOSFET, the IRF44Z needs 10V gate drive to turn fully on - this probably means its running hot and not delivering the full 12V to the strip.

To be fair (and this quite surprised me) the LadyAda tutorial recommends a MOSFET that isn't logic level either!

If you are driving a MOSFET from 5V you need one that has Rds(on) specified at "Vgs=4.5V" in the datasheet.  Most TO220 packaged MOSFETs are not logic level, note.  Also these days there are cheap MOSFETs with far better specs than the one's from the 1980's!!!  Look for Rds(on) < 0.02 ohms if you can, it'll run cooler.

Also my standard warning - ignore the current rating for MOSFETs, that's not useful information.  Go by the Rds(on) value and calculate the power dissipation using I-squared-R.  If you want to run without a heatsink the power dissipation should be less than 1/2 a watt, preferably even less.  Since the power is proportional to current squared, higher currents rapidly lead to "issues" and using a "10A MOSFET" for handling 5A is never sensible, you want one rated at 0.02ohms or less for 5A, 0.005 ohm or less for 10A, or you'll need heatsinking, fans etc.

For example here's a logic-level MOSFET: http://www.sparkfun.com/products/10213