Editorial Feature

LED Light Sensors

Oct 14 2013

Due to the high efficiency of LED sensor systems, they are highly sought-after for multiple applications. One particular application for LEDs is their use as photodiodes for light detection. While acting as photodiodes, LEDs are very sensitive to wavelengths that are equal to or shorter than the predominant wavelength they emit. For instance, a green LED is sensitive to blue light and some green light, but not to red or yellow light.

LEDs can be added to systems having only minor modifications in the circuitry. LEDs multiplexed in such a circuit can be used for both light emission and light detection. This sensor system can perform this dual function in the same circuit without changing its electrical or physical connections making them ideal for a number of applications including bidirectional communications and ambient light detection.

Image Credit: Photos.com

Working Principle of LEDs as Light Sensor

LEDs can be used as a light sensor by combining them with a microcontroller. Most of the modern microcontrollers consist of bidirectional I/O ports with configurable internal pull-ups or tri-state inputs. The circuit can provide a very accurate and precise measurement of the photocurrent with the help of a high-impedance input.

The LED charges to 5V in a detector mode, and this charge is sustained in the circuit by the inherent capacitance of the diode. The P1 section to the microcontroller is then converted to the high-impedence mode as a key step in the function of this sensor system. Under reverse-bias conditions, the LED acts as a capacitor that is in parallel with a current source. This generates a leakage current through P1. The voltage across the microcontroller is monitored by a timer-counter. The amount of light detected is proportional to the decay time. With the increase in amount of light received, the diode discharges at a much faster rate.

Benefits of LEDs as Light Sensor

LED sensors are becoming an ideal option for a light source as it is cost-effective to manufacture and apply, and it can also provide a digital measurement of a light source without the need to waste energy on amplifying the system.

LED light detectors have excellent signal-to-noise characteristics owing to signal integration over the measurement. They also exhibit enhanced sensitivity, which makes this sensor system an ideal approach compared to conventional photodiode systems.