LED Sensing:

Light emitting diodes are robust, low-cost, and energy efficient. LEDs cover an increasingly broad spectral range from UV to near infrared. LEDs can be both light emitters and detectors. For example, a LED that emits greenish-yellow light at the peak wavelength of about 555 nm detects green light at the peak wavelength of about 525 nm and over the spectral width of 50 nm. Almost every LED is capable to detect a relatively narrow band of wavelengths, with different sensitivity. When a LED is subjected to light, they generate a backwards biased current proportional to the light striking the diode, typical about 50pA. A precise measurement of the LED photocurrent is possible, using inherent capacitance of diode itself (typically picoFarads) and microcontroller I/O ports with configurable internal pull-ups states and built-in digital timer-counter.