Electrical Design Calculations

Motor Current Calculations

The resistance of the motor is meausred to be approximately 1 ohm.  We also attached 
1 ohm resistor in series with each motor to reduce the current.  The voltage through the 
motor is 7.2 volts.  The TLE H-bridge has a RDS-ON of approximately 0.5 ohms at each end.  
Therefore the total resistance is 3 ohms.

I = V/R = 7.2 V / 3 ohms = 2.4 amps

The maximum current through the TLE is 5 amps continuous.  Therefore we are ok.

Battery Life Calculations

Battery capacity : 1500 mAH

1500 mAH/battery * 2 batteries = 3000 mAH 

PIC16F690 : 600 ľA @ 8 MHz, 3.3V, 1100 ľA @ 8 MHz, 5.0 V,
Transceiver :160 ľA
Shift register  (74HC595) : 160 ľA
LEDs :  30 mA * 5 = 150 mA

Total current usage = 152.02 mA

The helm can operate for 3000 / 152.02 = 19.73 hours.