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Topic: The BEST way to control white stair lights? (Read 5 times) previous topic - next topic

orangeLearner

Hello everyone.

I have been thinking for a while about a stair lighting project. http://www.youtube.com/watch?v=Ir_cqS6oV5U I essentially want to make this except with LED strips going across the entire step instead of plain LEDs on the side since I have shoes on the stairs. I also have to cut down on the wires because I will not be able to rip open the wall and patch it back up, so I was thinking of using ethernet wire and power over ethernet, using maybe four wires for power.

For control, I was thinking about having using 1 or 2-bit shift registers on each stair since there is already code written to PWM shift registers. I have 14 stairs, and this would make it so that the number of wires running up the stairs remains constant, as opposed to just running a pair of wires to each LED strip, which would quickly become a cabling horror. This would take what, one wire for clock and one for data?

For power, I figured I could use MOSFETs. Should I bother buying the high current (30A) ones if all I need to control is a max of 2W per step? I also heard that logic signals have to be "buffered" along distances. Do I need to make a buffer circuit?

Is this the best way to go about this project?

AndrewStone

Here is someone a bit ahead of you http://www.thekanes.org/index.php/2010/09/27/led-stairs-the-chips-are-talking/.

What are your LED's power requirements?  You mention MOSFET so I am thinking that they are higher then the shift register can handle.  So you would not need a buffer unless you are locating the mosfet out near the LED.  Speaking of MOSFETs... I think you really want a high power constant current driver, as high powered LEDs + resistors are not a good combination since the LED changes as it ages.

Cheers!
Andrew
http://www.toastedcircuits.com Lightuino LED driver: 16 sources, 70 sinks, remote controlled.  Also high powered LED drivers.

orangeLearner

Darn. I would need a constant current driver on each stair? How easy is that to create? Is it simple enough to create with just a current mirror or do I have to buy something?

AndrewStone

I recently had 50 PCBs built of a simple 4 channel constant current driver using just a few components; an opamp, a darlington transistor and a few passives.  It has 4 inputs (one per channel) that you connect to the Arduino; if you set the Arduino pin mode to "input" (high impedence) the circuit is "on", if you pull it low (pinmode output, set to 0) the circuit is "off", and if you set high (pinmode output, set to 1) the circuit is no longer constant current; it becomes a normal darlington current "amplifier" (so don't do that with LEDs!).

   I'd be happy to share the schematics (you can breadboard it pretty easily) and/or sell you some PCBs, kits or assembled (it does use some SMT so its a good project to cut your teeth on SMT assembly if you want). 

You need to give the board a voltage just above what is required for the LED (so generally 4-5v -- the closer it is the more efficient the circuit), and it moderates the voltage to provide a constant current source.

Andrew
www.toastedcircuits.com
http://www.toastedcircuits.com Lightuino LED driver: 16 sources, 70 sinks, remote controlled.  Also high powered LED drivers.

orangeLearner

@Andrew

Sure, I'd love to see your circuit and PCB design. Right now I know I need a constant current circuit, but there seem to be many ways of building one including this one: instructables.com and I don't really know how to quantify the advantages and disadvantages of each. That link I provided does have a pretty simple circuit though. PWM-able too if you look down in the comments for where to hook it up.

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