I decided to make a digital indicator of the amount of fuel for a truck (bus), using a standard (rather mediocre) fuel level sensor...

Read the entire creation process and what came out of it in the article below.

Initial conditions:

• Truck (bus) with on-board voltage 24v
• Fuel tank for diesel fuel on 220l
• Fuel level sensor DUMP39
• Fuel level indicator EI8057M-3

Need to:

Make a digital fuel level indicator using a standard level sensor.

First, you will have to carefully study what a standard fuel level sensor, called a fuel level sensor, is. Let's dismantle it and examine it carefully.

As you would expect, there is a float, a rod, a variable resistor... wait, more about the variable resistor. As they say, it is better to see once than to hear a hundred times:

The design is both logical and clumsy. It is logical that the slider slides not directly over the variable resistance (which is quite delicate), but along the metal taps from it, but for such an increase in reliability you have to pay for discreteness. The clumsy thing about this design is that, as can be seen in the photo, in the middle position of the float we have a fairly large “dead zone”, due to the very wide central outlet from the resistance. Why this was done, we can only guess, but what we have, we will have to work with.

So, we rummage through the Internet and look for information. Here's what I dug up:

Float movement range - 412mm

Nominal resistance - 800 Ohm (according to another source, the nominal resistance is 761.0 – 193.5 Ohm)

Operating range from -40°С to +60°С

MTBF - 400 thousand. km to 95% wasting resources

Weight 160 gram, analogue - MAZ.

In general, not a lot.

We take the tester and measure it, and in the end we get the following picture:
Connection diagram:

Measured sensor parameters:

Total resistance - 767 Ohm

Additional resistance - 187 Ohm(it provides the minimum sensor resistance).

Left (from the photo) part of the resistance - 203 Ohm (13 taps to the slider), right side Ohm 376(17 taps to the slider).

Two metal sectors above the contact group - the left sector is not used, the right one goes to the fuel reserve lamp.

In general, I am giving such a detailed description only for those who are curious; we need the voltage value that we have at the output contact at different fuel levels. With the extreme left position of the contact at the output, we got 1.57v, at the extreme right position 3.28v, half a tank - 2.44v. At the beginning of the sector of switching on the lamp of the remaining reserve 2.95v.

More for the curious. The general connection diagram for the fuel level sensor looks something like this:
Reels L1A, L1B, L2- this is a deflection system of the fuel level indicator (essentially a milliammeter). The resistor is thermal compensation.

In fact, this is a diagram of a classic electromagnetic automotive device, specifically EI8057M-3- this is something else: there is an electronic circuit inside, the arrow is driven by a stepper motor, and all this is controlled using a microcontroller PIC.

In principle, this is enough to calibrate a digital indicator, if not for a couple of troubles:

1. Specified fuel tank capacity in 220l not true, in fact the tank holds more fuel.

2. In the extreme right position of the movable contact of the sensor, when there is supposedly no more fuel in the tank, in fact the float should already be below the tank level, which is, of course, nonsense (determined by the geometry of the tank and the fuel level sensor.

3. Having measured the geometry of the tank with a tape measure, we are convinced that it is a rectangular parallelepiped with slightly rounded long edges, dimensions 40x112x60 cm. Multiplying the sides accordingly, we get an internal volume of 268 liters, which, you see, is very different from the declared 220 l, and it is very doubtful that the internal partitions, mesh, fuel intake, etc. occupy almost 50 l.

4. As already written above, the resistance of the sensor over the length of its resistance is nonlinear.

What we do:

Fill the tank full and control the voltage at the FLS output. It turns out that after reaching the mark 1.57v The tank still contains a good twenty liters of fuel.

Remove the float and put the sensor in place. Naturally, the draft, devoid of a float, goes to the very bottom of the tank, look at the voltage - it is 3.02v! This is important because in fact, in this position there is no longer any fuel in the tank, and the moving contact has not yet reached the extreme position in 3.28v, while the standard device EI8057M-3 shows what's left in the tank 1/8 volume. (Putting the float in the central position, at standard EI8057M-3 we observe instead of the required ones 1/2 tank as much 5/8 level, with a full tank the standard device goes off scale).

We look at the graph of our fuel level sensor,

Let's take three points - the resistance of the sensor, the first point is its lowest resistance (moving contact on the left) formed by additional resistance in 187 Ohm(in the photo there is a vertical black rectangle), the second point at the middle position of the contact when connected in series 187 Ohm And 203 Ohm, i.e. 390 Ohm, the total resistance will accordingly be 390 + 376 = 766 Ohms.

(horizontally - resistance in Ohms, vertically - conventional units of length)

There is nothing pleasant in this picture; the sensor seems to be linear but has a significant kink.

With such a picture, we will either get accuracy in the middle, or at the ends of the broken line, or something in between by approximating:


Having received the formula with the correction and coefficient, you can, in principle, make something similar to a digital fuel level indicator, coefficient R 2 trend lines in 0,97 Of course it’s not bad, you can, in principle, use anything greater than 0.95.

But you can get your own conversion factor for each line, which will be more accurate:
We immediately measure the ADC value at the points we need so that 5% The tolerance for the divider resistors at the ADC input did not spoil anything for us and we get it in the range of an empty tank (ADC822) before 1\2 tank (ADC700):

(horizontally the received ADC readings, vertically the volume of fuel in liters)

Ranges from 1\2 tank (ADC700) to full (ADC456):

From the above we have the following:

1. As the amount of fuel increases, the resistance of the sensor decreases and the voltage drop across it decreases.

2. The sensor voltage delta is 1.45v, that at 10 bit ADC will be 56% which is more than enough to scale the ADC result to scale 0....220l and will allow you to simply digitize the result without using OU to adjust to the desired voltage range.

The scheme is incredibly simple:


Microcontroller Mega8, LED indicator on 3 discharge with a common cathode, input divider of two resistors R1, R2. Zener diode (in bourgeois zener "zener" diode :)) to protect the input MK just in case. I didn’t draw the power circuits, they are classic 0.1uF ceramics and some kind of electrolyte 100...1000uF as well as quenching resistors between the MK and the indicator, any in the range will do 80...100Ohm depending on the MK supply voltage and the brightness of the indicator. The voltage on board the car with the engine running was 27.5v.

My board layout:

On the right side of the board I placed a power converter that provides 5v at onboard voltage 10...30v the converter is assembled on MS3406 3 according to the typical diagram from the datasheet. throttle murata 1812. The zener diode indicated in the diagram is 3.3v I screwed up when wiring and soldered on top.

Why did I apply Mega8 when there is a much more convenient one Tiny26 and so on. ? because Mega 8 available 1kB RAM, why so much? The microcontroller not only measures the voltage at the input and displays the recalculated value on the indicator, it constantly records the measured values ​​in one of 256 memory cells, filling them in a vicious circle and after recording each cell, it calculates the average value over all currently available 256 cells.

The indicator is located outside the board on the car's dashboard and is connected to it 11 wire loop. The board is placed in a tiny case (the second one, the one with 4 wire terminals); excess plastic was removed from the case with side cutters.

The board is single-sided, without jumpers:

First, I unsoldered the PWM switch and checked the work, it works. varnished. you can continue building:

P.S. The project was created with the enormous support of Roman Viktorovich, for which many thanks to him, also thanks to the man Johnson from Ukraine for mathematical help and some ideas.

On vehicles equipped with an ECM, fuel is supplied by an electric fuel pump directly from the fuel tank. If the fuel runs out, there is a risk of failure of the pump itself. Therefore, the importance of the fuel level sensor on modern cars is very high. However, different types of sensors have different degrees of reliability.

Signs of a malfunctioning fuel level sensor

Naturally, the most common sign of device failure is the complete immobility of the needle when the ignition switch is turned on. A malfunction can also be signaled by raising the indicator arrow all the way towards the full tank. It is more difficult to diagnose inaccurate readings. In this case, the device shows the presence of fuel in the tank, the control light does not light up, and the engine does not start due to lack of fuel. Also vice versa. When refueling, it turns out that the tank is almost full, although the indicator indicates only three-quarters of the occupied volume.

Important! These signs do not always indicate a malfunction of the sensor; the dial gauge for measuring the amount of fuel in the instrument panel may also be faulty.

Pair structure: panel indicator and fuel level sensor in the tank

The indicator on the instrument panel is a miniature electric, reversible motor. Its action is based on the creation of an induction field in the stator windings. Simply put, the pointer is located on the rotor shaft, which moves around its axis under the influence of on-board voltage. The degree of rotation of the rotor is regulated by the resistance at the output of the windings. This is the “K” wire to the fuel sensor in the tank. The second wire on the old-style pair is an independent control light. When the level of the measured liquid drops to a minimum level, the contact closes to the housing and the light comes on, signaling a critical volume.

On more modern systems, the circuit in the pointer-sensor pair does not close to the housing, and the signal goes back to the device on the panel, which is already connected to ground. And when the potential drops to a certain value, it lights up a signal light integrated into the indicator circuit.

These are the most common fuel level monitoring systems on modern cars. For simplicity, these two types can be defined as:

• indicator with sensor mass
• feedback controller
• standard fuel level sensor device

Essentially, the fuel level sensor is a rheostat with variable resistance. A metal scale is applied to the ceramic plate, which acts as a spiral in a conventional rheostat. There are two soldered contacts at the corners of the plate. Depending on the type of sensor, the wires are connected. If the indicator and wiring are of the old type, one contact goes to the sensor mounting housing, and the second to the indicator, through the housing cover. The second wire is the test lamp wire and is connected to ground with a separate “track” in the empty position. A movable metal contact, tightly pressed to the sensor scale, acts as a “slider” of the rheostat. A float attached to it, depending on the fuel level in the tank, moves the “slider” along the scale. This is what changes the resistance of the pointer circuit, causing the arrow to move in the desired direction. The new system with a reverse signal works the same way. There is simply no connector for the warning lamp on the ceramic plate, and both contacts are connected by wires to the fuel level indicator in the instrument panel. Depending on the car model, the sensors may differ in scale resistance. All of them are marked with separate values, for example:

• DUT-1-01;
• DUT-1-03;
• DUT-2-03,

How to determine a faulty fuel level sensor

First of all, it is determined what exactly is faulty, the indicator on the instrument panel or the fuel level indicator itself in the fuel tank. To do this, it is necessary to provide access to the wire inputs and the sensor connector on the tank. As a rule, all vehicles have a technological hole for this operation. Depending on the model and manufacturer, the hatch is located in different places. It is indicated in the technical documentation of the machine. To check the functionality of the fuel level indicator on the panel, it is advisable to have the following on hand:

• control light (squeaker) with a ground detector;
• car tester;
• working fuel level sensor (matching VIN code).

The fact is that older models with a sensor “ground” to the body can be checked by shorting the wires supplying current to the body. However, with recent models with feedback this does not always work.

The process of checking the pointer on the instrument panel with the sensor ground

Disconnect the wires from the fuel level sensor in the tank. The connector must be freely accessible. Turn on the ignition. A stable “+” should appear on the sensor wires. This is checked with a test lamp and a tester. The voltage on the wires must be equal to the total network voltage. If both wires have normal voltage readings, you need to find out which one is the warning lamp and which is the sensor. To do this, short the wires one by one to ground. When one is short circuited, the critical fuel level light should light up. When the other one is being massaged, the fuel level indicator arrow should sharply move to the “full tank” position. If this is what happened, then the fuel level sensor is faulty and must be removed for replacement or repair.

Checking the fuel level indicator with feedback

On such devices, as a rule, only one of the two wires has a stable “+”, and the second has a weakly defined “ground”. This is determined by a control lamp with a “-” detector. In addition, often if the sensor or indicator is faulty, the arrow, on the contrary, moves to the “full tank” position. After checking with a test lamp that the wires are loaded, you need to connect a working sensor to them, raise and lower the fuel level float. The pointer arrow should move in accordance with the position of the float. And in the “empty” position the level indicator lamp will light up.

Attention! The rheostat drive rod must be moved slowly. A sharp jerk towards “full” or “empty” may well damage a working pointer.

Possible malfunctions of the fuel level sensor

Of course, the most common malfunction of fuel level sensors of all types is leakage of the float that controls the mobile element of the rheostat. Usually in this case the indicator arrow is constantly in the empty tank position.

Then the plate scale becomes dirty. This could be fuel deposits or elements of a worn-out runner. The arrow can freeze in any position. Or the accuracy of the fuel level indicator readings is greatly impaired.

Broken wiring both on the body and at the plate with the rheostat. And finally, mechanical damage to the plate itself. There may be some other exotic faults, but they are so rare that they are not worth considering. If there is a malfunction but cannot be diagnosed, it is better to replace the fuel level sensor assembly.

How to repair a fuel level sensor with your own hands

Repairing the fuel level sensor is quite affordable for the average car enthusiast. Parts of the device can be freely purchased at an automobile parts store. It is necessary to remove the sensor from the tank and study its characteristics. The marking is located on the front side of the plate directly above the rheostat scale.

If the float is broken, then this is the least of the problems. They change simply. Usually, this is a plastic barrel filled with air; it is simply removed from the holder socket and a new one is inserted. Sometimes this is a porous element and can be replaced in two ways. Remove the locking washer and put on a new one, securing it. Or change the float assembly with the rod, which is much simpler.

If the rheostat scale strips are dirty, you just need to clean them.

Attention! The record should only be cleaned with a soft cloth or cotton wool soaked in alcohol. Hard matter or any other object can damage the thin layer of the scale and the rheostat can be thrown away.

Unsoldered or torn wires can be carefully soldered into place or soldered at the fracture site, but a cracked, broken plate can only be replaced with a new one.

In most cases, no one suffers from this now. A non-working device is replaced and that’s it. Fortunately, their price is low. Moreover, on cars of recent years, the rheostat plate, and all other parts of the device, are secured with latches.

Inaccurate indicator readings can be deceived by adjusting the angle on the rod holding the float. By bending it in different directions you can ultimately achieve more accurate readings.

First of all, we can mention the electronic display for the instrument panel. It is installed together with a standard dial fuel level indicator on a panel in a free cell or directly on the torpedo. Three wires are output, “+”, “-” and “D”. The latter is connected to the sensor along with the standard device. On the sensor removed from the tank, the level readings are adjusted in digital format, from “empty” to “full tank”. This does not add any particular accuracy to the readings, but it does give a stylish look to the instrument panel and warms the owner’s soul. Signs are used in various shapes and it is quite possible to choose a sign to match the interior design of your car.

A completely different matter is the novelty in this matter (relative) - ultrasonic fuel level sensors. Here, as in most devices, the principle of receiving and transmitting ultrasonic waves is used. The sensor can be installed in the tank without violating its integrity. The signal can be transferred to a digital monitor on the instrument panel and even output to a computer or laptop via the GLONASS system. Today this is the most accurate way to obtain data on fuel level. However, this is still a rather expensive procedure, which also requires special knowledge and skills to calibrate the ultrasound emitter and specialized programming equipment. However, everything suggests that meters of this type will be widely used in the future.

In the video below you can see the replacement of the fuel control sensor on a VAZ Priora car:

Used with the original level sensor (in the tank), and instead of the standard pointer (on the dashboard).
This device (based on 16f676) displays the readings of the fuel sensor in the tank (40 l) on a two-digit seven-segment (with a common anad). Power supply from the car’s on-board network is 12 V. We connect the sensor in the tank to the “in” input.

Published 09/25/2012

Knowing the fuel level in the tank is not only “cool”, but sometimes vital. In some cases, it is difficult to assess the fuel level in the tank due to its location or lack of transparency. For such cases, there are fuel level sensors. Today, float sensors are the most common. The operating principle of such sensors is quite simple. The float mechanism, depending on the fuel level in the tank, changes the position of the moving contact of the potentiometer. The voltage reading on the potentiometer is measured and converted into human-readable form. However, it is not always possible to install a float sensor due to its size. In addition, in devices where roll is a normal condition, for example, ultralight aircraft, the float mechanism may become skewed and jammed. In addition, the position of the tank in the ground and flight positions may differ, which may alter the operation of the float mechanism. However, there are other ways to measure fuel level. I'm talking about capacitive fuel sensor. It is especially relevant if there is a need to get rid of moving parts.

Measuring principle and features

This method is based on measuring the electrical capacitance of the sensor, which, in turn, depends on the fuel level. The sensor used to measure the fuel level is called a capacitive fuel level sensor. The design of the sensor is quite simple and is nothing more than a capacitor. It consists of two plates, between which there is a gap that can be filled with fuel. The sensor can be made in the form of two metal plates or tubes inserted into one another. In this case, the surfaces of the two electrodes (capacitor plates) should not have electrical contact, and the gap between the plates should be freely filled with fuel when the sensor is immersed and just as freely released when the fuel level decreases. As fuel fills the space between the plates of the capacitor (sensor), its capacity changes. This method is only suitable for liquids that do not conduct electricity. This method will not allow you to measure the water level. Gasoline and other types of liquid fuels do not conduct electricity. By measuring the electrical capacitance of the sensor, you can estimate the fuel level in the tank. I would like to draw attention to some of the disadvantages of this measurement method. The fact is that the dielectric properties of the fuel can change when the chemical composition of the fuel changes. Those. When changing fuel type, you may have to calibrate the device. Despite this, this method allows you to install the sensor in the tank at an angle, or even mount it in the tank filler cap. The sensor has no moving parts, which is extremely necessary in some cases.

Is it safe to place an electrical circuit in a tank? Many people are concerned about this issue. What if there is a spark? Our sensor circuit is powered by a voltage of 5V, and the sensor is charged through a resistor of several megaohms. Under these conditions, spark formation is impossible. A voltage of 5V is negligible to cause a breakdown spark. In addition, an electric fuel level sensor already “floats” in the tank of any car. Low voltages and currents cannot cause a spark and ignition of the fuel.

I did not set myself the task of obtaining a super accurate sensor capable of measuring the fuel level to within 1mm and with an error of 0.1%, although this is quite possible. Considering that the sensor was created for devices where the fuel in the tank will be mobile, we are quite happy with a budget option with an error of 5%.

The sensor module circuit is based on measuring the charging time of the sensor. The higher the fuel level, the higher the sensor capacity, the longer it will take to charge the sensor (capacitor). The scheme works as follows. Uses built-in microcontroller ATMega8A analog comparator.
To the comparator input PD7 half the supply voltage is supplied through a resistive divider R3,R4. At the moment when the sensor is charged to this voltage, the comparator will operate. On the foot PD6 logical is set «0» . The sensor is discharged through a resistor R2. After which exit PD6 switches and works as a comparator input, the timer starts, and the sensor begins to charge through the resistor R1. When the voltage set at the input is reached PD7, the comparator is triggered, the timer stops. The timer readings are used for calculations. To ensure stability, the microcontroller must be clocked with quartz. The higher the frequency at which the controller operates, the higher the measurement accuracy. In our scheme ATMega8A clocked by quartz 16MHz. Measurements are taken continuously, averaged, and sent once per second via the serial port UART at speed 9600 as a numeric value. This is where the functions of the sensor module end.

As a sensor, I used two strips of foil PCB 1.5 mm thick with dimensions: 290 × 20 mm. The strips are glued together foil to foil through small non-conductive spacers. The distance between the plates is 1.5 mm. They can be made to almost any length. You can trim it if necessary. It is especially important to ensure a uniform gap between the plates along the entire length of the “capacitor”.

The display module is responsible for displaying the data received from the capacitive sensor module. This module can be designed according to your requirements. Data can be displayed on an LED bar, on a display, as in our case, on a dial indicator, or any other display device. If necessary, the sensor module can be connected to a computer via an adapter.

The display module works as follows. Numerical data is received from the sensor module via the port UART at speed 9600 , fuel level readings are calculated and displayed. But in order to perform a correct recalculation, the display module will need to know at least two sensor values ​​- the numerical reading of the sensor when the tank is empty and the numerical reading of the sensor when the tank is full. To do this, after installing the sensor, the device calibration procedure is performed. The display module remembers the readings when the tank is empty and full, stores it in its non-volatile memory and performs a recalculation in accordance with these data. Since the module does not require special performance, its microcontroller ATMega8A operates on frequency 2MHz from the built-in RC oscillator.

Instrument calibration procedure:
- the fuel tank must be empty, the device must be turned off
-press and hold the button
- turn on the power of the device
-release the button
- “SET 0” will appear on the screen. Make sure the tank is empty and press the button.
- “SET 100” will appear on the screen. Fill the fuel tank full and press the button.
- calibration is completed.

PCB example:

Sensor module board

The circuit of a digital fuel level indicator has a high degree of repeatability, even if the experience with microcontrollers is insignificant, so understanding the intricacies of the assembly and configuration process does not cause problems. The Gromov programmer is the simplest programmer that is necessary for programming an avr microcontroller. The Goromov programmer is well suited for both in-circuit and standard circuit programming. Below is a diagram for monitoring the fuel indicator.

The photo below is a montage photo.

Device functionality:

• is able to accurately display the current fuel level, accurate to the nearest liter, supports a fuel tank from 30 to 99 liters;
• displays information about the on-board system;
• works taking into account fuel fluctuations that are observed while the car is moving, the internal sensor in the tank makes multiple measurements and information is displayed based on the arithmetic average (measurement frequency can be set in the menu);
• The brightness of the backlight changes depending on the current level of illumination; there are two modes: day and night;
• There are two modes of indicator information display: normal and inverse.

Microcontroller Details:

R1 - 1 kOhm
R2 - 75 kOhm
R3 - 10 kOhm trimmer
R4 - 4.7 kOhm
R5, R6, R8-R11 - 10 kOhm
R23, R12-R15 - 3.3 kOhm
R24, R16-R19 - 1.8 kOhm
R20 - 2 kOhm * selected depending on the backlight
R21 - 240 Ohm
R22 - 1 KOhm * selected and set to constant
C1, C2, C15 - 0.01 μm
C3, C4, C6-C11, C13-C15 - 0.1 μm
C5 - 47 microns
C12 - 4.7 microns
L1 - 100 mH
DD1-LM7805
DD2 - ATMega8
DD3 - LM317T
VT1 - IRFZ44
LCD1 - Nokia 1110/1200/1110i/1112.

The diagram does not indicate the PC10 connector, through which the buttons are connected and the output for installing software on the microcontroller.

It is necessary to make two boards: one for the display; the second will be the main one. Both boards must be circular in shape and their case diameter must be 50mm. It is quite difficult to find the indicator of the mate for the connector, so it is rational to wire it for the cable. You also need to unsolder the connector from the mating part and solder it in its place, only solder a cable on the back side; the display itself can be attached using double-sided tape.

The main (main) board is double-sided, however, the reverse side is the base one, and on the second side there are stabilizers and one transistor; the main part of the parts is installed on the track side. The basic square holes are soldered with jumpers, the rest of the holes are drilled out.

In place of the disassembled connector, the two boards are connected using contacts. A threaded bushing is soldered under the main board; the boards are secured to the body with one screw. There are no buttons because from a practical point of view there is no need for them.

They are needed only when performing initial calibration, and therefore are output to the PC10 connector, which is located on the back of the case. Signals for programming the microcontroller are also output through this artificial connector.

Instructions for setting up the digital fuel level indicator.

1 step. The microcontroller is programmed in-circuit; for this you can use any programmer that is at your disposal.

Step 2. The fuse is set as follows. First you need to adjust the voltage readings. To do this, you need to connect the indicator to a voltage of 12-14V in order to configure it; we connect a voltmeter and trimmed resistor R3 to the same electrical power source, in which we set the values ​​​​that the voltmeter displays.

Step 3. Next, you need to perform software configuration of the device. First you need to set the tank capacity and calibrate it. Calibration of the fuel tank is carried out as follows: set the empty tank value to 0 liters and press the OK button. Then, pour 1 liter of fuel and set the value to 1 liter of fuel and press the OK button again.

This procedure must be repeated many times until the tank is full. Naturally, this process is quite time-consuming, but it must be completed once without fail.

During calibration, you can also record sensor readings, which will save a significant amount of time when performing any firmware. Other types of settings can be set in accordance with individual preferences.

The fuel indicator will allow you to rationalize your daily gasoline consumption and thereby save money.