Alternative uses for the second port
If you are running a single MaxxAir fan, the second port on the DIY controller is free to be used for other purposes. While the second port was designed with dual-fan setups in mind, the underlying hardware is flexible enough to support several useful secondary applications without any modifications to the board itself.
The second port exposes the same optocoupler-based inputs and outputs as the primary one. This means it can safely interface with external 12 V systems, push buttons, or logic-level signals over a simple RJ11 or RJ45 cable, depending on how you choose to populate it.
Using the second port for enhanced fan control
On Deluxe fans, both the RJ25 (RJ11) and RJ45 ports can be used at the same time. If your primary connection uses the RJ45 port, the second controller port can be configured as an RJ11 connection to the same fan.
This allows you to combine the strengths of both interfaces. The RJ45 port provides access to advanced features such as speed stepping, airflow direction, and auto temperature mode, while the RJ25 port gives you direct open and close commands instead of relying on toggles. Using both ports together results in more predictable behavior, especially when automating vent position or recovering from an unknown state.
You can read more about this dual use in the documentation of the fan ports.
Remote 12v switching
The optocouplers on the DIY controller are not limited to fan control. They can also be used to switch or sense low-current 12V signals elsewhere in your van. Because the optocouplers are electrically isolated, they can safely interface with external circuits without sharing grounds. Over the RJ45 port you get:
- 3 switched outputs at max 0,5A each
- A single 12v input
The outputs allow you to switch status LEDs or drive relays, and the input can be used to register a button press. The benefit is that you can run a single Ethernet wire to anywhere else in the van and add remote IO far away from your microcontroller.
Controlling your inverter
Some lower-cost 12V inverters include a phone-style control port for remote on/off functionality. These ports are typically designed to work with simple wall switches or indicator panels and operate at low voltage and current.
The second port on the DIY controller can be configured as an RJ11 interface and used to control these inverters in the same way as their original remote panels. From the inverter’s perspective, the controller behaves like a set of button presses, making this approach compatible without modifying the inverter or opening its enclosure.
This setup can be useful if you want to integrate inverter control into Home Assistant or another automation system, or if you simply want centralized control over multiple systems using a single controller board.
