Hydraulic proportional valve control, using an arduino

our 30 year old manlift uses descrete components produce square wave output (around 150hz) control hydraulic valve.  understand it, instead of outputting constant voltage 0 12 volts, outputs square wave duty cycle doesn't heat or something? understand how works: proportional valve 2 relay valves connected in series, 0 volts , 12 volts @ each end center tap connecting two, coming control.  neutral equivalent of 6 volts, tell me it's not 6 volts, 12 volts, cycled on/off 50% of time. forward, or 100% duty cycle turn 1 valve on, other off, moving full speed in direction. , 0% duty cycle opposite direction. 

this sounds uno can do, , won't cost $700, new old school pcb!  i'm needing replace 1 of joysticks, , thinking simple thing atmega328 chip.  attached schematic way original joystick worked.  it's powered 741 opamp ic.

couldn't have atmega produce pwm mosfet same thing?  atmega read joystick pot, , pwm accordingly.
in schematic, there's 2 mosfets or transistors being used. pnp & npn.  need both of these?  if explain why there's two, understand it. 

the output has 6 amp diode on keep 12 volt backfeed out other systems.  just heavy pnp mosfet, , pwm uno chip?

i'm thinking along line of this power mosfet sparkfun (this 1 npn, have pnp equivalent)
  needs handle 3 amps of current @ 12 volts, i'd need beef traces on board make.

thanks much.  if do-able, can save $$$!

i think overthinking solution. bet can use ne555 , $1.00 worth of parts work.

paul

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