Íslenskar landbúnaðarrannsóknir - 01.09.1978, Side 62

60 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
Electrode Electrode Electrode
ELECTRODE SPACING
Fig. 3. Equipotential lines around the electrodes based on measurements in a model.
put to the preamplifier. When a fish un-
balances this circuit, the signal is
amplified, detected (i. e., the signal com-
ponent present in the 500 Hz carrier is
extracted), and filtered by a bandpass fil-
ter.
The signal passes a threshold circuit
that eliminates all low level noise. Other-
wise it is evaluated by a pulse length cir-
cuit and a length counter, to decide if the
flsh is swimming up- or downstream. A
doppel pulse appears when a fish swims
across the mat. This latter circuit contains
a timer that must be adjusted for each
stream to match the average swimming
speed of the fish. This timing sets a limit
for the counting speed and for how close
individual fish can swim for separate de-
tection. Finally, a print request circuit op-
erates simultaneously. The output from
the counter is printed on a paper strip and
the recorded data have the following form:
Hourly printout: Day of month — Hour
— Total count.
Printout upon activation: Minute —
Second - Period - Signal strength -
„Mark.“
The counter is an up-down counter and
the hourly printout shows the difierence
between the recorded up and down
counts. Period is the passage of time in
units of 0.5 s. Signal strength is an arbit-
rary number. The ,,mark“ printout can be
offour different types, i. e., in addition to
plus and minus, which indicate an up-
stream or downstream count. It is possible
to distinguish an approach either to the
upstream or downstream electrode, al-
though both electrodes are not crossed.