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Rarely Asked Questions – Issue 108, August 2014
LEDs are Photodiodes Too
Q: I read RAQ #45, Glass Diodes May See the Light – and Hum, which discusses 100‑/120‑Hz LF noise caused by a glass diode’s photosensitivity. I need a cheap photodetector, but a 1N4148 doesn't seem to work. How should I connect it?
A: 1N914/1N4148 diodes have enough photosensitivity to cause hum, but not enough to use as photocells. Their sensitivity is more in the infrared than the visible spectrum, and even in bright sunlight their photocurrent is only about 10 nA. Glass diodes are not a practical substitute for solar photovoltaic panels! Interestingly, an old-fashioned flashlight with an incandescent bulb excites two or three times more photocurrent in these diodes than direct sunlight, and a mains-powered 60-W incandescent bulb produces about 7% photocurrent modulation at 100-Hz. This suggests that the likely source of hum in RAQ#45 is from both incandescent and fluorescent lighting.
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.
An elegant application is where an LED is driven by an analog microcontroller. The same LED may be used as a photodetector by disabling the digital output driving it and sensing its photocurrent output. If the microcontroller has dual-purpose analog input/digital I/O pins, like the ADuC7023 or the Atmel ATMega controllers used in Arduinos, this can be done with an LED and two resistors—and just one processor pin.
A semiconductor diode can be connected as a photocell in two ways: photovoltaic mode and photoconductive mode. Solar panels work in photovoltaic mode; light shines on them, the anode becomes more positive than the cathode, and a current proportional to the incident light flows in any circuit connected between the anode and the cathode. The diode is forward biased and its capacitance is several times larger than its reverse-biased capacitance.
In photoconductive mode, light shining on a reverse-biased photodiode causes a photocurrent proportional to the incident light to flow. It is best to use photoconductive mode for ac signals as the frequency response is better, but light measurement in photovoltaic mode is very simple, as shown in Photodiodes and Other Light Sensors.
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