What's the best way to detect zero crossing in this design?

The design that I am talking about:

I want to be able to detect the zero point with an MCU of which pin can withstand only 3.3 V. I'll forgo optocoupler because A. I want everything to be as cheap as possible and B. I want everything as simple as possible.

As you probably have guessed no, I don't have a 100% understanding of the schematic, so this design could very well have featured a zero-crossing detecting mechanism yet I failed to find it completely but I doubt it.

The incoming voltage max will be 110V. I want to detect the zero-crossing so I can turn on and off Q1 and Q2 alternatively on a regular cycle to make the throttling of power possible.

So my questions:

1. How do I do the zero crossing detection?

My knuckle-dragging plan: attach one pin to a resistor 500K to limit the current, then attach the resistor to the center point surrounded by L1, Q1 and Q2 and D13, yes, that point attached to the K pin of Q1 and Q2, negative pole of D13 and one side of L1. I'll keep sampling it with ADC and decide when the zero point has arrived, at which point I'll make the switch.

• An optocoupler would be cheap, but unreliable due to delay, a comparator with local supply to directly control Q1&2 would be more advanced, but more reliable. I have to admit I'm having a hard time understanding the schematic. I see a +5V on the AC side, I see a series RC to 5V (startup?), half-bridge with SCR and 2.2$\mu$H inductor, what is it all meant to do? In the lower right corner it says "light switch", but then SCR_IGBT... what sort of light? – a concerned citizen May 2 '18 at 8:21
• Hello citizen, your concern is duly noted. It's a dimmer used in tandem with its load (a lightbulb of course) and the V-DC will be connected to a viper06 chip. Instead of a comparator I'm thinking about using ADC to constantly sampling certain point and decide the wave form, any suggestions? – Matt Cox May 3 '18 at 4:38
• People answering here do it on their own free will, there's no need for sarcasm. I had to see your other question to see that it's an eval board (a link would have been nice), which doesn't say too much, unfortunately. But you say you want it as simple as possible, even suggest opto. If you can get one fairly fast, you'll have less delay, and it will do the job, but how do you know the uC in the eval board doesn't do what you want, already? Do you have more info regarding the schematic and what does it do? – a concerned citizen May 3 '18 at 7:21
• More info? Sure! Um... I want it to do exactly, well, almost exactly what it supposed to do, dimming, on and off etc. but using my own mcu. I want to be able to control that "bridge" (so I can get on with the dimming whatnot ) but clearly in order to do that I need the zero-crossing detection. And here is the full specs page, knock yourself out :D st.com/en/evaluation-tools/steval-ild004v2.html – Matt Cox May 4 '18 at 5:09
• As to the likely question "Why dont you use their mcu?" Well, they did not really offer the source code so... – Matt Cox May 4 '18 at 5:10