In this project, I will be upgrading the fan remote on the wall (called a thermostat by the manufacturer) with the latest version of the remote... and I will add wireless remote capability as well. The thermostat I have is shown to the right in the photo below. It is no longer available and replaced by the thermostat on the left. Since the new thermostat has more "real estate" to build the wireless remote upon, and if you have the newer thermostat - you cannot likely buy the older one, I decided to swap them out.

As far as I can tell, these thermostats are compatible with several fan models as shown here: MaxxAir Fans. Look at the column for "Optional Wall Control" and you will see which fan models are compatible. The thermostats seem quite common in the OEM market, but not so much in the retail aftermarket.

I have seen prices for these anywhere from just under $8 to almost $40. I bought mine for $7.94 at the link here:

RVAutoParts had two similar looking thermostats on their website: P/N 00-03550K, and 00A03550K. The "A" version was slightly more expensive; $8.16. So I bought both to see the difference. Oddly enough, both thermostats were identical. I don't know if "A" is a revision or some other difference, so if you are looking to buy one, go for the 00A03550K part number.

Opening both "thermostats" reveals they are not thermostats at all, they are just a circuit board with 2 resistors and 4 switches. However, we will call this item a "thermostat" in this text to differentiate between the wall switch and wireless remote. In the above photo, the new thermostat is on the left, while the older thermostat from my RV is on the right.

The older thermostat uses tactile switches, while the newer one - as shown above, uses a printed circuit board with etched switch contacts, along with a carbon pyle membrane to make the connection when it is depressed. Both methods are commonly used in the electronics industry, and I can work with either.

What makes remote control of the thermostat possible is the recent introduction of a micro 4 channel receiver in the marketplace. The major difference is the micro-receiver does not include any relays.

There are 4 outputs that correspond to each of the 4 buttons on the transmitter. When a button on the transmitter is depressed, the corresponding output pin raises to 5V. Since the output current is limited to 10mA, it can only drive low-level circuits.

The receiver has the typical Latch, Momentary, Jog type arrangement as do the other generic type relays.

And their cost is under $10... Perfect!

Reverse-engineering the circuit for the thermostat revealed that it is fairly simple, however since mistakes can still be made, especially when traces go under components, I built a breakout board to verify the thermostat wiring schematic.

This breakout simply allows me to measure the voltage on each pin to the remote without needing 4 hands. After a bit of testing, I figured out the schematic for the "thermostat".

This is typical of the behind-the-scene stuff you don't typically see when I do a project.

To add wireless remote capability, I simply need to build a new circuit board with the switch pads on the backside, then add relay "dry contacts" to each switch.

The resistor looks to function as a "pull-down" resistor. The sense pins (pins 2 and 3 on the RJ11 connector) simply introduce either 5V or GND (through the resistor). However, when no switch is depressed, there is no voltage present on the sense line. This likely connects to a "window" style input rather than a logic circuit on the fan's remote board. How the fan control board works is still a mystery as I would have had to tear the fan apart in the middle of the winter to figure it out.

For that reason, I went with an old-school method of using relay contacts connected in parallel with each of the thermostat switches. I would have liked to have used a more elegant-type solution such as a MOSFET/OptoIsolator (sometimes known as a Solid State Relay) to control each switch, but without knowing how the fan control board works, I felt simple relay contacts was the safer solution. The only downside to using the relays is the receiver board was not sufficient to drive them, so I had to add a switching transistor to each relay.

While using DPDT relays seems overkill, they were the smallest/cheapest combination I could find from a US supplier (other countries may find different sources). For example, all of the SPST relays I found were for RF switching, and were $6-8 each. The relays I am using cost about $2 each. Since 4 relays are needed, this can be a farily large expense.

Hint: I typically order a higher quantity of parts than needed for a project. Higher quantity often brings a lower unit cost, and I then have spares for repair or if I ruin a part installing it. And since I tend to use the same or similar parts in my projects, this helps create a bench-stock of parts for future projects.