is a dual-display digital thermostat module generally well-regarded for its versatility and affordability
A: Your 12V power supply is too weak. The relay coil causes a voltage dip. Upgrade to a 2A or 3A regulated 12V supply. xh-w1219 manual pdf
Many electronics hobbyist sites, like Instructables or Random Nerd Tutorials , host the manual as a PDF. like Instructables or Random Nerd Tutorials