PHOTOSENSOR
A
Texas Instruments OPT101—a monolithic photodiode
and single-supply transimpedance amplifier—is
used to sense the incoming solar insolation level.
Natural sunlight ranges in intensity from 0 to
approximately 1,000 W per square meter, but the
OPT101 puts out its maximum voltage at roughly
10 W per square meter of incident insolation.
With hardly any light striking the sensor, it
reached its upper limit. It was thus necessary
to attenuate the intensity of the light striking
the photodiode to increase the range over which
the sensor could differentiate intensity.
Ideally,
this would have been done with neutral density
filters, but I didn’t have any on hand. I used
the next best scientifically accurate and available
tool: electrical tape. Two layers of electrical
tape covering the photodiode were found to be
quite effective. This solution enabled the sensor
to give reasonably reliable readings over the
full range of expected intensities (see Photo
3).
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(Click
here to enlarge)
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Photo
3—The photosensor is covered with two layers
of electrical tape to provide a larger range
of sensitivity. The inset shows the sensor
without the tape cover. |
The
microcontroller’s ADC has a maximum value of 255.
Multiplying by a factor of four roughly converted
the ADC reading to watts per square meter. Ideally,
this system would be calibrated and the factor
would be more precise than four, but this approximation
gave reasonable results with a range of readings
from 0 to 1,020 W per square meter.
I
used the ATmega32-based STK500 development board
because of its integral flash memory and switches.
However, if this system were commercialized or
rebuilt, it is fairly obvious that the STK500
would not necessarily be used. It has many features
that are unnecessary for this project. A far simpler
and more compact board can be designed.
