Strategies for powering an ATtiny micro controller and 40W peltier from mains
Would using the 40W SMPS to power both sub-circuits be a viable option? If so, how?


Yes, that sounds likely. Your microcontroller circuit is likely to draw very little current. Unless energy efficiency is paramount, this means you can use a cheap linear regulator like a 7805 to get 5V from a 12V supply.

The biggest draw on the 5V circuit is likely the relay coil. To solve that problem, I'd suggest not using a 5V relay. Instead, use a 12V relay, and drive it via a transistor, like this:







simulate this circuit &ndash; Schematic created using CircuitLab

This way, the current necessary to run the relay coil is drawn from the efficient 12V supply, and your 7805 won't get hot.