acmainspower
This is an "everyday" layman question, not a technical question.
I have an AC ceiling fan with three power settings (like some light-bulbs have three brightness settings). I also have a wall switch which is circular rather than on/off.
So I can either "ask" for less power (by changing the fan's setting) or "send" less power (by rotating the circular switch).
Is there an electrical difference between the two?
Why not consider placing your small Arduino board and its power source in the electrical box where the light bulb is located?
From your diagram it appears that the powered 220VAC feed and return line are both in the electrical box at the light fixture.
Edit to Add an Alternate Idea to try out. There is too little data regarding the Lifx bulb to know if this is even feasible at all. So you would have to simply try it.
The idea is to remove the wall switch from the AC circuit and instead wire it into the MCU board. Then wire a small transformer across the AC lines that come to the switch box. This would likely have a low turns ratio and would operate off the current that passes through the special LED bulb unit. An AC power converter on the other side of the transformer would convert AC to DC power.
The LED power may be limited by thermal considerations. Both the power supply and the LED are prone to failure. The former because of possible shoddy transient protection and inevitable death of filter capacitors (exacerbated by the typical high temperatures). LED life will also be greatly foreshortened by high junction temperature.
These things are a trade-off. My suspicion is that the LED rating of 3W you mention is not really achievable (with any reasonable lifetime) in the environment of the light bulb, and everything (driver and LED) is really close to the line. Here's a snippet from a Chinese "3W" LED datasheet:
The Power Dissipation is absolute maximum 3W at \$T_A = 25°C \$. It will have to be derated above 25°C at the bulb, so I think you can see that calling that thing a "3W" LED is somewhere between irrationally optimistic and an outright lie. While the bulb maker might be perfectly happy to run it right at 3W, if they die so fast that they are getting 40' container loads of lamps dumped back at them, they'll not be so happy.
The only thing I really expect to be true of the bulb is the (initial) lumen output (you can expect them to fade over time). If the lumen output falls short, especially out of the box, that is false advertising and I would return them. So far, from reputable makers such as GE, I have not been disappointed. I can't directly measure lumens, but I use lux meter to check light levels on work surfaces and so on, and can reasonably well compare halogen and LED bulbs with similar angles.
Best Answer
Yes, there is a difference.
The circular thingy on the wall, called a dimmer, chops up the 60 Hz power sine wave in such a way that the overal apparent voltage available to the motor is less.
The knob on the fan causes different coils inside the fan motor to be switched in and out to trade off more torque versus lower power consumption in the motor.
Use the latter (the switch on the fan) to control its speed if you can. The dimmer does some nasty things to the power line waveform. This will not only make the fan run less efficiently, but also cause some radio interference. The people that designed the fan added different taps to the coils inside the motor for a reason. This allows still running the motor reasonably efficiently at the different speeds. The downside is that you only get a small number of fixed speed settings, whereas the dimmer can control the fan speed smoothly. However, it's a fan. Three speeds should be good enough.