Electronic – Isolated mains (AC) presence detector

The Phidget board that you refer to has digital outputs only.

The voltage-controlled dimmer has an optically-isolated analog input, which takes 0-10V on the non-mains side and generates an analog signal on the hot-side (the side connected to the mains) which is read by the dimmer board micro and used to control the dimmer.

The main point is that your USB board (and whatever circuitry you intend to use to control the optoisolator current) must only be connected to the photodiode (non-hot side of the optocoupler). If you try and interface it directly to the dimmer micro, plug the circuit into the mains then plug a computer into the USB port, you're going to have a catastrophic failure on your hands (since you'll be introducing an earth connection to the primary circuitry) or worse, end up electrocuting someone.

You must make sure that inside your plastic box, no part of your Phidget USB board is located near the mains areas of the dimmer board. In North America, depending on the voltages involved and the exact standard, 4 to 8mm spacing is fairly common.

Use wires with 600V insulation rating to connect between the Phidget and the optoisolator, in case they happen to come near any of the mains circuitry along their travels.

While technically Ethernet provides (barely) enough isolation, I would not rely on it to provide for your safety. You also have other issues to consider.

One issue is that the Ethernet transformer has a parasitic capacitor between the primary and secondary. If your MCU circuit jumps up 100-ish volts when you connect it to the AC line, a pulse will be sent out the Ethernet cable. Many devices have protection on their ports to prevent damage, but many devices don't. I have blown up many devices in this way.

Then there is your personal safety while debugging this type of circuit. One tiny slipped probe while debugging could make for a very bad day.

Of course your real problem is in doing current sensing without requiring expensive isolation transformers. I suggest that you look for hall effect current sensors. Here's a link to some.