Hey Guys - So I got some excellent advice for my project from these lovely folks: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1266251470
Quick recap - I need to switch 120v and about 5A from an Arduino. The load is 4 incandescent lights per "switching unit". The answer I got was solid-state relays. That was great, but those are quite expensive - running $15-20 a piece, and I need 10. The link in the above post had a cheaper source, but they're sold out.
Here you may find some cheaper options for SSRs:
http://www.herbach.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=HAR&Product_Code=TM03RLY4706&Category_Code=RLY
http://www.mpja.com/products.asp?dept=133
http://www.electronicsurplus.com/leftnav/relays/solidstate.cat
So I did some more looking, and happened upon the TRIAC.
From reading the description, this looks perfect. Lord knows I'm no electrical engineer - the reason I'm using the Arduino is it gives me a computer interface to circuitry - but if I understand this component properly, I can simply get one rated to switch 120v/5a or better, and switch it on and off with the Arduino. It looks like I'll even be able to get a good bit of control over the brightness of these lights - if I turn on the triac at the local maxima of the voltage curve, I should be at effective half brightness.
Yes, but to get that amperage rating, you will likely need to attach it to an appropriately sized heat sink. Also, I would use an opto-coupler to control this; I wouldn't switch it directly with the Arduino. As far as being able to control dimming: Yes, you can do this, but you need a method to detect the zero-crossing (so you can time off of that and only turn on when you need to); I believe there are special opto-couplers for this, as well as other methods (I've only read about this stuff; I've never used them - so take this all with a grain of salt - you are not talking with a hands-on TRIAC expert here).
(Please tell me if I'm dead wrong about any of the above, by the way).
So I went to my component supplier over at Newark.com and found their TRIAC page. But I'm intimidated by all the options they offer. I'm not concerned about the characteristics of the load - an incandescent won't add power back into the system like a fan would - so much as the specification. I'm a little suspicious of all these transistor-looking things and more than a bit unsure that they'll be able to handle 5A.
Provided you have them heat sinked properly, you should be OK (I would look into TRIAC circuits as well - look for something incorporating a 555 timer to blink a lamp using a TRIAC; that will get you far enough (if you understand the schematic) to hook a TRIAC to the Arduino (once again, thru an optocoupler - like a 4N25 or 4N26).
Am I way off the mark here, or is this something I'll be able to use?
Well, I think if you take care, and don't do anything "stupid", its worth a shot to try; just do your research carefully before you hook anything up, and make sure you put the proper heat sinks on them. Isolate the Arduino from the TRIAC using optocouplers; it would be wise to keep the AC control side of things as isolated as possible from the Arduino side of things - do so by including an "air gap" between the Arduino and the optocouplers via a small length of ribbon cable; the optocouplers should be on their own board, connected to the TRIACs control lines, with the load lines of the TRIAC connected to two separate screw contact bus strips (use proper gauge wire, or if etching a PCB, proper thickness traces - for connections).
Put it all in a metal box, with nylon standoffs, ground the metal box properly, etc; when you test it, you may think about using an isolation transformer (oh, make sure you have proper fusing and/or on-board circuit breakers installed on the AC side of things).
Keep a fire extinguisher handy, too...good luck!
