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— | university:courses:electronics:electronics-lab-led-sensor [07 Feb 2022 15:17] – [Activity: LED as light sensor] Doug Mercer | ||
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+ | ======Activity: | ||
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+ | =====Objective: | ||
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+ | The objective of this Lab activity is to explore the use of LEDs as a photodiode light sensor and the use of NPN and Darlington connected NPN transistors as interface circuits for the light sensor. | ||
+ | |||
+ | =====Background: | ||
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+ | When exposed to light photodiodes produce a current that is directly proportional to the intensity of the light. This light generated current flows in the opposite direction to current in a normal diode or LED. As more photons hit the photodiode the current increases causing a voltage across the diode. As the voltage across the diode increases the linearity decreases. | ||
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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 " | ||
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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. | ||
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+ | =====Materials: | ||
+ | ADALM2000 Active Learning Module\\ | ||
+ | Solder-less breadboard\\ | ||
+ | Jumper wires\\ | ||
+ | 2 - 2N3904 NPN transistors ( or SSM2212 NPN matched pair )\\ | ||
+ | 1 - 100KΩ resistor\\ | ||
+ | 1 - 2.2KΩ resistor\\ | ||
+ | 3 - LEDs ( multiple red, yellow and green colors )\\ | ||
+ | 1 - Infrared LED ( QED-123 ) | ||
+ | |||
+ | =====Directions: | ||
+ | On your solder-less breadboard construct the LED light sensor circuit as shown in figure 1. Notice that the LED diode, D< | ||
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+ | {{ : | ||
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+ | <WRAP centeralign> | ||
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+ | =====Hardware Setup:===== | ||
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+ | {{: | ||
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+ | <WRAP centeralign> | ||
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+ | Use the variable positive power supply from the ADALM2000 module set to +5 V to power your circuit. Use scope channel 1 to monitor the voltage at the collector node of Q< | ||
+ | |||
+ | =====Procedure: | ||
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+ | Insert a red, yellow or green LED into the circuit as shown one at a time. Try exposing the three different color LEDs from your ADALP2000 Analog Parts Kit to different light sources such as standard incandescent, | ||
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+ | {{: | ||
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+ | <WRAP centeralign> | ||
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+ | {{: | ||
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+ | <WRAP centeralign> | ||
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+ | {{: | ||
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+ | <WRAP centeralign> | ||
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+ | =====Step 2 Directions: | ||
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+ | Change the circuit on your breadboard to the Darlington configuration shown in figure 6. 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< | ||
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+ | {{ : | ||
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+ | <WRAP centeralign> | ||
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+ | =====Step 2 Hardware Setup:===== | ||
+ | |||
+ | {{: | ||
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+ | <WRAP centeralign> | ||
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+ | =====Step 2 Procedure: | ||
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+ | Repeat the same procedure of inserting the various LEDs into the circuit for D< | ||
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+ | {{: | ||
+ | |||
+ | <WRAP centeralign> | ||
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+ | {{: | ||
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+ | <WRAP centeralign> | ||
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+ | {{: | ||
+ | |||
+ | <WRAP centeralign> | ||
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+ | =====Questions: | ||
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+ | 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? | ||
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+ | How does the sensitivity of the Darlington connected configuration compare to the single common emitter configuration? | ||
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+ | <WRAP round download> | ||
+ | **Resources: | ||
+ | * Fritzing files: [[downgit> | ||
+ | * LTspice files: [[downgit> | ||
+ | </ | ||
+ | |||
+ | **For Further Reading:** | ||
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+ | https:// | ||
+ | https:// | ||
+ | https:// | ||
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+ | **Return to Lab Activity [[university: | ||
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