Alright, this is what I think I've found. Correct me if I'm wrong please
Edit-I hope I'm not too far off track, by the end of this post I think there's a solution that might work with an extra resistor. What do you think? I have a knack for popping zeners so I cant test my idea right now LOL
As I was going lower on R1 values, the difference on the gauge got smaller and smaller. Reason being, the lower the resistance, the higher the mA at the base of TIP31, but as the mA's get higher, the change of the collector voltage gets smaller as the curve flattens out.
To get to the point where I need to be to get the gauge to read full, I've figured the mA input needs to probably be about 10ish mA to get a low enough output voltage at the collector to read the tank full. I hope that doesnt screw up the empty reading too badly, but I'll figure that out as I go.
This is how I've sort of gathered the ~10mA figure.
Two resistors I've tried-
a)3900
hm:
b)560
hm:
Two zener's I've tried-
a)3.6v
b)3.9v
Results for each-
aa)0.923mA
ab)1mA
ba)6.43mA
bb)6.96mA
Gathering that with the AB(1mA) combo went to about exactly 3/8'ths tank, and BB(6.96~7mA) was dead on at 3/4, what would I do to find what would give me a full tank considering the collector emitter voltage graph on the datasheet is a curve, so I dont think I can just make a straight line from 3/8 to 3/4 to 1 and change the numbers at the same ratio. If I did though, I would probably need that ~10mA base current to read a full tank on the gauge. If that's right, looks like it's close to something like .16-.18 base/emitter voltage.
Couldn't I just use the 3.9v zener, 560
hm: R1, and put in an extra resistor between the gauge and collector to lower the mA to the gauge that extra little bit? Like 1
hm: or less?
Perhaps there is a better suited transistor all together?
Edit 2-The difference between collector emitter voltages between what I have now and what I think would be closer, is so close even a 1
hm: resistor would overshoot it depending on the real world resistances. If in a perfect world I have 0.19v now, and .5
hm: resistance, the mA is .38 and reads 3/4 tank. I guess I should measure that to be sure next time, then I can have this for sure, I think. If I want to get the mA that 0.17v with .5
hm: resistance would make (.34), I'd only need a difference in resistance of +.56
hm:... umm but that doesn't seem right...? Of course this is still all speculation on what it looks like on the graph, and that a straight up 10mA base current is what the gauge needs to read full.
Im just trying to figure out some stuff on my own, right or wrong, but need to get pushed in the right direction again
I hope I didn't write too much to confuse the situation. It's what I get for thinkinga bout it at midnight!