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The R3000 is adopted because we have a lot of these in stock, and also because it’s a diode that has a 3~4 V forward voltage drop (as opposed to 0.7V on standard diode). Other than that, you can use standard diodes such as 1N4007.
The TVS should be at least 43CA or above — it needs to be larger than the highest voltage under normal operating situations. Because a 24V AC source often has a no-load voltage of up to 30V AC, and that has an instantaneous voltage of up to 30 * 1.414 = 42.4V, you need TVS that’s rated at least 43V or above. So 51CA is fine, but 33CA or 36CA is not.
Z0109MN for triac is fine. You can use it to replace BT1308W. Because the way OpenSprinkler works is that it holds the gate on constantly to keep the triac open, the 4Q triggering is not an issue for OpenSprinkler.
There are actually two zener diodes — one power zener connected in series to R3000 and the purpose of it is to further drop the voltage by another 2~3V (because MC34063 has a maximum input voltage of 40V, I try to keep the input voltage safely below that limit). The other is connected to the 5V rail after the PTC fuse. The second zener is 5.6V, and is used to protect the 5V line (in case of shorting).
I will post an image of the smt version when I get time to take a picture.