I'm trying to measure the voltage on my fuel gage to determine the fuel level.

The circuit can be modeled as follows:

                 +-----------+
                 |           \
                   |    95 Ohm /
                   |           \
                   |           /
   Battery   -----         |
    11-15V    ---          |--------------+ V Level
                   |           |                        |
                   |           \                        \
                   |           /                        /  0 Ohm Empty
                   |   180 Ohm \                        \  90 Ohm Full
                   |           /                        /
                 +-----------+--------------+
                               Gauge          Sender

The 95 and 180 Ohm resistances are associated with the
coils in the fuel gauge. The Sender is electronic and
the 0-90 Ohm ratings are for a nominal 12V. There's
also a 12V supply connection to the Sender. The problem
that the battery voltage varies, moving the output of the
Sender all over the place.

I thought of using two A/D channels: One to monitor the
Battery and the other to measure the Fuel level. The
correct level could be found from the ratios. If anyone
has an idea of how to do this with one A/D channel, I'd
be interested.

Also, I'd like to hear anyone's ideas on a circuit to
interface this to a PIC to accurately scale down to safe
levels and protect from nasties like voltage spikes and
reverse voltages.

Thanks,

Chris

Attachment converted: wonderland:WINMAIL.DAT (????/----) (00003A89)