Decrypt This

Circuitry

Helm:
[Helm PIC] [Shift Reg Clock] [Shift Reg Data] [Shift Reg LED] [Safety Switch] [Reed Switch] [Speed, Direction, Water] [Key Switch] [Key Pad]

Ship:
[Ship PIC] [Ship Slave] [Power Regulator] [Rudder Motor] [Blue LED] [SPI Clock] [SPI Data] [SPI Slave Select] [Red LED] [Propeller Motor]

Overview

Helm Operating Lifetime

Due to the extended length of overseas flights on Air Force One, the satchel was designed to operate for a minimum of 8 hours. Measurements and calculations to ensure this performance follow:

State	Operation	Indication	Current
0	Standby and Unarmed	LED Cascade Style 0	0.18 A
1	Waiting for iButton	LED Array Fully Lit	0.30 A
2	Pair Broadcasting	LED Cascade Style 1	0.23 A
3	Paired with Ship	Single Paired Ship LED	0.12 A

The power cell consists of 4 rechargeable NiCd AA-sized batteries, which provide 2.3 A Hours of current. With the helm set in broadcast mode (highest current draw), expected lifetime is 7.67 hours; in paired mode (lowest current draw), expected lifetime increases to 19 hours. We anticipate actual usage to fall between these two extremes.

Ship Design Considerations

The propeller motor has 0.2 Ohm resistance at 7 V, which implies a current draw of 35 A. The initial choice of power mosfet to control the motor was an IRLZ34N, but its peak operating current is 30 A. The final choice was an IRF3205PBF, which has an operating current of 98 A. For use at Tj = 25 degC, with Vgs = 7 V and Vds = 5V, the resulting Id = 110 A. For Tj = 175 degC (the mosfet's maximum operating temperature), Id = 108 A. While the IRF3205PBF is rated to operate at 175 degC, this temperature is too high for other system components: the 2N7000 and 2N3904 mosfets and PIC16F690 microcontroller have maximum operating temperatures of 150 degC. Therefore, the design had to attend to the duration of exposure to maximum rating conditions. Regarding voltages, the PIC and XBee both operate at 3.3 V, so the voltage was jumped up to 5 V for the slave PIC and servo control board.