>
> I've been playing with arduino off and on for a while and this seems like
> an application for that.  it could probably be configured to read the water
> contacts such that there would be more leeway as far as all the connections
> being perfect and variations in water source.

The resistor network that the panel compares against is:

B+ --------150K---100K---R---100K---Y---100k---G---150K--------- B-

That makes the green light at 25% of B+ or less, yellow from 25-42%, red
from 42-58%, and no LED above 58%.**

The network for the sender is similar:

B+ is fed through one megohm to the green sender wire at the top bolt, and
the bottom bolt is grounded.

So B+-----sender wire+top bolt---/\/\/\/(black box)----next
bolt----/\/\/\/---third bolt----/\/\/\/----B-

As the water rises it first touches only the ground bolt, for no LED.  Then
it successively shorts the bottom bolt for red, middle for yellow, and top
for green through the water to the ground bolt.  I don't have a magic box
to measure the values, but they have to be such that the resulting voltage
divider with the tank water in parallel with the bottom n segments will
cause the output voltage to be between the thresholds.

Since both networks are connected to B+ the absolute voltage doesn't matter
-- it's the proportion that counts.

The voltage lights behave similarly except they compare against a network
fed by a 5.1v zener diode.

  And then you could also set
> the voltage level lights to switch at specific voltages.

You need proportions rather than absolute voltages unless you supply a
regulated voltage to the sender.

The existing board is simple and works well, sometimes with a tweak
resistor added to the comparison network.

The later float wand of course gets rid of the uncertainty of water
resistance at the expense of mechanical fragility/leak susceptibility and
losing the lowest (no LED) level indication.**

**Note that the '80-81 panels with four water LEDs G Y R R and no flame
detector will light the bottom red in this case, rather than showing no
LED.   They will also shut off the water pump at that time.

Yrs,
d