Will that work? He said the fuel sensor rheostat (variable resistor) has one side grounded internally and only the one lead available, so I don't think he can rewire it so as to have both terminals available to use as you have drawn.Originally Posted by SPDCHK
Lefty
Measurement changes behavior
yes it is internally grounded.... thanks for bringing that up didnt know that that would have an issue THANK YOU LEFTY!!!!
I must have missed the one wire part.
I actually also have the resistor value wrong as well. It should be 8.62 Ω. (Good luck finding one with that value in any shop).
8.62 Ω should give 0.7V drop across the resistor when the tank is empty and at fuel sensor measuring 132 Ω, the volt drop over the 8.62 Ω resistor should be exactly 5.0V.
Again, it's just an example.
ahhh its starting to make sense a little to me... but i dont under stand what wiring a 8.62 ohm resistor in line with ground would do for me, but if it will make all this work i can get a resistor in there , but it is atill just going to common ground.
as for the two resistors that it is using for ref, do those change now that you have changed the value of the one coming off the signal
It's dificult for everyone that is just starting out. I am also just learning the basics.
The resistor values for the ref do not change, they are just a voltage divider setting up a 5V reference.
If you do not understand what a voltage divider is, try googling it, it will help you understand alot of what this circuit is doing.
How do you know that the fuel level meter gives those exact Ω values? Did you measure it, or did you get the information from a datasheet or something similar?
Nevertheless, the sensor can still work, but the signal now needs to be amplified. As can be seen from the attached schematic, I now changed the sensor and voltage divider resistor network around, to cater for the grounded sensor signal and the feedback signal (for lack of a better word). I.e. the ONE wire that comes from the sensor (and I assumed here that the Ω measurement was made with one lead of the DMM (Digital Multi Meter) connected to the car chassis (i.e. ground) and the other lead was connected to the sensor).
The wire from the sensor is wired in series with R3 (1.452KΩ) I got this value calculating a volt drop of 1V over the sensor if the tank is empty (i.e. sensor = 132Ω).
With the sensor measurement at 12Ω, the circuit simulator gave an output of 5.03V, and with the sensor measurement at 132Ω, the output was 0.11V. The OPAMP output can be wired directly into the LM3914 signal input.
I hope this will help you. Don't feel too bad about not knowing too much. I was in the same boat about a month ago. Our forum colleague, Mr. Eric Gibbs helped me with the designs of OPAMP circuits.
Good luck.
That should do it, nice job. The op is scaling and reversing the voltage direction to make it compatible with the display chip. However one improvement might be to regulate the +12VDC down to say +10VDC and recalculate the resistors. Otherwise as the car's voltage varies somewhat with speed, idle, etc the reading is bound to move around some, but maybe not with just 5 'step' display.
Lefty
Measurement changes behavior
HOLLY CHIT!!! this is getting all kinds of complicated....as for how i know the resistance values , i did use my Fluke and moved the arm . all you guys have been really cool, and i wish i could express my appriciation.
now that, thats said....how the hell do i build this thing now??? i have the LM3914 hooked up as shown with the R1 as 1.2K and R2 as 3.8K and the cap. at 2.2uf. as for SPDCHK's diagrams(to amp it up) i dont understand the R3 value is that the same as 1.45K and what is that SPAN??? god i am sooo sorry that you have to walk me through this. and if i connect all this together it should work ???
ZERO and SPAN is terminology used in instrumentation where an input signal (the signal that you want to measure) is converted into something more usable by electronic devices.
Your fuel gauge is a typical example. When the fuel tank is empty, the sensor gives out a certain value, and the same for when the tank is full. BUT, the value from the sensor is not usable for normal electronics. The ZERO setting is to adjust the minimum (or zero) value, and SPAN is for the full scale value. In this case 0 volt (ZERO) 5 volt (SPAN)
Leftyretro made mention of perhaps using 10V i.s.o. 12V. Personally I would still use a 7812 voltage regulator in a car. (Usually the charging and regulating voltage of a car battery runs around 13.4V). Mind you, I have never done this, so I might be wrong.
Basically, if you connect the two circuits as posted it "should" work. However, theory and practice very seldom agree at the first attempt. You will have some teething problems, circuit bugs and short circuits. Don’t worry, it’s normal.
okay that made a WHOLE lot more sense now you put it that way
okay but i still need to know about that R3 value in the amp curcuit is that 1k45 the same as 1.45K resistor??? anyway i will go build this today thanks a ton
The 7812 needs at least 14.5V at the input. I would say to use at least the 7810, which requires 12-5V, or the LM2931AZ50R as 3v0 suggested earlier. Automotive environments tend to pretty harsh to electronics, as there is a lot of noise and voltage spikes.
Although Not Precise because of Battery Variations, This Circuit should do what you want.
For better accuracy, You could possibly use a slightly lower or higher valuse for the 150 ohm resistor.
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oh you totally screwedm me up now... so would this still need that amp circuit that SPDCHK had designed....i will have to give this a try tonight
i have a 1.2K and a 3.8K resistor will that be okay?? i dont want to run to the store today but i might have to
Your diodes are backwards (or should be connected to +5 instead of ground) in the first circuit. And check the value of R2, if you would. I'm guessing you copied most of it from the "typical application" circuit, in which R2 is 3.83k instead of 2.83k.Basically, if you connect the two circuits as posted it "should" work. However, theory and practice very seldom agree at the first attempt. You will have some teething problems, circuit bugs and short circuits. Don’t worry, it’s normal.
Last edited by Darth Bagel; 6th December 2007 at 05:11 PM.
The diodes are not necessarily connected backwards, although pull up resistors will be needed. The reason that they are connected like that is that the voltage drop across the sensor is inversely proportional to the quantity in the tank.
EDIT: This is only true if the opamp is not used.
Last edited by superbrew; 6th December 2007 at 05:30 PM.
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