TM 1-1520-238-T-7 10–43 10–7. SYSTEM DESCRIPTION (cont) 10–7 SDC M70-208 TGT 2 SENSOR PILOT DIM / TEST PANEL AIRCRAFT POWER BUSES BATTERY TGT 1 SENSOR FORWARD FUEL CELL SENSOR AFT FUEL CELL SENSOR FSC PILOT / CPG INSTRUMENTS 28 VDC BUS 1 28 VDC BUS 2 MANUAL DIM / BRIGHT AUTOMATIC DIM / BRIGHT AUX PWR LAMP TEST LOW FUEL WARNING LOW FUEL WARNING TGT 1 SENSOR DATA TGT 2 SENSOR DATA EMERGENCY 24 VDC Figure 10–18. SDC Functional Block Diagram (g) The FSC (fig. 10–19) operates on 28 VDC from the SDC except during refueling. During refueling the FSC operates on 24 VDC from the battery. (h) The fuel cells act as the sensing capacitor for the system. The capacitive value is determined by the level of fuel between the inner and outer tubes of the sensor, density of fuel, and the shape of the inner tube. (i) The FSC derives its operating voltages from an internal power supply. A 6 kHz oscillator provides a LO-Z voltage across the fuel tanks. The amount of current the tank passes from the LO-Z unshielded line to the HI-Z shielded line is directly proportional to the mass of fuel in the tank. The forward and aft signal conditioners amplify and rectify the HI-Z signals, producing zero to 5 VDC to the fuel indicators. Internal empty adjustment is made at transformer centertaps at the oscillator. Internal full adjustment is made in the dc amplifier. The FSC contains built-in-test (BIT) circuitry. BIT provides a known signal to functionally check the FSC by full fuel cell simulation. BIT is activated by an external test switch on the FSC. (j) The pilot fuel quantity indicator receives operating and lamp supply voltages from the SDC. The FSC supplies aft and forward fuel cell data to the indicator. The pilot dim/test panel provides display lamp test, digital blanking and display lamp dim/bright automatic control. The pilot’s indicator edge-lighting is controlled by the pilot EXT LT/INTR LT panel.