[Editor's note: This post from James Bryant at sister site Planet Analog introduces a use for LEDs many of us have probably not considered: as either photovoltaic or photoconductive photodiodes.]
At about 2¢ each in large quantities, LEDs cost about five times as much as diodes, but they are much more sensitive as photocells. With the sun falling directly on it, the photocurrent of a red 5 mm LED (1000 mCd @ 20 mA) is over 20 μA. In the sunny tropics this might keep a clock battery charged. They're not well suited for power generation, but LEDs are convenient photodetectors at about 10% of the price of purpose-made ones.
An LED's spectral sensitivity depends on its color. They sense wavelengths shorter than or equal to their own emitted wavelength. This depends on the properties of the encapsulation. Light from colors that it absorbs will not reach the LED. White LEDs contain a phosphor to convert monochrome light to white light and do not make good photocells.
Manufacturers do not characterize LEDs as photocells, so minor design changes that have minimal effect on their behavior as LEDs may cause major changes in their characteristics as photocells. When using LEDs as photocells, characterize them yourself, and use conservative designs so that your circuit still performs well with any changes. This makes mass-produced circuits using LEDs as photocells demanding, but they are very useful in small batch or single system designs.
Read the rest of the story at Planet Analog.
— James Bryant has been a European applications manager with Analog Devices since 1982. He holds a degree in physics and philosophy from the University of Leeds. He is also C.Eng., Eur. Eng., MIEE, and an FBIS. In addition to his passion for engineering, James is a radio ham and holds the call sign G4CLF.
— Keith Dawson , Editor-in-Chief, All LED Lighting