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Helpful Hints
Helpful Hint #1: Using Inexpensive LEDs as Optical Detectors
For
some applications, an inexpensive, readily- available, LED can be used
as an optical detector. Although not generally touted as optical
detectors, most LEDs can be used as detectors without harming the
device in any manner. In fact, most LEDs can perform double-duty in the
same circuit without changing the LEDs physical or electrical
connections.
The trick is to electrically bias the LED in the
proper current-voltage (I-V) quadrant for operation as a detector and
to detect an appropriate range of wavelengths.
The LED, as the name (Light Emitting Diode) implies,
is electrically a diode and can be used as a detection device similar
to a photodiode. LEDs are first cousins to photodiodes and have some
similar optical and electrical properties.
Rule of thumb #1: LEDs will not respond as rapidly to optical rise and fall times and PN or PIN photodiodes. |
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A
properly biased, good quality photodiode will have sub-nanosecond time
response and be able to detect the rise and fall time of light pulses
shorter than 1 nanosecond in duration, an LED used as a photodiode may
only be able to detect 100 microsecond optical rise times and may not
do well in detecting pulses shorter than a millisecond. However, if you
are trying to detect if the room lights were turned on or if an outside
door was opened during the day, this might be the ticket to an
inexpensive, readily available optical detector. |
Rule of thumb #2:
LEDs will only detect light of wavelength shorter than the wavelength
of light that the LED would emit if it was put in a circuit that
forward biased the LED. |
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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. |
Rule of thumb #3:To
use the LED as an optical detector, do not forward bias the LED into
quadrant # 1 of the current-voltage (I-V). (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). |
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Photodiodes
are typically operated in quadrant 3 of their I-V characteristics. In
this mode, a small reverse bias voltage is applied to the device and
the incident light linearly increases the leakage current proportional
to the intensity (actually the number of above bandgap photons) of
light falling on the photodiode die. Although LEDs are not intended to
experience large reverse bias voltages, most can be reverse biased by a
few volts (3v to 7v) and operate in the photodiode mode. Make sure you
limit the magnitude of the reverse current so that you do not damage
the LED. The photodiode mode will give the best time response and the
most linear sensitivity to light. You can expect sensitivities of a few
tenths of a microamp of current for each microwatt of optical power
directed at the LED (or photodiode die).
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 make the LED act as a current generating device
proportional to the magnitude of light incident on the LED die, use the
"short circuit mode". External circuit conditions and the amount of
light incident on the LED die determines the current-voltage operating
point of the device. If you keep the generated voltage across the LED
low by using circuit techniques to effectively put a short circuit
across the LED, the generated current will be approximately linearly
related to the amount of light incident on the LED. This becomes
similar to the photodiode mode. The simplest approach to this circuit
is to put a relatively small (know value) resistor across the two
terminals of the LED and measure the small voltage generated by the
current flowing through your known resistor. Experiment with the size
of the resistor; the best value will depend on the amount of light you
are detecting and the optical arrangement to capture this light onto
your LED. Try 100 ohms, 1,000 ohms and 10,000 ohms. If you are using
the NEEM-112 to detect this signal, use one of the +/- 2.5 volt
channels for 16 bit sensitivity (approximately 75 microvolts
resolution).
To make the LED act as a voltage generating device,
operate in the open-circuit mode by connecting either no load resistor
or as large a resistive load to the LED terminals as possible. In the
no load resistor mode, the voltage measuring instrument will serve as
the load. If the input impedance of the measuring instrument is high
enough, the circuit conditions attached to the LED will appear as an
open circuit condition. If you are using the NEEM-112 to detect this
signal, use any of the voltage sensing channels including either of the
0 to 5 volt channel, either of the +/- 2.5 volt channels, or the +/- 10
volt input channel. In this voltage generating, open circuit mode, the
LED will generate an open circuit voltage between 0.0 volts and
approximately 1.6 volts - depending on the amount of light incident on
the LED die and the color of the LED selected for this measurement.
Remember, in the open circuit mode, the magnitude of the voltage is NOT
linearly related to the amount of light incident on the LED die. |
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