Voltage controller?

[Daniel Houlton]

Hi.  Me again.  Sorry, but I've got more questions.  For my turbo, I don't
yet have an intercooler and don't know when I will.  In the meantime, I've
planned on building a water injection system using basically a boost
pressurized tank and a couple pumps to provide a pressure differential to
inject water after the turbo.

Instead though, it's very likely that I'll be getting an ERL AquaMist from
a friend to try out.  This is the $500 job I think.  Uses a high pressure
magnetic/diaphram pump or something.  I won't be getting the controller 
they sell for it, just the basic system.

I'm pretty sure I read on their page that the pump pressure is controlled
by a 0 - 12V signal.  It could be it's just driven by 0 - 12V, but I
thought it was a seperate signal.  Power is through a seperate 12V 
supply.  Maybe not though.  This is how their controller controlls it.
How would I produce this signal from a MAP sensor?

Say I get a 2 Bar sensor.  These are linear from 0 - 5V representing
vacuum - ~15 psi (over ambient) right?  Now if the MAP sensor put out 0 - 12V
instead and the pump took a 0 - 5V signal, I could just use a pot and
direct connect the sensor to the pump.  I need to go the other way though.
I need to step up the 0-5V MAP signal to a 0 - 12V signal for the pump.

Any suggestions how to do it?  What about if instead of a 0 - 12V signal
I need a 0 - 12V power (presumably much higher amps than the MAP can
supply) source?  What I need is something like a 30 amp relay that can
supply a variable voltage instead of full voltage or none.  I'm sure 
something like this exists, but I'm (obviously) not an EE so I don't know
what to look for.

Oooh!  What about an "electronic" pot?  Something that can control
resistance from a voltage signal instead of physically turning a dial?  I
could then provide full, high amp voltage to the pump through a relay and
run it through this "electronic" pot.  The pot would adjust the resistance
according to the MAP output.  Map signal goes up signalling pot resistance
to go down and the pump gets more voltage resulting in higher water 
pressure.

A seperate Hobbs switch could control the relay to arm the system at 
whatever boost pressure I want.  Is this totally off the wall or is it
doable?

thanks
--Dan