MOSFET in place of relay?


Joined Jan 15, 2015
A few problems you are likely to have. As Max pointed out you have 5 relays on that board they are TX2SA type Panasonic relays. The ones you have are dual coil latch on and latch off so a brief pulse is applied to one coil or the other to close and open the contacts. All of the logic on that board is handled by an Atmel Atmega32L 8 bit AVR micro controller. That's the ball game chip which based on inputs determines outputs including the latch and unlatch pulses to the relays. You do not need a 600 VAC 40 Amp rated triac. You would need a circuit switch which is pulse on and pulse off.

Rather than try and modify the controller I would either live with the noise or replace it with something hopefully quieter and you must have some serious hearing because it should only click when it is doing something. :)



Joined Jan 23, 2018
Is that circuit board in side the actual thermostat?? Or in a control box?
I find it hard to imagine that those relays are loud. I do see that the board is quite rigidly mounted, so my suggestion id to use an elastomeric sound deadening material, applied to the other side of the board opposite the relays.
It may be that whatever that package is mounted to is acting like a sounding board and making the sound much louder.
Since those are latching relays you will not be able to make a simple solid state replacement for them.

And, what is so bad about hearing your relays click? If that were the biggest problem I could find I would be very glad indeed!


Joined Sep 24, 2015
I have three clocks throughout the house that tick every second. I can hear them no matter where I am in the house. It's a background noise and it doesn't bother me in the least. I also have a Revers Osmosis drinking water system with a prelate pump. It also ticks every time it clicks over from "Inhale" to "Exhale" and back as it pumps water at a higher pressure. Where most RO systems run a pressure about 40 to 60% inlet pressure, the prelate pump allows me to pump my tank up to nearly full inlet pressure. That thing ticks whenever it's producing water, and can be faintly heard. Some people complain it can be heard loudly but mine is in the basement. And yes, when I'm in the basement, if it's pumping I can hear it.

I also hear dog barking regularly. Neighbor's hound dog howls every time he hears sirens. Behind me barks at the letter carrier. Other neighbor has a yappy dust mop dog that goes out then yaps at the back door to get back in. THAT is the most annoying noise I hear on a daily basis. Clicking relays that click when heat or AC comes on, then clicks when heat or AC goes off is no where as constant as a clock ticking or the prelate pump pumping. Dogs not withstanding are a nuisance. Especially when the dog down the street starts barking at 3:00 AM at a cat walking slowly by the fence knowing the dog can't get to it.

I can live with the occasional tick of the relays. My swamp cooler (Evaporative cooler) clicks when it comes on. When it's blowing on low it's more noisy than the relay. When on high it's a lot more noise. I guess what I'm saying is that your relay clicking noise may just be something that bothers you alone. Anyone else in the house complaining of the noise? I think (my opinion - that's all it is) is that you're obsessing over a very very minor issue. And I agree, building a new way of controlling your HVAC unit isn't going to be as easy as you think. Best solution has already been offered - sound dampening. Just don't trap heat. That could shorten the life of the unit.


Joined Jan 23, 2018
One more consideration is that just unsoldering those relays from the PCB will be a challenge that is beyond the soldering abilities of most people. So while the circuit is fairly straight-forward to implement, it would take a fair amount of space, and the soldering portion of the project would quite probably result in the destruction of that circuit board. You can certainly apply some noise dampening materials to the relays as a reasonable approach, and that should be adequate, and muc less risk of destruction.


Joined Jan 23, 2018
Not only are relays cheaper, they are far more tolerant of voltage and current transients, and they easily provide far better isolation between the control circuit and the load circuits.
Both of these details are rather important.