Download Detection of Harmful Algal Blooms

Detection of Harmful Algal Blooms
Matthew Barboza, Supervising Professor Dr. Sungyong Jung
Summer REU National Science Foundation AMIC
PHOTODIODE
Abstract
The purpose of this project is to create a remote
sensor array that will detect the presence of toxins in
large bodies of water released during harmful algal
blooms. The proposed solution will consist of a sensor
that will: i) provide pulses of light through an optical
antenna array; ii) diminish the intensity of light as the
toxin concentration increases; iii) provide a signal that
corresponds to the presence of toxins. My role in the
project is to develop a prototype circuit that will be
used as a proof of concept. Testing has been done to
ensure the selected components are working as
expected.
a)
 For generic p-n diode: Diffusion current = Drift Current
 Reverse biasing a diode reduces diffusion current, resulting in a net
reverse current
 For photodiode: Window allows photons into material, a percentage of
photons create electron-hole pairs (EHPs)
 Negligible increase in Diffusion current, dramatic increase in drift current
 Effectiveness of photodiode - Responsitivity: the ratio of reverse current to
power of incident light. Expressed in amps/watt
The FDS1010 has a speculated responsitivity of .5 A/W for 780 nm
wavelength light.
Introduction
A hazardous algal bloom occurs when a sudden
increase of nutrients in large bodies of water
leads to a rapid growth of first-order organisms
that can cause harm to their environment, e.g.
the release of the toxin microcystin. Our plan is
to create a remote sensing node that will
monitor for the presence of specific toxins and
wirelessly transmit any detection to a central
location. This will be done by providing a pulse
of light directed through a film that reduces its
intensity when the target toxins are present. The
reduction of light will be measured by a
photosensor to provide a signal that will
correlate to the concentration of toxin..
c)
a)
a) Array of sensor nodes
transmitting data to
central location
b) Proposed integrated
circuit for microcystin
detection
b)
ADC PERFORMANCE
a) ADC test set-up
The microcontroller is programed to convert a voltage signal in the range
of 0 to 3.2 V into a 10-bit number. The signal was created from a
potentiometer, which has its bulk resistance located between the voltage
supply and ground. The upper four bits of the result are used to drive
four output pins of the controller connected to leds. This allows me to
see what the ADC is measuring in .2 volt increments. By sweeping the
wiper across its full range and measuring at what input voltages the
output leds change their value, I can get an accurate representation of
how close the ADC result is to the applied voltage..
b) ADC test schematic
c) ADC test results
The result from the ADC gave an accurate
representation of the input voltage. Any
discrepancy can be attributed to the
potentiometer.
The photodiode showed both a noticeable
increase in current with an increase in light and
a stable output regardless of a varying bias
voltage.
Further testing will include a reliable control of
light with the laser diode and the use of the
transimpedance amplifier to allow the
microcontroller’s ADC to indirectly measure the
response from the photodiode.
b)
Materials
o A dsPIC33FJ128MC802 microcontroller from
Microchip.
o The microcontroller will be programed using C code
and the manufacturer provided development
environment, MPLAB X, and a separately purchased
ICD 3
o D7805I laser diode: 5 mW optical power output with
780 nm wavelength
o OPA380 transimpedance amplifier from DigiKey
o FDS1010 photodiode from Thor Labs
o Agilent E3620A Dual Power Supply, Agilent 34401A
Digital Multimeter, DSO-X 2004A Oscilloscope
Materials
Circuit
close-up
Summary and Conclusion
b)
PHOTODIODE RESPONSE
a) Photodiode test set-up
The photodiode was placed in series with a measured
resistance and a voltage source. As the voltage source
increased, the diode’s reverse bias voltage was measured
and the reverse current was calculated from the voltage
across the resistor. Several data points were collected to
obtain a current-voltage relationship. The process was
repeated under a fluorescent lamp to see the effect of
increased lighting.
b) Photodiode test schematic
c) Photodiode test results
Acknowledgements
a)
c)
Thank you to Dr. Sungyong Jung and
Mohammadreza Moghadam for their patience,
guidance, and support, and to the National
Science Foundation for their financial
contribution: grant #EEC-1156801.