John A. Tuttle wrote in MMD 04.04.28:

> I'm not sure what "0" or "-1" bars would equal.

I had problems looking at the link that John sent but I think I finally
got to it.  It states that this is a Vacuum-Pressure switch. The  -1
number relates to vacuum while the + numbers are pressure.  The 0 value
would then be no vacuum or pressure or open to the atmosphere.

Spencer Chase wrote in MMD 04.04.28:

> Simple on-off control can be achieved by driving a comparator circuit
> with the output of the sensor, but hunting is likely to be a problem.

I agree 100% with Spencer's statement.  To get the fastest response
from this type of system requires using a switch rather than a speed
control.  If the vacuum motor were completely silent, then this
wouldn't be that big of a problem because you wouldn't hear the
changes.  Of course we know this isn't the case.

It would be nice if the vacuum from the motor would respond immediately
with voltage, but it doesn't.  The inertia of the motor and turbine is
the problem.  Using a proportional control system is good but it can
add even more time for the motor to compensate for the change.  Your
biggest limitation will be the response time from the motor.

As a first try, you might try using the vacuum switch to switch between
full power and use a motor speed control in parallel to keep the motor
from spinning down to quickly.  This might help reduce the problem with
the hunting problem without complicating the electronics.  Trying this
would give you an indication as to where to go next or if the system is
good enough for what you want.

Pete Knobloch (Tempe, Arizona, USA)