Just an interesting fuel gauge scheme

Fuel Level Sensor (FLS). Assembly, circuitry, production


Greetings dear readers! Several years in a row I have written on the subject of our service monitoring vehicles, the equipment that we produce, revealing the internal aspects of production and work in general. In this article, I want to tell about the full cycle of production of such a very important element of the systems of GPS monitoring and control, such as a fuel level sensor (the searchers know it as a remote detector). There will be a theory, all the drawings and diagrams for the assembly of this product. Who is interested – read on.

0. Introduction

Looking ahead, I say there will be three articles, in this one, I will talk about the easiest option for determining the level of diesel fuel (only diesel, the use of gasoline vehicles is absolutely prohibited, because it is explosive). In the following articles, if the reader will certainly be interested, consider the digital fuel level sensor, and at the very end, I plan to lay out a diagram and firmware device for monitoring, which described in this article.

1. A little bit of theory

The most popular fuel level sensors is an electrical capacitor, consisting of two tubes placed inside each other, set a tank of fuel, the level of which is measured. Diesel freely penetrates into the space between the tubes, the signal of fuel level change in the tank is a change in the electrical capacitance of the sensor.


When the fuel level in the tank changes, the relative dielectric permittivity of the space between the condenser shells changes, because the dielectric permittivity of fuel and air is generally different. And since the capacitance is directly proportional to the dielectric constant of the insulator, the electrical capacitance of the sensor changes as a result. Sensors are mostly made of aluminum or copper, because they are the least susceptible to corrosive media. Of the many ways to measure the capacitor capacity value and the subsequent conversion of its capacitance into a proportional change in the DC voltage output, the pulse-width method was chosen as being simple and reliable enough, yet providing the necessary level of measurement accuracy. Immediately a disclaimer is required, this is the simplest in terms of finances and simple enough in terms of assembly of the diesel fuel level sensor method.

2. description of the fuel level sensor circuit diagram

Fig. 2. Schematic diagram of the fuel level sensor (FLS) (big diagram here)

To increase stability and accuracy all circuit elements are used with a minimum temperature coefficient. Resistors are used with 1% tolerance, microcircuits are chosen with improved parameters in contrast to domestic analogs, for example SE555N instead of NE555N and LM358D instead of LM258D. On the chip U1 SE555N and elements R1, R2 and C1 assembled a master oscillator. Since it strongly depends on the stability of the readings, the precision polystyrene capacitor K71-7 1% is used as a C1 capacitor, usually installed in Soviet color TV sets in the master oscillator of the main scan. We can replace them with something modern, but the availability and price of these capacitors makes them very attractive, and they were born in the distant year, when the USSR was very good at monitoring the quality of manufactured elements. From the output of the 3rd chip U1 rectangular pulses start a single-oscillator, assembled on the chip U2 SE555N. A sensor placed in the fuel is used as a capacitor of the single-oscillator, so its capacity will depend on the fuel level, and consequently, the pulse width at the output of chip U2 will also vary with the fuel level. To ensure a linear dependence of the pulse width on the level of filling the sensor with fuel, the fuel sensor receives charging current from the current stabilizer made on the chip U3.2 and the transistor Q1 BC856BT. Also by changing the charging current adjustment of the circuit for different sizes of sensors is carried out. Setting of the circuit is done by selecting resistors R6 and R7, to get 1.8-1.9 volts at the output of the circuit, “dry” sensor. From output 3 of chip U2 pulses come to the integrator assembled on the elements R8 and C6. Then the integrated voltage formed on the capacitor C6 goes to the low-pass filter made on R10 and C10. Then DC voltage goes to a DC amplifier, made on the chip U3.1. From the output of 1-st chip U3.2 signal through a filter made on the elements R17, C12, C14 and C15 goes to an output. The resistor R16 is used to prevent self-excitation of the amplifier when working with a capacitive load. The divider is made with resistors R9 and R11 to provide the necessary DC bias to operate the DC amplifier in linear mode. The voltage regulator to power the electronic circuit is placed according to a classic circuit on a U4 LM317MDT chip. As a result, at the output, we get an analog signal an empty tank 1.8V full 6.0V (there is a dependence on the height of the DUT), which is linear and directly proportional to the fuel level in the tank / tank / storage. Then, by applying a Kalman filter, you can remove fuel spikes, output the average flow rate calculation, etc.

Locking the rear speed in motion on the "snake" selector

In reality, it will look something like this:

Fuel level + speed graph.

Drawing of fuel level sensor, materials

FIG 3. Drawing of fuel level sensor (link to large drawing)

Already mentioned that mostly aluminum is used, as you can see from the drawing, the outer tube is soldered any convenient way to the “head” of the fuel level sensor. In the manufacture of their sensors we use welding, as we have access to it, though not the most aesthetically beautiful option, but reliable and time-tested. Inside we use aluminum rod, for fixation of which a thread is cut in the upper part. Grommets are used special fluoroplastic, which is the most tolerant to diesel fuel.

4. bottom line

The vast majority of fuel level sensors on the GPS market of the CIS and the world are based on this solution. Each manufacturer makes its own changes to increase the accuracy of fuel level measurement, such as an accelerometer, temperature sensors, digital signal processing, etc. The scheme I presented is the simplest, ready to work, as they say, in the field without any complications. The esteemed reader with straight hands may well make any modifications that can be used for their own purposes as well as for commercial purposes.

PS. A little erotica about how this kind of goodness is installed on the technique can be seen here.

Digital indicator of fuel level and battery voltage for the car V.3 (ATMega8 and a display from Nokia 1110i)

About 1 year ago I published my first article on Datagore about digital fuel gauge and it is already more than 2 years since I use it. And not once has it failed me, to go to the gas station when the tank is 2 or 3 liters left is the norm, and it is not an extreme and not a show, when you know that these 2 or 3 liters are just enough to get to the next few gas stations and you take it easy, no comparison with a flashing light regular device. That’s the end of philosophizing – to business!

How to change the antifreeze in Kia Rio video guide


Comrade, consider the datagoric recommendations

Useful and tested hardware, you can take it.

Tested in the editorial lab or by readers.

Probably not clear why the actual version V.3, when there was no version 2, actually was, here it is

It was not very good, they used MC33063 switched regulators, which cause ripple in both directions and I couldn’t get rid of them. Since the idea of a KIT was born, it was decided to make a new version, with reliable power supply, with protection of all input circuits and with parts that meet the conditions of use, first of all the temperature range -40…+125°C. So the new 3rd version appeared, made almost according to all rules, with an updated firmware.

Unfortunately the kit turned out to be not in demand, but a lot of time was spent on it, and now it gathers dust on the shelf, or rather in its folder. And so not to waste efforts I lay out all the documentation of the project, I will be glad if anyone useful.

From Igor (Datagor): In the analysis of personal correspondence, comments to the first article, and after conducting random surveys it was found that people want not just a very good-quality gas gauge, but also a clock with an alarm, etc. etc. (and shob was inside a little Chinese and ran for beer) that turns this wonderful and completely independent development into another onboard computer (OBD). At the same time, for the people was willing to pay not more than 500 rubles in assembled form. But it does not fit in any gate at all. We did not do BC and we did not start the subscription to the kit on such a sad background. All the same, Sergiy (HSL) deserves our regards and thanks! The quality of his designs is at the highest level.

Top 10 Causes of Check Engine and what to do about it

So in order.

↑ Scheme

The difference is in the connection of the AREF signal (reference voltage), in the A5 version it is taken from the power bus +5v, in the A2 version it is taken from an internal source. The basic modification is A5, A2 is made to extend the functionality, in case the basic modification fails to calibrate the tank. On the board, this is done by different setting of elements R11, C4, C6 in more detail this will be described below in the instructions. Connector for the display board is also used for in-circuit programming.

The schematic of the display board

It is a universal block with a display, control elements and a regulator to power the display. So, it can be used with other devices as well.

↑ Boards

The connector for the display board is also used for in circuit programming of the MCU.

Display board

The display board is mounted on the board with the double sided adhesive tape.

↑ Technical specifications

Supply voltage 8-30V Backlight night mode voltage 10-20V Fuel sensor resistance (recommended) 250-500 Ohm voltage display range 0,1V Voltage display range 8-30V Fuel amount display range 1 liter. Supported range of the tank capacity 30-99 liters. Inertia range 1-10 sec. Gradation range of brightness 0-255 units. Contrast range 1-15 units.

↑ Main mode capabilities of the unit

  • Vehicle mains voltage with 0.1 volts accuracy, operating voltage range 8-30 volts.
  • Fuel remaining in the tank with the accuracy of 1 liter, permissible measuring range is 30-99 liters. Recommended resistance of the sensor in the tank is 250-500 Ohm.
  • The device is connected to the following points: ground, power supply, sensor in the tank, instrument panel illumination or lights.

↑ Individual configuration of the device

  • Possibility to set the tank capacity from 30 up to 99 liters.
  • Possibility to calibrate the selected tank by liters.
  • Possibility to smooth the consequences of sensor swinging in the tank by tenfold fuel level measurement and displaying of averaged value, with the choice of measurement time from 1 to 10 seconds.
  • Ability to set the brightness of the display separately for day and night mode. The operating mode is determined by the fact of turning on the lights and the dashboard backlight.
  • It is possible to set the normal or inverse mode of the display.
  • Possibility to set the display contrast level.
Can not open the gas tank hatch Polo sedan

↑ Description of device operation and controls

Controlled by buttons Menu, Ok, Up, Down Menu – enters Setup mode in basic mode. In settings mode you return to the previous menu without saving any changes, and exit from settings mode. Ok – Only works in setting mode. Enter the selected item, saving the current parameters in non-volatile memory. Up – Only works in settings mode. Moves up through the menu items, increases the current value. Down – Works in settings mode only. Moves down through menu items, decreases the current value.

Operation modes Main mode

The unit enters Main Mode 2 seconds after applying power to the unit. Voltage value readings appear at once, the remaining fuel value readings appear with a delay due to the inertia setting, 1-10 seconds.

Setting mode

The settings mode is used to configure the device for specific operating conditions. To enter the settings mode press the Menu button

Menu items Tank capacity

allows you to set the volume of the tank in use. Use the Up/Down menu buttons to change from 30 to 99 liters. In order to save the selected volume, press the Ok button. To exit the menu without saving the changes made, press the Menu button.

allows you to calibrate the tank capacity polytrically. The calibration is carried out after selecting the required tank volume in the Tank capacity menu. Liters – in this item using buttons Up/Down sets the necessary value of the cell of liters to record the calibration value. The calibration value is written with the help of the Ok button. Sensor – shows the current value of the residual fuel sensor. When you press the Ok button, this value is entered into the current memory cell selected in the menu item Litres . In memory – shows the value stored in the memory corresponding to the cell currently selected in the menu item Litter.


allows you to set the period for measuring the residual fuel. With the menu buttons Up/Down it changes in the range of 1 – 10 seconds. During the selected period, 10 fuel measurements are taken at regular intervals, after which the average value is calculated.

Change of a fuel filter of Volkswagen Polo sedan.

allows you to set the brightness of the backlight for day and night. The fact of day and night is determined by turning on the lights and the dashboard backlight.With the Up/Down buttons select the desired item for Day/Night adjustment. To enter the mode of changing the selected value it is necessary to press the Ok button, and then use the Up/Down buttons to set the required value of backlight brightness from 0 to 255. To save the set value it is necessary to press Ok , to exit the current item without saving the changes it is necessary to press Menu button.

allows you to select the normal/inverse display mode. Selection of the desired item is made by buttons Up/Down . The selected value is saved with the help of the button Ok . To exit the current item without saving changes press Menu button.


button allows you to set the desired contrast of the display. Use the Up/Down menu buttons to change the contrast from 1 to 15. The selected value is saved with the Ok button. Exit the current item without saving by pressing Menu .

↑ Wiring and Initial Setup

Connect the unit according to the marking. [-] Ground, it is desirable to choose a good ground connection. [+] On-board power plus, 12 volt is connected to any point on the on-board power supply after the ignition switch. [G] Lights, connect to power supply circuit of lights or dashboard backlight [F] Fuel sensor, to avoid influence of native sensor, it is desirable to disconnect it and connect device directly to sensor line in tank. Turn the ignition on, connect a voltmeter in parallel to the power supply and check the indicator voltage reading, if necessary tune the indicator with the trimmer R2

↑ Program settings

  1. The tank is drained
  2. Set the value in liters to 0 and press Ok, go to calibration menu with Up/Down buttons
  3. Fill up with 1 liter of gasoline and set the value in liters (liters) to 1 by pressing Up/Down buttons and press Ok
  4. Repeat last two last points until the tank is full. Set the time lag, day/night light intensity, inverted light intensity, contrast at your own discretion.

↑ Product modification options.

There are two firmware versions on the CPU board: A5 and A2. The differences are in the measurement range of the input signals. The default setting is modification A5. Modification A2 is for non-standard connection cases and if it is not possible to calibrate the tank with the modification A5, this modification concerns the following elements R11, C4, C6

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