Here is an interesting paper on “Very Low-Cost Sensing and Communication Using Bidirectional LEDs”. Not new (2003), but this technical report is very detailed.
The abstract to the document by Mitsubishi Electric states: “A novel microprocessor interface circuit is described which can alternately emit and detect light using only an LED, two digital I/O pins and a single current limiting resistor.”
Thanks Squonk! Via the contact form.