I needed a AC Zero Cross Detector (ZXD) for my DIY Reflow Oven so that I can switch the Heating elements and fans at the appropriate time (When AC power is at Zero).
Some SSR's allow you to switch anytime, some WON'T switch until zero cross, either way, switching AC at a time other than Zero Cross isn't great.
ZXD is generally useful, and there are lots of applications where knowing the Zero point on your AC power is advantageous, or necessary. BUT this is 240V (or 110V) which is potentially lethal, some designs are NOT safe, for example Atmel has a reference design which feeds High Voltage AC directly into the pin of a MCU. This is both OVERKILL and POTENTIALLY LETHAL.
The Dextrel.net design was the best I have seen, it is simple, produces a solid, reliable pulse of a known duration directly around zero cross, uses very little power of it's own to work, and most importantly is safe. The digital signal is optically isolated to at least 5000V to the AC side. So provided some common sense is used in wiring up and attaching the ZXD it is perfectly safe, and produces a nice digital pulse which can be interfaced to any electronics. The output is Opendrain, so by applying the appropriate IO reference voltage to the digital port the device can interface to any MCU, 5V, 3.3V, 1.8V, anything.
The original designer even had a great detailed write up to explain how his design works. All I have done is faithfully implement the schematic and make a board based off of it, using thru hole components which makes them easy to build.