The nest runs from 24VAC too, ie typically 30V after rectification, but some HVAC systems can be badly wired and go higher.
24VAC is the standard for a US thermostat control loop (same as most irrigation systems)
As has been noted in other threads, the ATSHA204 has been replaced by the ATSHA204A, which can be found in stock on Mouser.
It just dawned upon me that the RT424F24 relays are latching. Wont this implicate, that if a relay have been turned on and at the same time something happens to the imp that puts it out of function (don’t know what) and the 24VAC stays on, the relays will stay on and the HVAC unit, or in this case, the solenoid will stay on until the 24VAC is shut of?
If this is so I could potentially find my self in an unfortunate situation, where I have poured ALOT of water on some floor.
Maybe better to use this relay instead, would also save me alot of FETs?
http://www.digikey.com/product-detail/en/RT424024/PB969-ND/1095293
Yeah, you may want to change to momentary relays… makes it easier as no shift register required!
Ok two questions:
I already sent my gerber files to ITEAD studio PCBA team (stupid me). However when I receive the assembled boards from them, will I be able to directly swap the RF424F24 (latching relay) with the RT424024 (non latching from the link above)? The pin connections would still be correct, right? (except for all the “off mosfets” would be of no use)
And just to be clear: A Momentary relays is a “normal” non-latching relay that requires a constant “on-current” to be activated?
Don’t think you can swap. I can’t see the pinouts but the latching one appears to have two coils vs one.
Yes momentary requires coil current to close the NO contacts.
So we received the boards a while ago (did the changes to relay that Hugo made aware of). They work as they should except for one thing: the SCL / SDA lines to the i2c temp humidity sensor are swapped… Must have been thinking UART or something 
I am planning on making a relay board with 4 relays too, maybe with temperature sensor, maybe not… I am planning on using an imp002 for it instead, just to have a few more pins to connect to things to monitor. Temperature sensor could then be connected to that instead, or a switch if it should monitor a door, or what ever it could be.