I am working on a thermostat project for a heating/cooling system in the house. The original plan involved having three relays (like this one: https://wiki.wemos.cc/products:d1_mini_shields:relay_shield) controlling heating, cooling and fan. The relays are controlled by an atmega328p which is connected to wifi through the serial line and esp8266. There are two problems that I ran into,
(1) these relays require 5V to power the coil, which means I need to have a dual voltage on the board and two voltage regulators (both atmega and esp8266 are working off 3.3V), and
The question is, what would be a better way to drive the furnace that would make it impossible to engage both heating and cooling lines at the same time.
Thermostat control lines are 24V and my furnace draws about 1.5A on either control line.
I am attaching a diagram of the furnace wiring from https://www.electrical-online.com/thermostat-wiring-explained.