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I've started working on a project to control my christmas lights via and arduino. I've read a lot of blog posts about doing this, seen a lot of videos, and done a good bit research. The only problem that I am running into is being able to dim the lights.

My initial plan was to connect the arduino to some shift registers and have the shift registers control SSRs. I'd probably toss an optocoupler between the shift register and SSRs to help further isolate the AC from the control. This setup, however, dose not allow me control the brightness of the lights, only if they are on or off. I've looked into TRIACs and kicked around the idea of switching the SSRs on and off very quickly (kinda like a DC speed controller).

Dose anyone have any ideas how to approach this?
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How about some more description.
How many strings, how much current, 120VAC, etc.
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Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
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I'm planning roughly 40 channels. The maximum a channel would pull would be 5 amps (for the floodlights), but a typical channel would be roughly 1 amp. The majority of the lighting is going to be rope lights with a few strands. All of the lighting is going to be 120 VAC.
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How much of that would you want as on/off, how much dimmable?
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I'd probably toss an optocoupler between the shift register and SSRs to help further isolate the AC from the control
No need an SSR will be opto isolated already.

If you pick the right SSR then that will do proportional control. Don't even think about just randomly turning the SSRs on an off rapidly, it will end in tears.
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How much of that would you want as on/off, how much dimmable?

Ideally I'd like to be able to dim everything, but the minimum would be around 25 channels.
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but the minimum would be around 25 channels.
Wow, have a look on ebay you might get some proportional SSRs for $30 - $40 if you are lucky but that is $750 - $1000 on the SSrs alone.
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Would there be any other way besides a proportional SSR to accomplish this?

Something like a TRIAC?
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An SSR is a triac!
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I did a kick ass Halloween display one year with 12 Cyber Lights and 8 AF100 strobe units  smiley-evil  (Google them)

Have a look on eBay for "dimmer pack"  there are a few NSI units going for $60 and one Leprecon for $40.  You can not buy the part cheaper and these are all set ready to go!

The protocol of choice would be DMX512 since all the software and equipment is designed to use it.  AMX is a Strand protocol an I would avoid it.  Midi also sucks.  DMX512 can have a 5 pin or 3 pin XLR connector.  Read the manual for the equipment you get. 

If you look into the DMX protocol you should be able to make the Arduino send or read it.  I always liked having the ability to live edit any project I was working on. 
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An SSR is a triac!

Well I feel a bit dumb now. I was kicking around the idea of just buying a Mega, so I could eliminate the shift registers. If I did that, I'd have 14 PWM outputs, so I'd scale back my lighting a bit to just use the 14 PWMs for the dimmable channels. My question is, what would the PWM drive? A proportional SSR, or is there something else I can use?

I've worked with speed controllers for DC applications and understand the idea behind them, however, with AC wouldn't I just be able to modify, or clip the wave and get the same results?
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with AC wouldn't I just be able to modify, or clip the wave and get the same results?
No with AC you need to phase angle control the waveform. PWM works far too fast. You need to detect the zero crossing point of the AC waveform, then delay a little, then turn on the triac or SCR. That is what a proportional SSR does for you. You can do that in hardware or software.
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