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Wicked Kool WRX Startup Circuit Sam Schoofs Kyle Schlansker
Goals • To design a circuit for Kyle’s Subaru WRX • It must turn on an inverter and computer inside the car when the car is turned on. • It must turn off both the inverter and computer when the car is turned off.
Specific Circuit Goals • The inverter will power the computer. • The computer will be used to play music (in the future, DVDs, games, etc). • The computer should be turned on 30 seconds after the inverter is turned on to allow the inverter to fully charge.
Switching • Two switches needed to be designed. • The first had to turn on the inverter when the car was started. • The second had to turn the computer on 30 seconds after the inverter was turned on. • A Basic Stamp 2 microprocessor will be used to control the switching of the circuit.
Switch Options • For the inverter switch: • Solenoid switch • BJT switch • MOSFET switch • For the computer switch: • Photo FET Optocoupler
Schedule • Plan of Attack • Brainstorm different methods of implementing our circuit goals. • Select the optimal method. • Find parts that meet our design criteria. • Simulate our circuit, with cadence, using the chosen parts. • Test a preliminary circuit in the lab. • Implement working circuit in the future.
Switch Selection • The solenoid was more complicated and prone to mechanical failure. • The BJT was not practical because of the limited source current from the microprocessor. • The MOSFET could handle the amount of current and was easily implemented with the microprocessor.
Switch Parameters • The inverter would need 40 amps DC (max) sustained current. • A low drain-source resistance was needed to minimize the voltage drop from the car battery to the inverter. • The MOSFET needed to be able to saturate at a VGS of around 8 V and cutoff at 0 V.
MOSFET Selection • Googled for hours • Asked Chuck for search recommendations • Searched Newark’s website until one could be found to meet the requirements.
Computer Switch • A photo FET optocoupler was selected to ensure that no feedback could destroy the microprocessor.
Safe Physical Implementation • Op-Amp
Unsafe Physical Implementation • Inverter-Battery hookup
Full “High Power” Test Setup Car Battery Our version of “Chassis Ground” Scott’s 1337 80mm heat sink and fan (thanks 319!) Digital Readout of Battery Voltage (12.2 V) Test motherboard (Scott’s “strange” Tyan trinity) Op-Amp circuit Motherboard Power Switch Photo FET Optocoupler
The Next Step • Put both switches on a PCB with BS2 • The PCB is already made. • The code for the BS2 is already written. • Install the setup in the car • The inverter and computer will go in the trunk. • The PCB will be mounted near the drivers seat.