Water level indicator in the tank. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Indicators, detectors

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The proposed device is intended for remote measurement of the liquid (water) level in a tank, for example, a water pump. The circuit controls up to 4 intermediate levels. Basically liquid. may or may not be conductive. In this case, reed switches are used as sensors. For water, you can take both reed switches. and contact sensors (pins) made of stainless steel.

Water (conductive liquid), filling the tank, alternately connects the electrodes to a common wire, to which the lowest electrode is connected.

In the case of reed switches, floats are located inside the tank, on which magnets are fixed. When the liquid level rises, the floats together with the magnets rise, and the reed switches open one by one.

The circuit is physically divided into two parts - a transmitter and a receiver. For data transmission, a two-wire line up to 10 km long is used. The transmitter uses pulse-width modulation (PWM). Thanks to this, the device is operational when the parameters of the connecting line change over a wide range and does not require constant adjustment. The block diagram of the device is shown in Fig.1.

Transmitter (pic 2).

(click to enlarge)

The clock pulses come from the network. To do this, a diode is added to a standard power supply unit (it is switched on by the anode to the diode bridge, and by the cathode to the smoothing capacitor) The DA1 timer serves to increase the steepness of the clock pulse edges and increase the noise immunity of synchronization, (the pulses generated by the timer are fed to the input of the 2-stage frequency divider DD1. DD2 From the outputs of the divider, the pulses are fed to the address inputs of the multiplexer D03 and cyclically connect the inputs Y0 ... Y3 DD3 to its output Y. The inputs of the multiplexer are connected to the sensors.

The block of reed sensors is made according to the scheme in Fig. 3

Thus, each of the 4 sensors is given the same time to poll it, i.e. data (voltage levels) at these moments pass to the output DD3 (Fig. 2) and then to the inverting input of the comparator DA2. The second input of the comparator (non-inverting) is connected to a variable resistor R7. The comparator compares the voltage levels at the inputs, and if the voltage at the inverting input becomes lower (the sensor is shorted by water or the reed switch is triggered). a high level appears at the output of DA2. Resistor R7 can be adjusted sensitivity depending on humidity, insulation resistance and water. When using reed switches, the R7 engine is installed approximately in the middle. In principle. R7 can in this case be replaced by two resistors of the same resistance (5,1 .... 18 kOhm)

From the output of the comparator, the signal goes to the connecting line. The common wire is the "-" power supply. Since the output of the comparator is open collector, a pull-up resistor R8 is needed. In order to "not communicate" with clock pulses from the network, the DA1 timer can be switched to self-generator mode.

The DA1 switching circuit in this case is shown in Fig. 4.

Receiver (pic 5).

(click to enlarge)

At the input of the receiver there is a divider R1-R2, which increases the noise immunity of the device. The input switch on the transistor VT1 inverts the input signal. Signals with a level below 4...5 V are ignored. The second stage on VT2 inverts the signal again. Next, the signal is fed to the pulse-width converter time-voltage on the elements R5-C1. A full charge of this capacity occurs in 6 ... 10 s.

Speed ​​is determined by the speed of filling the tank. A 1-level comparator is assembled on the DA4 chip. Switching levels are set by resistors R6. R10 From the outputs of the comparators, the control voltage is supplied to the bases of the key transistors VT3 ..VT6. which switch the indication LEDs. Resistors R15 .. R18 in the collector circuit are rated for a current of 15 mA at a supply voltage of 12 V. This current is quite sufficient for super-bright LEDs. Anodes will be common to the LED, they are connected to the "+" power supply.

For automatic control of the pump, I used an analog of a lockable thyristor on transistors VT7 ... VT10. Input X7 is connected to the output (X6...X0). corresponding level at which it is necessary to turn on the "thyristor" (when "8" appears at the desired output), and input X14 - to the level at which it is required to turn it off (when "G at the output"). A pump starter can be connected to output X300 (current switching - no more than XNUMX mA) or a relay, and its contacts - to a powerful starter.

The power supply of the receiver and transmitter is local (6.15 V). Current consumption of the receiver - 10.. 80 mA, transmitter - 7... 30 mA.

When setting up the transmitter, zero potential is applied to all sensor inputs (imitation of a full tank). The transmitter output should have a level close to the supply voltage. By connecting one sensor to the minus (the rest are in the air), they control the presence of a quarter of the supply voltage at the output of the transmitter.

Setting receiver is reduced to the selection of resistors R6 ... R10, setting the levels of operation. At the time of tuning, resistors R6 and R10 are replaced by variable resistances of 1 ... 3,3 kOhm. Resistor R6 achieves stable burning of all 4 LEDs with a full tank, R10 - stable burning of 1 LED with one sensor closed to minus.

Author: V. Khvostik, v. Tsaredarovka, Kharkiv region.

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