ELDONET – EUROPEAN LIGHT DOSIMETER NETWORK - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 95

Figure 2 . Hardware of the ELDONET instrument.
eliminate temperature-related drifts and potential aging. This preventive measure is not
necessary for the other two channels since their dark values are stable within one digital
unit over a wide range of temperatures. The shutter function is controlled by the built-in
microprocessor using switches which signal the end positions of the shutter movement.
The signals of the three photodiodes are routed to separate operation amplifiers with
precision field effect transistor (FET) inputs featuring very low offset voltages and
minimal temperature dependence of offset and amplification. All signals from each
stage are low-pass filtered to minimize high frequency noise. External noise is further
reduced by careful shielding in a Faraday cage. All supply voltages are highly
stabilized. The last stage of each amplification channel is a switch operation amplifier
which can be switched to 1 time, 10 times or 100 times amplification, depending on the
amplitude of the incoming signal, under software control. This measure increases the
dynamic range of the radiation measurements to 1:409500. In addition to the three
radiation channels the instruments have separate temperature sensors for the external
and internal temperature. Underwater instruments possess a pressure sensor to monitor
the water depth. All signals are routed to a 12 bit analogue/digital (A/D) converter with
8 parallel channels.
Since the instruments are installed over a wide variety of geographic locations
from polar regions to the tropics, they experience a wide range of ambient temperatures
from -50° to +50°C. In order to limit this range, the interior is maintained at an elevated
temperature of 35° C controlled by a heater operated by a fuzzy logic. Thus the internal
temperature can fluctuate only in a limited range of about 15° C.
All internal functions are controlled by a microprocessor. It generates the pulse
width modulation for the shutter servo, it polls the A/D channels and averages the data
over 256 measurements. Furthermore, it automatically adjusts the amplification of the