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John Cooper wrote:

>Also, with the float hanging down away from the support arm, the reed
>switch inside is OPEN. With the float in line with the body of the sensor,
>the swich is CLOSED.
>
>Any suggestions on which way I should install the thing?

Seems to me the orientation of your float switch is going to be determined
by the location and the logic of the circuit you're trying to control.

Scenario #1... Simple low level indicator
Pick a low header fuel level you feel comfortable with (1/2 tank?) and
install the switch float up so when the level drops it comes inline with
the switch body it simply lights up an indicator and sets off a siren <g>
to remind you to top off your header tank manually.

Scenario #2... Autofill trigger
If you want to use it to trigger a timer that starts a relay to autofill
your header, I'd locate it at the 1/2-2/3 full level in the tank and
install it so the circuit is open (float up) until the upper 1/3-1/2 of
your fuel is consumed.  At that point the float would close the switch,
activate the timer and turn on the pump.  Naturally you set the timer so
the pump runs long enough to top off the tank.

Scenario #3... Constant full control
Mount the float switch at the top of the tank, float down, so when the fuel
level drops far enough for the float to open the reed switch the circuit
will open.  Use the float switch to control the pump relay coil and use a
pair of NC contacts in the relay to control the pump... anytime the float
opens the circuit (fuel off the full mark) the pump will come on and top
off the tank.  This is kinda scary as that pump is going to be cycling on
and off a lot, and I suppose has a larger potential for failure as a
result.  I imagine there are other failure modes as well, but still, it's a
neat way to keep the tank topped up.  

Scenario #4... Full auto fill with auto shut-off
Assuming you have one in the approximate middle of the tank to indicate low
fuel and to energize the pump relay, (this time without a timer), a second
float could be installed near the top of the tank (float up again) in
series with a latching circuit (using one of the NO contacts in the relay
again in series with the coil) on the relay that controls the pump... that
way when the tank is topped off the float will rise, open the reed switch
which de-enrgizes the pump relay and shuts off the pump.  You could also
wire in a pushbutton in parallel with the lower float to force pump relay
activation manually if you wanted to.

I imagine there are probably a dozen or more other possibilities for using
these switches, but these are the first 4 that popped into my head.  I
kinda like the autofill/autoshutoff method, although it hardly meets the
KISS criteria, it's really not that complicated and doesn't rely on _too_
many parts.  What say the rest of you out there?

   <Marv>