Wiki

Below is the source of this page. To edit this source as well as other pages on the wiki, you must login.
======Activity: LED as light sensor - ADALM2000======

=====Objective:=====

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:=====

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.

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).

(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.)


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.

=====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<sub>1</sub>, is reverse biased i.e. opposite to how it would be connected as a light emitter. The photo generated current will flow into Q<sub>1</sub> as base current and will appear in the collector multiplied by the current gain, ß of the transistor.

{{ :university:courses:electronics:aleds_f1.png?600 |}}

<WRAP centeralign> Figure 1 LED and single common emitter NPN light sensor </WRAP>

=====Hardware Setup:=====

{{:university:courses:electronics:led_single_emitter-bb.png|}}

<WRAP centeralign> Figure 2 LED and single common emitter NPN light sensor Breadboard Circuit </WRAP>

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<sub>1</sub>.

=====Procedure:=====

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, florescent and LED lights held at differing distances from the LED sensor. Observe the voltage waveform seen at the collector of Q<sub>1</sub>. Try inserting the infrared LED from your kit and observing how it responds to the light from the different sources. Try increasing the sensitivity or gain by increasing the value of R<sub>L</sub> to 200KΩ or 470KΩ.

{{:university:courses:electronics:led_single_emitter_wav1.png|}}

<WRAP centeralign> Figure 3 Red LED and single common emitter NPN light sensor - Led light max distance </WRAP>

{{:university:courses:electronics:led_single_emitter_wav2.png|}}

<WRAP centeralign> Figure 4 Red LED and single common emitter NPN light sensor - Led light medium distance </WRAP>

{{:university:courses:electronics:led_single_emitter_wav3.png|}}

<WRAP centeralign> Figure 5 Red LED and single common emitter NPN light sensor - Led light min distance </WRAP>


=====Step 2 Directions:=====

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<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.

{{ :university:courses:electronics:aleds_f2.png?600 |}}

<WRAP centeralign> Figure 6 LED and Darlington connected NPN light sensor </WRAP>

=====Step 2 Hardware Setup:=====

{{:university:courses:electronics:led_darlington-bb.png|}}

<WRAP centeralign> Figure 7 LED and Darlington connected light sensor Breadboard Circuit </WRAP>

=====Step 2 Procedure:=====

Repeat the same procedure of inserting the various LEDs into the circuit for D<sub>1</sub> and measuring the response to the various light sources and record them in your lab report.

{{:university:courses:electronics:led_darlington_wav1.png|}}

<WRAP centeralign> Figure 8 Red LED and Darlington connected light sensor - Led light max distance </WRAP>

{{:university:courses:electronics:led_darlington_wav2.png|}}

<WRAP centeralign> Figure 9 Red LED and Darlington connected light sensor - Led light medium distance </WRAP>

{{:university:courses:electronics:led_darlington_wav3.png|}}

<WRAP centeralign> Figure 10 Red LED and Darlington connected light sensor - Led light min distance </WRAP>


=====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 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?

<WRAP round download>
**Resources:**
  * Fritzing files: [[downgit>education_tools/tree/master/m2k/fritzing/led_light_sensor_bb | led_light_sensor_bb]]
  * LTspice files: [[downgit>education_tools/tree/master/m2k/ltspice/led_light_sensor_ltspice | led_light_sensor_ltspice]]
</WRAP>

**For Further Reading:**

https://en.wikipedia.org/wiki/LED\\
https://en.wikipedia.org/wiki/LED_circuit\\
https://en.wikipedia.org/wiki/Photodiode

**Return to Lab Activity [[university:courses:electronics:labs|Table of Contents]].**
Analog Devices Wiki

Page Tools
