I designed motion detectors for a couple years and once spent a few weeks researching the use of LEDs as light detectors. What I found was the wavelength each LED could detect and the sensitivity was inconsistent from LED-to-LED and inconsistent across LED batches. We were producing 50k-1M units per year so those findings made using the LED as a sensor a deal breaker and we eventually just added a $0.12 photo-sensor to the boards to detect light.

All of that said, I designed a few demos (using the LED on the motion detector as a light sensor) that were pretty cool:

- I connected the amplified LED input on a motion detector to the microcontroller and sampled the LED in between PWM cycles (we PWMed the LED to save power). Even when the LED was on you could shine a flashlight at the motion detector light-pipe and trigger events or a test mode.

- On a motion detector that had a bi-color LED (red/green) in the same SMT package I connected the amplified LED input from one LED to the microcontroller and pulsed the other LED at a non-visible rate. If you placed your finger on the light pipe it would reflect the light from the pulsing LED to the other LED and you could detect when someone touched the light-pipe.

- A motion detector that could receive serial data over the LED to configure parameters.