I'm planning to build a digitally-controlled light dimmer circuit. I'm not trained in electrical engineering, and I don't want to do something that an electrician would immediately recognize as dangerous and stupid.
Please suggest a reasonable home set up for prototyping circuits involving 120V AC power.
As far as lab equipment goes, a 1:1 safety transformer (AC mains : AC mains) is worth a lot. They aren't cheap, but I would not want to work without one. Mine is home-built and uses two 250 W transformers back-to-back.
The trick of the 1:1 transformer is this: Current from its secondary winding can only go back to the other end of this exact winding. As long as you touch any circuit connected to the 1:1 transformer with one hand only, you are safe because the current from your finger can not go anywhere. You are a bit like a bird on a wire. A regular wall outlet is referenced to earth, just like you are when standing on the floor: In a fault, current from the live pin of the outlet runs through you, the floor and to earth, which is equal to the other end (neutral) of the wall outlet.
Keep in mind that current always needs a loop to flow in: Any energy that comes from the transformer can go back to this transformer only, and can not go anywhere else, especially not to the ground via your body.
You don't have to probe a live circuit. Get test leads with insulated clips on them. Clip onto the circuit with power off. Turn it on, stand back, and look at the reading. Never use an oscilloscope on a non-isolated circuit. I would also suggest some kind of pilot light in the circuit so you know when the power is on. (Just unplugging is not enough if you have large capacitors that store high voltage, but I don't think you will for a light dimmer.)
If you're not comfortable with it, don't do it. I wouldn't mess with AC unless you're totally confident in what your doing.
If you must, use the one hand rule, and keep one hand in your pocket while working with live AC. Also use insulated tools.
Apart from what @bt2 said
Since it would be impossible to turn you into an electrician in a single answer, I'll recommend some reading.
For the US, home wiring is governed by the NEC (National Electrical Code). It's a huge, detailed, expensive publication. For a DIYer, I strongly recommend purchasing the most recent edition of Wiring Simplified. It's probably available at your local (or big-box) hardware stores (I've seen it at Ace Hardware and Home Depot), or here on Amazon, should be around $10. I use it when planning any electrical job at my house, parents' cottage, or workshop.
From the Amazon description:
Revised and completely updated for the 2008 National Electric Code, this small manual continues its 75-year history of demonstrating how to install safe, convenient, and economical wiring. Encouraging readers to tackle jobs small and large, the guide covers everything from repairing a table lamp to wiring a whole house. After introducing the basics—standards, codes, safety practices, and an overview of how electricity is measured and delivered—chapters show how to design a layout for lights, switches, and receptacles; run a line from a utility pole; install wires, circuits, and grounds; and handle special projects such as replacing old wiring and wiring detached garages and accessory buildings.
There is a section on safety, and the book recommends good practices, but it's not a replacement for training. It's a good reference for knowing what you're working with, though, and common-sense rules (like the one-hand rule, disconnecting the breaker, and wearing rubber-soled shoes) together with this book will make you a much better electrician.
The inimitable Dan of Dan's Data has an entertaining article on avoiding electrocution. The follow up mail on the subject contains further tidbits.
I recommend going to your local hardware store and buying a light switch with a neon tube in the switch. Also buy the metal junction box to install the switch in and a faceplate. Run your AC input into one side of the switch, and the other side to whatever circuit you're playing with. This is a good way to be able to see at a glance whether power's on, and to have an easy cut off. At work, I have this installed on a wooden board with the output wires going to a pair of terminal blocks. This makes it easier to change what's connected. Whatever you do, make it perfectly clear which switch position is 'on' and which is 'off' for safety.
If you make it plug in and not hardwired you can prototype anything. If you want to sell it you will need to have your device meet ul or some other governing body that oversees your perticular type of electrical device.
If you take a regular rotary dimmer switch (Potentiometer/variable resistor/rheostat) and hook that up with a stepper motor you could get away with not knowing a whole lot more than how to control a stepper, how to keep up with the stepper's position, and how to wire up the dimmer switch (very easy). Oh, and you've got to power the whole switch control setup somehow.
If you want to do more than that, like use solid state switches or pretty much anything else living on a board you've designed, you've got more issues, and even your power supply may become a lot more complicated since you don't want your control circuits to become a possible path for shorting out the system. High voltage can cause arching at some distance normally, and with residential AC you've also got to worry about extremely high current from lightning.
If you want this system to be energy efficient then you've got a lot more problems ahead of you. The easiest way to dim the lights is by introducing resistance, but is wasteful, and if you want the lights to run off AC it's difficult to get around easily.
You might want to have a look at the NEC (National Electrical Code) if you are in the US.
I'm not an EE or electrician, btw.
