By using the same LED as a receiver too? https://makezine.com/projects/how-to-use-leds-to-detect-light/ https://makezine.com/projects/how-to-use-leds-to-detect

By using the same LED as a receiver too? https://makezine.com/projects/how-to-use-leds-to-detect-light/ https://makezine.com/projects/how-to-use-leds-to-detect-ligh...