Glow Fuel Sensor

[erwabo]

Okay...so I did some other testing tonight and I found it was my voltage sensor that was really throwing stuff off. So I found that the full battery voltage is actually passing through the fuel. Im getting the full 7.4 volts. The 20ma current isn't making it though, but the voltage is. Whats REALLY strange is this voltage sensor. Its a Frsky FBVS-01 and it has voltage division ratios

There are three voltage ranges to choose:
•1S lipo for battery voltage range from 0V~6.6V
•up to 3S lipo for 0V~13.2V
•up to 5S lipo for 0V~19.8V.

Please check the battery voltage division ratio for each option:
1S 2:1
2S - 3S 4:1
3S - 5S 6:1

So the output voltage is 8.1v on the fully charged battery, so with the 4:1 ratio, my voltmeter was showing 2v as expected. I changed it to 2:1 and output was showing 4v as expected, however the telemetry output was always .6-.7 volts from the receiver...so I found out its just a voltage divider and you have to input the maximum voltage value for it to reflect the actual voltage.

Anyhow, I may still try to get the LED lit, but its definitely isn't lighting with full voltage and that probably would change the drawing again right?

THat being said...based on the volume of liquid in the tank, my voltage sensor is giving me a range of voltage from 0 to 1v (per my voltage divider) on a full tank. I can set my transmitter to give an audible "fuel is full" if that sensor reads 1volt which would is probably going to be a lot easier than the LED idea. The whole point of the LED was for it to light up when the fuel was full so I could stop winding the crank and not have to worry about fuel spilling out into my vent or flooding my carb had I forgot to close the throttle all the way

 

[erwabo]

I checked the resistance of the entire circuit again and depending on where I put the lead it went anywhere from 25k to 200k....which was interesting...might as well build a mini resistance float for the tank =)

 

[mjmccarron]

First let me apologize. When I first responded to this question I was packing for an out of town trip (work related). I copied the link to the type of circuit needed as I didn't have the time to draw it out and post it for you. That left you with a vague idea of what was needed. Craftydan picked up the ball and did a superb job of clarifying. Thanks Dan! I breadboarded the circuit this afternoon and it works as expected. In any event, please let us know how it works out for you. It's a neat idea.

Mike

 

[erwabo]

First let me apologize. When I first responded to this question I was packing for an out of town trip (work related). I copied the link to the type of circuit needed as I didn't have the time to draw it out and post it for you. That left you with a vague idea of what was needed. Craftydan picked up the ball and did a superb job of clarifying. Thanks Dan! I breadboarded the circuit this afternoon and it works as expected. In any event, please let us know how it works out for you. It's a neat idea.

Mike

No problem! Yeah Craftydan did more than I ever expected. Thanks a ton!! I need to order those couple parts and then I can test this in real life. Im thinking about getting copper posts instead of steel ones...thinking they may be more conductive. I did get a chance to build a custom screen from my Taranis this weekend for this model. I built a fuel gauge and have it tied to my voltage sensor. This works when filling and draining the tank. When the tank reaches full I will get a "Fuel tank is full" voiced from my transmitter and when it gets about 80% empty I get a clip from Top Gun " We gotta land this thing, we are way low on gas, you understand!" Here is my LCD from the LUA script I made(looks better in person)

 

[erwabo]

So when looking at the resistors that im ordering (may just get a box of them) does the wattage matter in this case? Im seeing 1/4 watt to 1 watt on these resistors. Thanks

 

[Craftydan]

erwabo,

Glad I can help


P = I²R, or P = V²/R

Expected current through the LED is 20mA, so the wattage through the 180ohm resistor is expected to be 72mw . . . well under 1/4W (250mW).

Not sure what the current through the 100k resistor is, but the voltage can't be greater than 7.4v, so it can't draw more than 0.55mW from that battery.

1/4W is overkill, but we buy what's cheapest that fits the bill, not the part just slightly better than failing

 

[erwabo]

"Not sure what the current through the 100k resistor is, but the voltage can't be greater than 7.4v, so it can't draw more than 0.55mW from that battery."

Well the voltage when FULLY charged is sometimes 8.1v and something REALLY interesting I found is that when current is applied I get almost full voltage passing through the leads on the tank itself. If I measure the same voltage at the solder joints where it the wires enter the PCB of the voltage sensor, that voltage drops down to the .6 and .7 readings im getting on the transmitter (by passing the multiplier) Those wire lengths are only 6" but they are VERY thin wires. Thinner than your normal servo cables. Very odd.

Anyhow should I go with 1/2" watt since voltage could potentially go over 7.4v.

 

[Craftydan]

erwabo,

Feel free to over-spec -- we're talking about a few penny's difference, after all -- but look at the units again . . .

the 100kohm resistor would consume 0.55mW of power . . . that's 0.00055w. Unless you're considering plugging this into your dryer socket, 1/4W is plenty

 

[erwabo]

Well I just ordered my parts...from amazon and of course the MPSA29 will be a week or so, but that's fine...my fuel port and double power switch is arriving around the same time so it should work out. Gives me time to finish monokoting the wing

 

[erwabo]

So typically these little transistors are used in a breadboard or a PCB, should I put shrink tube on each lead once I solder the wire since they are so close together?

 

[Craftydan]

If you're building dead-bug style (without a board), you will want to insulate all the exposed connections. Heat shrink will do, as will liquid electricians tape (similar to plasti-dip) or even a touch of hot glue.

I'd avoid using actual electricians tape -- over time it gets gummy and falls apart, and the fuel will probably accelerate that.

 

[erwabo]

Yeah I hate electrical tape...always turns into gooey mess. I'll let you know if it works when I get my parts, if it does ill buy a fresh tank and tubing and paper clip terminals...a lot more conductive than my steel pushrods

 

[erwabo]

Okay so here is a video of the working gauge I made on the Taranis. The voltage decreases faster when draining, so I went a little too fast and drained too much fuel out but you can see the low gas alarm actually hit when the needle was at about 1/8 full, but it works as expected.

The reason I want the LED is so I don't have to turn on the transmitter or unscrew tank hatch to check the fuel, I can flip on the power to the battery, if the light isn't on, I need to add fuel to fill her up =)

Here is the link : https://youtu.be/D_rSFHGMih0

Let me know what you think! About 24 hours of work into this so far...12 of them writing the LUA script to read the voltage LOL

 

[erwabo]

Well I got everything but my transistor. Got the power switches hooked up, the tank prepped and the resistors inline. Probably wont be here for a few more days, but im hoping to have this working and POC by next weekend, then I can build my production tank and complete the plane =)

 

[erwabo]

Transistors arriving today, I will build this circuit this weekend and pray it works, if it does ill put a demo video with the LED and the Taranis Lua gauge working in tandem. I really hope this works =)

 

[Craftydan]

Bueno! Good luck with the build and test (and don't let the smoke out )

 

[erwabo]

SUCCESS!!! You guys are genius....well technically, I'm just electrically ignorant...but genius sounds better. I still cant believe that little $2.46 transistor did the trick. At first it didn't work, and I had the anode and cathode switched on the LED, but as soon as I resoldered those...it lit! Ill post it all after I clean it up. That was my prototype so its a wire mess and still needs shrink tubes and sized for the plane.

Can't thank you guys enough! I would have never figured that out!

 

[Craftydan]

Great to hear erwabo!

I'll be the first to admit "not genius", but this stuff isn't obvious either. Engineers are the wizards of our age, but I've come to learn wizards aren't always as clever as it seems -- they've just learned enough of the right tricks to get what they need done.

Glad you've gotten what you need