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

(2) more serious, there is a chance that due to a programming error or a hardware problem, both heating and cooling lines can be triggered simultaneously, which could be a very costly mistake. enter image description here

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.


  • \$\begingroup\$ It might be possible to use optos and relays arranged to block each other's circuit with a relay but it's an expensive solution. It might be cheaper to detect a fault with a low current optocoupler on each output to disable both and beep and alarm. But if the controllers use reeds or relays, they usually fail open circuit. \$\endgroup\$ – Sunnyskyguy EE75 Apr 7 at 18:55
  • \$\begingroup\$ What could happen if the relay is connected through a MOSFET and a transistor fails? \$\endgroup\$ – Cattus Apr 7 at 21:43
  • \$\begingroup\$ That might be an unreliable furnace input. I would expect significant transient protection to make it reliable or use relays for remote switching to make it robust yet carry little current so it's good for 1 million cycles. This is conducting AC not DC so a diode bridge would be needed to use a transistor. \$\endgroup\$ – Sunnyskyguy EE75 Apr 7 at 22:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.