Detecting I2C transfer with LED
It can be done with two N-channel MOSFETs or one P-channel MOSFET (depending on what you have locally / can get / is in your bits box):







simulate this circuit &ndash; Schematic created using CircuitLab

In the left schematic the first MOSFET acts like a NOT gate, and the second is a simple low-side switch. The pair together make an active-low low-side switch.

The right hand schematic, which is somewhat simpler, uses a single P channel MOSFET as a high-side switch. This is normally driven with an N-channel MOSFET to invert the logic, since it is naturally active-low. You don't want that, so you can just connect direct to the gate.

Note that all MOSFETs are "logic level" with a threshold voltage below 3V (or -3V in the case of the P-channel).