There is no reason other than I've already had PCBs made and soldered everything on. :)

Which opamp are you using?

@dDebug, the rise/fall times do matter sometimes, but in this case they likely don't. And also, your oscope probes are skewing the results.

@dDebug, it doesn't matter, the rising edges should have the same distance at every level.

The output doesn't seem bad. It's probably better in reality than your scope is showing, since the probe is loading the line. What you should check is the period -- measure the distance between two consecutive rising edges.

Right, I'll check for the 101st time, but I'm pretty sure it's just a typo in the question :) Anyway, it turns out that I've accidentally mistook LIVE wire for NEUTRAL and vice versa. Except that the switch is really connected to LIVE. Now it finally makes sense :) Your explanation of R3 is also quite reasonable. C4 remains a mystery to me.

@PlasmaHH, there's quite a difference. You need only a transistor to control a P-channel MOSFET.

You don't actually need the transistor or the diode, both are integrated on the relay (this is the one, isn't it? proton.cz.avg.com/plan/31399/?detail=746216654982669497). You do need to connect the 5V ground to the Arduino ground.

Configuration is a euphemism for "loading the bitfile".

There are AC components in a squere wave. Opamps are linear elements, which means that they act on each frequency separately; however, each frequency is amplified with different gain and phase shift. This means that different components of the square wave will be amplified differently, leading to a distorted output. In practice, this will yield a nearly-square wave, but with vertical edges slightly skewed and rounded.

Thank you for answering! What do you mean by "seems like a bootloader+FLIP"?