I think I've settled on a final circuit:
The main difference here is that Q4 is now a MOSFET (this guy:
NTE 2980 Datasheet). Due to the fact that when I attempted to use a NPN (same one as the other 3: 2N4401 I had to use too much current to turn it on which affected the sense pin). Using the MOSFET makes the wakeup circuit use 0 ma current because its switched by voltage, not current.
Here is what the prototype looks like: LOL
(Of course the wife took a look at that and said: "So is this what its going to look like stuck to the side of the kitchen cabinet??" LOL Answer: "um NO! Its going to be soldered up inside a box next to the black tank out of sight").
The green LED on the far right is 1/3. The yellow LED is 2/3 and the clear LED is Full. The black part to the left of the Full LED and its transistor is Q4 the MOSTFET. The two resistors on the far left going vertical and then a third one going to the top "rail" (+5V) is the 67k and 120k resistors representing the ones in the probe harness.
Measuring the current: when operating with a 12V power supply about 30ma when asleep 1.4ma (according to my fluke).
Next steps:
- Wait for all the snow to melt (with the weather this week we'll be on our way).
- When I can bring the RV home I'll install the foil on the side of the tank
- Use the prototype circuit to calibrate the max capacitance value
- Solder up the main circuit
- Test circuit on RV
- Mount the box
- Find spare 12V fuse on the forward fuse panel (looks like there are a few)
- Connect up circuit
- Press "Black tank" button on the panel
I have the code setup such that it blinks the next level indicator via a PWM of sorts in 10 steps (10% on/90% off for 1/3 .1 full, 20% on/80% off for 1/3 .2 full, 30% on/70% off for 1/3 .3 full for example). Thus looking at the panel you'll get a resolution of approximately 31 points (10 steps to 1/3, 10 steps to 2/3, 10 steps to full and full).
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