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university:courses:alm1k:alm-lab-led-sensor [20 Jun 2018 16:26] – [Background:] Imprecisions, quadrant 4 and 3 seems to have the same definition. REMOVE the (?) tag after appropriate correction, please. Michel Walshuniversity:courses:alm1k:alm-lab-led-sensor [24 Mar 2022 16:13] (current) – [Step 2 Directions:] Doug Mercer
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-======Activity: LED as light sensor ======+======Activity: LED as light sensor, For ADALM1000======
  
 =====Objective:===== =====Objective:=====
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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 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 Analog Parts Kit).
  
-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 positive, 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 positive, 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.
  
 ====Materials:==== ====Materials:====
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 =====Step 2 Directions:===== =====Step 2 Directions:=====
  
-Change the circuit on your breadboard to the Darlington configuration shown in figure 2. Be sure to turn off the power supply before making any changes to the circuit. With the Darlington connected transistors the emitter current of Q<sub>2</sub> becomes the base current of Q<sub>1</sub> such that the photo generated current from the LED D<sub>1</sub> is now multiplied by ß<sup>2</sup> and will appear in the load resistor R<sub>L</sub> from the collectors of Q<sub>1</sub> and Q<sub>2</sub>. Because of this much higher current gain we can use a much lower value load resistor.+Change the circuit on your breadboard to the [[wp>Darlington_transistor|Darlington configuration]] shown in figure 2. Be sure to turn off the power supply before making any changes to the circuit. With the Darlington connected transistors the emitter current of Q<sub>2</sub> becomes the base current of Q<sub>1</sub> such that the photo generated current from the LED D<sub>1</sub> is now multiplied by ß<sup>2</sup> and will appear in the load resistor R<sub>L</sub> from the collectors of Q<sub>1</sub> and Q<sub>2</sub>. Because of this much higher current gain we can use a much lower value load resistor.
  
 ====Step 2 Procedure:==== ====Step 2 Procedure:====
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 ====Questions:==== ====Questions:====
  
-How well does the red LED respond to the various light sources? Does it respond to another red or yellow or green LED used as a light emitter? How about the yellow and green LEDs? Is the infrared LED sensitive to the same or different wavelengths of light compared to the visible light LEDs? Which is the most sensitive to standard household lights such as incandescent and compact florescent bulbs?+How well does the red LED respond to the various light sources?\\ Does it respond to another red or yellow or green LED used as a light emitter?\\ How about the yellow and green LEDs?\\ Is the infrared LED sensitive to the same or different wavelengths of light compared to the visible light LEDs?\\ Which is the most sensitive to standard household lights such as incandescent and compact florescent bulbs?
  
-How does the sensitivity of the Darlington connected configuration compare to the single common emitter configuration? Are the minimum and maximum voltages the same for both configurations? If not why?+How does the sensitivity of the Darlington connected configuration compare to the single common emitter configuration?\\ Are the minimum and maximum voltages the same for both configurations? If not why?
  
 **For Further Reading:** **For Further Reading:**
  
-https://en.wikipedia.org/wiki/LED\\ +[[wp>LED|LEDs]]\\ 
-https://en.wikipedia.org/wiki/LED_circuit\\ +[[wp>LED_circuit|LED circuits]]\\ 
-https://en.wikipedia.org/wiki/Photodiode+[[wp>Photodiode|Photo diodes]]
  
-**Return to ALM Lab Activity [[university:courses:alm1k:alm-labs-list|Table of Contents]]**+**Return to [[university:labs:intro_ee|Introduction to Electrical Engineering]] Lab Activity Table of Contents**\\ 
 +**Return to [[university:courses:alm1k:alm_circuits_lab_outline|Circuits]] Lab Activity Table of Contents**
  
university/courses/alm1k/alm-lab-led-sensor.1529504768.txt.gz · Last modified: 20 Jun 2018 16:26 by Michel Walsh

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