Brake fluid level indicator in a car. Free Technical Library article

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Brake fluid level indicator in a car

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Modern cars are equipped with hydraulic brakes, one of the disadvantages of which is the risk of sudden leakage of brake fluid from the drive, which can lead to an accident.

Installing a brake fluid level indicator on a car creates a certain convenience in operation and significantly increases traffic safety. The indicator can be installed on all types of domestic vehicles with a hydraulic brake drive and a supply voltage of 12 V with a negative on the ground, in the hydraulic system of which the GTZHA-2 "Neva" liquid TU6-09-550-73 is filled. If a different brand of brake fluid is used in the hydraulic system, then the ratings of the elements, of course, will differ from those shown in the diagram.

Brake fluid level indicator in a car. Brake Fluid Level Indicator Diagram

The schematic diagram of the indicator is shown in fig. 1. Its base is a multivibrator based on transistors T2 and T3. The load of the multivibrator is the Tf1 telephone capsule. Transistor T4 contributes to a clearer fixation of the operating state (open - closed) of transistor T2.

When the dipstick in the tank is immersed in brake fluid, a bias voltage is applied to the base of transistor T1, and it is open. In this case, the base and emitter of transistor T2 have the same potential, and this transistor will be closed. As a result, the multivibrator does not work, and the phone Tf1 is de-energized. Diode D1 protects the base of transistor T2. When the level of brake fluid in the reservoir drops, the dipstick is in the air. As a result, transistor T1 closes and transistor T2 opens. Now the multivibrator will operate at a frequency determined by the time constant of the R5C1 chain (about 400 Hz). An audible signal warns the driver of a leak or insufficient level of brake fluid in the reservoir.

Brake fluid level indicator in a car. Indicator circuit board

The indicator is assembled on a printed circuit board made of foil fiberglass or getinaks with a thickness of 1,5 mm. The PCB drawing is shown in fig. 2, and the entire pointer assembly - in Fig. 3.

Brake fluid level indicator in a car. Location of parts on the board

It is undesirable to use germanium semiconductor elements in the indicator due to their low thermal stability. Resistors - MLT-0,25, capacitor - MBM. As Tf1, you can use an electromagnetic telephone capsule of any type with a winding resistance of 65-1600 ohms, for example, TA-4. The telephone capsule is attached to the printed circuit board in the following way. Two holes with a diameter of 35 mm are drilled in the capsule cover at a diameter of 3,3 mm with a countersink on the inside for the countersunk head of the M3 screw. The capsule cover is fixed with two M3 screws on the board from the side of the elements, then the capsule is screwed into the cover. The printed circuit board with the phone is placed in a plastic or metal case.

An indicator correctly assembled from standard parts, as a rule, does not require adjustment. In some cases, it may be necessary to adjust the sensitivity by selecting the resistor R1. The sound frequency is regulated by changing the resistance of the resistor R5.

Brake fluid level indicator in a car. Sensor design

Rice. 4. The design of the indicator sensor (brass L62)

The sensor is mounted in the plastic cap of the brake fluid reservoir. One possible design of the sensor is shown in Fig. 4. The sensor is made of L62 brass. The length of the sensor depends on the distance from the reservoir cap to the mark of the minimum brake fluid level in the reservoir. For a VAZ-2101 car, it is 33-35 mm. If, when installing the sensor, the vent hole in the tank lid is closed, a hole with a diameter of 1,5 mm must be drilled next to the sensor to communicate the inside of the tank with the atmosphere.

If the reservoir for the brake fluid or the pipeline coming from the reservoir is metal, then no further modifications to the reservoir are necessary. If the tank and pipeline are made of non-metallic materials (for example, for VAZ cars), then the brake fluid in the tank must be electrically connected to the vehicle ground. To do this, in the tank at any point below the minimum level of brake fluid, it is necessary to drill a hole with a diameter of 4,1 mm, insert an M4 screw with a cylindrical head into the hole, laying sealing rubber and metal washers on both sides of the tank wall, and connect the screw to ground with a flexible stranded wire (MTB, BPVL, etc.) with a section of 0,35-0,5 mm².

This need is due to the following. Although on domestic cars of all types, the brake fluid is electrically connected to the car body (metal brake master cylinder, metal pipelines, etc.), the point of its connection in some cases is far from the sensor, so the electrical resistance of the liquid column in the sensor - body section is a few megohm . So, in VAZ cars, a polyethylene tank is connected to the main brake cylinder by a dielectric pipeline, and the resistance of the sensor-case section in cars of this type is so high that transistor 77 does not open. For cars "Moskvich-408", "Moskvich-412" there is no need to ground the liquid - the reservoir is located directly on the main brake cylinder, and the liquid contacts the body close to the sensor, so the resistance of this area is small.

There is no short circuit along the walls of the tank between the sensor and the housing due to the high resistance of this section, even with the maximum possible presence of liquid residues on the walls. In addition, tanks made of plastics are poorly wetted by liquid, and the walls of the tank are located vertically. Therefore, as experience has shown, it is possible to place two sensors in the tank lid, and connect one of them to the body. However, the distance between the sensors for greater reliability of the device should not be less than 15 mm.

In this case, setting up the device comes down to choosing the optimal sensitivity by selecting the resistor R1. In some cases, with a significant amount of liquid remaining on the lid, the generation frequency of the multivibrator may change slightly.

When installing two sensors in the reservoir cap, you can use the same indicator to monitor the level of brake fluid in the reservoir and coolant in the radiator. To do this, a similar sensor is installed in the radiator (or in the expansion tank), isolated from the housing, and all sensors are connected in series: one sensor in the brake fluid reservoir cap is connected to the indicator input, the other to the fluid level sensor in the radiator. When the liquid level in the tank or in the radiator drops, the circuit opens, and the indicator warns about this with a sound signal.

Publication: cxem.net

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