Depth/Temperature Measurement (Sen Board) - Furuno CN-24 Service Manual

2.1.4 Depth/Temperature Measurement (SEN Board)

Depth Measurement
Refer to the simplified depth measurement circuit shown in figure 2.8.
1) The pressure sensor for depth measurement constitutes a Wheatstone Bridge.
It balances at 0 m and the voltage difference between "+" and " - " becomes zero.
(In fact there is 0.6 mV approximately, so R10 adjusts the off-set voltage to obtain 0 V.)
2) When the sensor (S1 and S2) detects water pressure, the resistance of one sensor
increases and that of the other decreases, developing a voltage proportional to the
depth across "+" and "-" terminals.
3) The voltage detected by the sensor device is once amplified at U1 (1/2) and (2/2), then
led to differential amplifier U2 (1/2). The output of U2 (1/2) detected as DEP-1 at TP3 is 1
to 9 V which corresponds to the depth of 0 - 2000m. U1 (1/2) and (2/2) are an amplifier of
high input impedance and employed to increase measuring accuracy.
4) The succeeding differential amplifier U2 (2/2) works to enhance the measuring
resolution as much as five times. Namely the output at TP4 (DEP-2) varies from 1 to
9 V five times while DEP-1 at TP3 varies from 1 to 9V.
This is controlled by the CPU; the CPU monitors TP3 (DEP-1) voltage and changes
the reference voltage of U2 (2/2) at depths corresponding to every 400m (400m,
80cm...160Cm). Refer to Figs. 2.9 for the operation.
Fig. 2.8 Simplified Depth Measurement Circuit
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