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Chapter 5: Functional Circuit Description
Sine and Cosine fluxgate output signals are measured between the Yellow & Red and
Green & Red fluxgate terminals respectively. The Red terminal is biased at 2.5 Volts. The
micro controller selects the input signals using multiplex chip IC8. These are then
presented to the input of the dual slope integrator (IC9a and capacitor C61) for
measurement. IC9b is a comparator that generates a micro controller interrupt (ADC-INT)
at the end of the integrator discharge cycle.
Rate Gyro
The Type 150 and 400 Course Computers can be fitted with a Rate Gyro (yaw sensor)
either externally (via connector block TB8) or internally (via connector CON1).
In either case, the signal is biased around 2.5 Volts (zero yaw condition) and changes by
±22mV (sign is dependent on direction of turn) for every 1°/sec of turn rate. The output
signal has a total operating range of 0 to +5 Volts.
External Gyro
12 Volt power is supplied from VBUS via a current limit circuit built around transistor
TR45, to the Red gyro connection. The rate signal returns to the Course Computer via the
Yellow terminal.
Internal Gyro
The internal Rate Gyro option is powered directly from 5V-ANA. The output and external
gyro are connected to the same part of the circuit. This makes the output of the internal
gyro available for direct measurement (for diagnostic purposes) at the Yellow terminal of
the external gyro connector block.
In either case (external or internal gyro) the signal is buffered by voltage follower amplifier
IC13d. The difference between this signal and a 2.5 Volt bias voltage is amplified by
differential amplifier IC13c (gain = 3.9). The raw gyro signal (RATE-GYRO) and the
amplified signal (RATE-GYRO x3) are presented to two channels of the built in Analogue
to Digital Converter (ADC) inputs of the microcontroller IC15.
Rudder Reference
The rudder transducer connects to connector block TB12 on the Course Computer.
Transistors TR57a and TR57b current limit the 5 Volt supply to the Red terminal. The
transducer output signal enters the Course Computer on the Blue terminal and is buffered
by voltage follower IC22a. The output of this unity gain follower (RUDDER-REF) is fed
to one of the ADC inputs of the microcontroller.
Differential amplifier IC13b, amplifies the difference between signal RUDDER-REF and a
2.5 Volt bias. The gain of this amplifier is 1.5 and it's output signal (RUDDER-REF x1.5)
feeds into another ADC channel of the microcontroller.
RUDDER-REF x1.5 is further amplified by differential amplifier IC13a. Once again, this
amplifies the difference between RUDDER-REF x1.5 and a 2.5 Volt bias. The gain of this
Type 150/150G/400/400G Course Computers Service Manual 83156-1
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