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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Liquid level detectors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Home, household, hobby Level switches are the most common automation devices. The principle of operation of these devices is very diverse and is determined both by the physical properties of the medium and by the tasks set. When signaling the level of bulk solids, ultrasonic radars, mass meters of a container with a product, systems based on measuring the attenuation of an ultrasonic wave propagating along the wall of a container from a transmitter located at the level of the alarm setting to a receiver located horizontally at a certain distance are used. There are even radioisotope devices that shine through the wall of the container, on the opposite side of which there is a Geiger counter. There are many more devices for signaling the level of liquids. If the liquid is not electrically conductive, then, in addition to the above, capacitive sensors are used - when a pair of adjacent electrodes, insulated with a fluoroplastic film, crashes into the tank at the alarm level. When the liquid covers the electrodes, due to its dielectric constant, the electrical capacitance increases, the change in which is measured by the electrical circuit of the sensor. The most commonly used float and vibration alarms. For float sensors, the designs of which are the greatest variety, when the float rises, a reed switch or a non-contact element is activated. The vibration alarm is based on an LC v low frequency generator. A mechanical resonator in the form of a tuning fork is located in the inductance field. When the liquid touches the petals of the resonator, the attenuation of mechanical oscillations greatly increases and generation is disrupted. The alarm circuit gives an external signal when the required level is reached. Signaling the level of electrically conductive liquids, in addition to the above methods, is most often carried out using control electrodes. The principle of operation is to measure the electrical resistance between the reference electrode and the common wire. To eliminate the effect of polarization of the electrodes, in which the electrode is covered with a film of electrolysis products, which is a poor conductor of electric current, control is carried out exclusively on alternating current. Most often, this method is used to measure the water level. Typically, the voltage at the reference electrodes is about 6 V, and the electronic circuit operates when the resistance in the reference electrode circuit is below 3 kΩ. The alarm circuit has a symmetrical input for both half-waves of the control voltage. The voltage allocated to the measuring resistor is rectified and fed to the input of the threshold element, at the output of which a relay or contactless element is connected. Since in everyday life it is most often necessary to signal the water level, and circuits using control electrodes are the simplest and most accessible for self-repetition, only such systems will be considered in this section. The author has developed several dozens of such schemes, mainly for use in industrial enterprises, but many designs are perfect for signaling the level in storage tanks, wells, pools, drainage pits, etc. In this section, for simplicity, only two-channel signaling devices of varying complexity and on a different element base are considered, but the number of channels can be made any. For example, the industry produces three-channel signaling devices, and in the household, five-channel or more devices may be needed. The circuit of any signaling device necessarily outputs an alternating current to the control electrode, which is then fed to the threshold element, which has a hysteresis characteristic. At the output of the threshold device, an electromagnetic relay or non-contact elements are connected. Due to the diversity of the element base, there can be a huge variety of such circuits, but for simplicity, only relay output circuits will be considered below.
The simplest signaling device can be built on electromagnetic relays (Fig. 1). Only DC relays are used in the circuit, since the AC relay has practically no hysteresis and the signaling device will be accompanied by a large bounce when the electrode touches the water surface. The disadvantage of the circuit is the mandatory use of low-power relays and low sensitivity.
The transistor circuit has much better characteristics (Fig. 2). To eliminate relay chatter at the moment the electrode touches the water surface - due to insufficient threshold properties of the circuit, electrolytic capacitors are installed in the transistor base circuit. A Schmitt trigger can be used as a threshold element, as is done in industrial devices, but this greatly complicates the circuit.
Thyristors and triacs have good threshold properties. The thyristor circuit is simple and reliable (Fig. 3), but there are problems with the voltage on the output relays. A simpler and more reliable circuit (Fig. 4) is obtained using imported triacs of the BT134 ... BT138 series. You can use any relays or other loads in the circuit, for example, signal lamps or a bell.
If relays or other elements for a voltage of ~ 220V are used, then the circuit in Figure 5 is best suited. Threshold elements are best built on microcircuits. You can use the K561TL2 chip - a Schmitt trigger, but due to its scarcity, circuits using it are not considered here.
Almost all logical elements have good threshold properties, but for simplicity we will consider only K561PU4 (Fig. 6). Sometimes signaling devices are required, in which the output relays are activated in the absence of water and released when the electrode touches the surface of the water. Such a scheme is convenient for remote monitoring of the signaling device's health - it is enough to turn off the power and n.c. the relay contacts will close (Fig. 7).
If a suitable transformer with two output windings is not available, a circuit with one secondary winding can be used (Fig. 8). Depending on the voltage of this winding, output relays are selected. The circuit has good characteristics and the inverse characteristic of the state of the output relays. If you want to control the water level in a tank where splashes and waves are always present above the surface, it is best to use a circuit with level signal damping. The circuit should operate only after a steady appearance of a signal from the electrodes and not turn off the output signal during a short-term level loss. The quad comparator circuit has very good characteristics. The circuit uses a transformer with one secondary winding. If it is necessary to use powerful output relays in the circuit, then additional transistor amplifiers will be required.
The industry produces many level switches, but not all of their schemes can be called successful. The author many times had to bring serial products "to mind" (Fig. 10). Because circuits of industrial devices are often unreasonably complicated and inaccessible; in the future, they will not be considered. Only the simplest signaling circuits with good characteristics are considered here. Based on them, you can create many other schemes that the author does not publish here and suggests that readers do it themselves. Author: Kravtsov V., kravitnik.narod.ru
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