Christmas lights

[codemonkeyrawks]

I want to drive some Christmas lights from a Arduino but not quite sure how to do it and multiplex the pins together since the arduino only has 12 pins. I am thinking of using a 12 relays and need to output 40 lines. Any Suggestions

[tinsmith]

Here's one. Do you really need to set it up as a 40-element matrix display? Strand lights usually don't. Common effects like the "racing" cycle tend to use sub-arrays tied to a common signal.

LEDs also don't need to be at logic high all the time in order to appear on. Even a fairly low duty cycle will usually look like it's persistently lit, but you'll start losing some apparent brightness if you go too far. Because of this, a common technique for arrays of LEDs is to set up an m x n matrix and strobe over it.

[tyblu]

Shift registers are great for this type of low-update rate and pin count situations. 4094 is serial to parallel converter. You'll need a driver if the lights aren't LEDs, like the ULN2803A.
More resources: http://www.lightorama.com/products.html
http://www.avrfreaks.net/index.php?name=PNphpBB2&file=printview&t=18054

[badSCR]

I lave light-o-rama....  Very good company to do business with.   

I use there controllers for my display.     I am a very happy customer, and my neighbors love my display.
Buy your lights and stuff when they go 75% off for the after christmas sale.

[House91320]

I am blanking on the name but there chips that will do this.

[Murphy]

Look into using solid state relays, they can be switched by just 5v.

Or make your own circuitry using triacs or mosfets if you need it cheap.

[tinsmith]

I am blanking on the name but there chips that will do this.

Well, there's the TLC5940 16-channel chainable PWM controller. But it's probably overkill for this.

[oPossum]

8 bit power shift registers

TPIC6A595
TPIC6B595
TPIC6C595