Jeff,
<< low pressure switch to provide an "automatic" mode to the electric
fuel boost pump, so that if the engine driven pump failed, the electric
one would be powered up. ... It would likely mean an engine hiccup,
followed by the fuel pump on LED illuminating - as soon as the electric
pump provided sufficient pressure, the low pressure switch would open
and the pump would shut off, and then back on, and then off. Ooops :-(
>>
My circuit published in the May 2008 issue of Kitplanes magazine will do
what you describe including switch conditioning and relay driving. It
is a simple programmable timer. Set it to run for say 4 minutes. This
gives you plenty of time to notice the LED and turn on the manual pump
switch. You would need some blocking diodes for the manual switch and a
cockpit accessible fuse/circuit breaker to be able to override a
stuck-on failure.
With the system you describe there should not be an engine hiccup. When
the fuel pressure starts to drop there is still a bowl's worth of fuel
in the carburetor. The pressure switch should turn on the pump and
refill the carb long before the bowl is depleted.
Email me off list and I'll send you a revised schematic for this
function.
Tom
