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university:courses:electronics:electronics-lab-led-sensor [07 Feb 2022 15:17] – [Activity: LED as light sensor] Doug Merceruniversity:courses:electronics:electronics-lab-led-sensor [20 Nov 2023 04:55] (current) – [Background:] Eric hungerford
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 In addition to emitting light, an LED can be used as a photodiode light sensor / detector. This capability may be used in a variety of applications including ambient light level sensor and bidirectional communications. As a photodiode, an LED is sensitive to wavelengths equal to or shorter than the predominant wavelength it emits. A green LED would be sensitive to blue light and to some green light, but not to yellow or red light. For example, a red LED will detect light emitted by a yellow LED and a yellow LED will detect light emitted by a green LED but a green LED will not detect light emitted by a red or yellow LED. All three LEDs will detect "white" light or light from a blue LED. White light contains a blue light component which can be detected by the green LED. Recall that visible light wavelengths can be listed from longest wavelength to shortest wavelength as Red, Orange, Yellow, Green, Blue, Indigo, Violet (remember the mnemonic "Roy G. Biv"). Violet is the shortest wavelength light with the most energetic photons and red has the longest wavelength light with the least energetic photons of all of the visible colors of light. LED's with clear plastic encapsulation will be more sensitive to broad-spectrum illumination (like general room lighting) than LED's with colored encapsulation (such as those included in the ADALP2000 Analog Parts Kit). In addition to emitting light, an LED can be used as a photodiode light sensor / detector. This capability may be used in a variety of applications including ambient light level sensor and bidirectional communications. As a photodiode, an LED is sensitive to wavelengths equal to or shorter than the predominant wavelength it emits. A green LED would be sensitive to blue light and to some green light, but not to yellow or red light. For example, a red LED will detect light emitted by a yellow LED and a yellow LED will detect light emitted by a green LED but a green LED will not detect light emitted by a red or yellow LED. All three LEDs will detect "white" light or light from a blue LED. White light contains a blue light component which can be detected by the green LED. Recall that visible light wavelengths can be listed from longest wavelength to shortest wavelength as Red, Orange, Yellow, Green, Blue, Indigo, Violet (remember the mnemonic "Roy G. Biv"). Violet is the shortest wavelength light with the most energetic photons and red has the longest wavelength light with the least energetic photons of all of the visible colors of light. LED's with clear plastic encapsulation will be more sensitive to broad-spectrum illumination (like general room lighting) than LED's with colored encapsulation (such as those included in the ADALP2000 Analog Parts Kit).
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 +(Beware that phosphor-coated LEDs are increasingly common. These LEDs actually have a blue emitter, but a phosphor coating causes the blue light to be converted to any other color. If you try the following experiments with such an LED you may find very poor results lighting it from an identical LED, even though they appear to be of the same wavelength! Because, e.g., the blue LED, used as a photodiode, is being lit by the longer wavelegth orange phosphor.)
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 To use the LED as an optical detector, do not forward bias the LED into quadrant #1 of the current-voltage (I-V) curve. (Quadrant 1 is when the operating voltage and current are both positive.) Allow the LED to operate in the solar cell mode, quadrant #4 (operating voltage is positive, current is negative), or in the photodiode mode quadrant #3 (operating voltage is negative, current is negative). In the solar cell mode, no applied bias voltage is used. The solar cell (or LED in this case) generates its own current and voltage. To use the LED as an optical detector, do not forward bias the LED into quadrant #1 of the current-voltage (I-V) curve. (Quadrant 1 is when the operating voltage and current are both positive.) Allow the LED to operate in the solar cell mode, quadrant #4 (operating voltage is positive, current is negative), or in the photodiode mode quadrant #3 (operating voltage is negative, current is negative). In the solar cell mode, no applied bias voltage is used. The solar cell (or LED in this case) generates its own current and voltage.
university/courses/electronics/electronics-lab-led-sensor.txt · Last modified: 20 Nov 2023 04:55 by Eric hungerford

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