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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Electronic level gauge. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur [an error occurred while processing this directive] In the practice of radio amateurs, it sometimes becomes necessary to control the liquid level in closed tanks. Such analog or digital control with a minimum discreteness allows for ultrasonic and capacitive level sensors. However, these rather complex and expensive devices are used only in industry. The electronic level gauges described in the article are discrete and are designed to control the level of liquid in various containers. The proposed electronic level gauges allow you to control the volume of water or aqueous solutions, including softened water, which is used in heating systems. The principle of operation of level gauges is based on a significant decrease in the resistance between the sensor contacts when they are immersed in a liquid. The scheme of the first version of the level gauge is shown in fig. 1. The voltage divider, formed by one of the resistors R1 - R6 and the resistance of the sensor connected to one of the inputs 1 - 6 and common wire 7, sets the logical voltage level at the input of the inverter. The latter simultaneously acts as a buffer element. The outputs of the inverters are connected to LEDs, on the basis of which the level scale is built. To protect against possible interference and impulse noise, an integrating circuit R7C3 - R12C8 is installed at the input of each inverter.
The switched on LED corresponds to the open state of the sensor. Thanks to this solution, the current consumption when all sensors are immersed in water or solution, i.e. when the container is completely filled, does not exceed 2 mA. The level gauge can be powered from any stabilized source with a voltage of 4 ... 15 V. When powered at low voltage (4 V), it is necessary to select resistors R13 - R18 to ensure the desired brightness of the LEDs and resistors R1 - R6 according to the criterion of reliable operation of inverters of the DD1 microcircuit. The total current consumed by the level gauge when all indicators are triggered, i.e. when the tank is empty, it can be estimated from the calculation of 4 mA for each volt of the supply voltage IW- Thus, with a supply voltage Upit = 4 V, the current consumption will be 16 mA, and at 15 V - 60 mA. A sketch of the printed circuit board of this level gauge is shown in fig. 2.
Level sensors are placed on a cylindrical pipe made of dielectric material. Each sensor is made of two strips of copper foil 10 mm wide spaced from each other at a distance of 10...20 mm. One of the strips is connected to a common wire, the other - to the input of the level gauge. There are six sensors on the pipe. The wires of the senders are passed inside the pipe and connected to the cable using a seven-pin connector. Thanks to this design, one indicator unit can be used with various sensors. Each sensor is calibrated for its own volume of liquid and connected to the indicator unit with a cable made of seven PEV-2 wires with a diameter of 0,3...0,5 mm. In the author's version, the cable length reached 50 m. Since the display unit operates at a constant voltage at the logic inputs and has a large input impedance, there are no fundamental restrictions on the cable length. When repeating the design, you can use any CMOS microcircuits, the elements of which are included as inverters, for example, K561LA7. It is only necessary to change the wiring of the printed circuit board conductors and the number of controlled levels in accordance with the number of inverters in the microcircuit. It is permissible to use several microcircuits to increase the number of controlled levels. The proposed level gauge will not work with TTL series microcircuits, for example, K155 or K555, since these microcircuits require a significant input current. The values of the resistors at the input of the inverters can be changed over a wide range: R1 - R6 - from 5 to 100 kOhm; R7 - R12 - from 100 to 750 kOhm. The capacitance of capacitors C3 - C8 should be sufficient to attenuate impulse noise and mains interference. It can reach 1 uF. Resistors R13 - R18 set the desired brightness of the HL1 - HL6 LEDs, which can be any LEDs that emit light in the visible region of the spectrum and provide sufficient brightness with a current consumption of not more than 10 mA. The display unit can be supplemented and improved by installing a decoder and a seven-segment indicator. Since there are no standard decoders for displaying the status of sensors corresponding to the above diagram, it was decided to use the K155REZ RPZU chip as a decoder. At the same time, the number of controlled sensors was reduced to 5 (according to the width of the address bus of the microcircuit). The scheme of the second version of the level gauge is given in Fig. 3.
The system of sensors and input buffer elements, which are the inverters of the DD1 chip, is similar to that used in the first version of the level gauge. At the input of the decoder, assembled on a DD2 chip, data comes from the outputs of DD1. In accordance with the firmware presented in the table, the seven-segment indicator HL1 displays information about the maximum water level in the tank.
Since the RPZU K155REZ chip used in this design is critical to the supply voltage, the device shown in Fig. 3, should be powered from a stable voltage source of 5 ± 0,25 V. In the indication mode, the current consumption reaches 100 mA, so it is not advisable to use an autonomous power supply. A mains rectifier is recommended to power the indicator. On the board of the level gauge, made according to the second scheme (Fig. 4), there is a place for installing an integral stabilizer K142EN5, the use of which will allow you to connect the indicator unit to a rectifier with an output voltage of up to 15 V.
About construction details. As an indicator, you can use any seven-segment indicator with a common cathode. An indicator with a common anode can also be connected to the output of the decoder. In the latter case, it is necessary to invert the data in the RPZU K155REZ programming table and turn on current-limiting resistors at each RPZU output. The indicator anode is connected to the positive power wire. The decoder can also be performed on EPROMs of other types and capacities, as well as on the simplest FPGAs, for example, PAL16L8 and similar ones, without triggers in the output circuits. Author: I. Tsaplin, Krasnodar
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