LM555 is a widely used analog-digital hybrid (i.
e. bipolar) integrated circuit. Based on the LM555 control circuit, this paper studies the timing and control integrated circuit. LM555 timer constitutes a multi-resonant oscillator.
In the control circuit, the duty cycle of output waveform is regulated by the potentiometer, and the start-stop time of load operation is controlled.
The counting of encoding circuit gives the trigger counting pulse. The high and low levels of the trigger control the absorption and disconnection of relay, and then control the start-stop of load. The design realizes the weak-current pair as a whole. Control of high-power load operation. The design has the advantages of precise control, economic durability and high sensitivity. It has wide application value and market development prospects.
LM555 time-base circuit is mainly used in all kinds of timing control circuits, which is a typical application of timing control of fish pond aeration. In order to cope with the complexity of the timing control circuit of the current market demand and technical requirements, the clock module, coding module, timing/counting module and relay driver module are mainly designed in this product. In practical application, it is necessary to specify and precise the working time of the load. This product has important use in saving cost and maximizing output value. Usually, under the background of high voltage and high voltage, weak current control is prone to run out of control. Analog-digital hybrid circuit can overcome this phenomenon. It has the advantages of simple structure, easy control, good stability and strong external interference. Based on the timing chip LM555, this paper discusses the design of automatic timing control switch circuit for fish pond. As shown in Fig. 1 and Fig. 2 below, it is the overall block diagram and flow chart of the control circuit. The system is mainly composed of logic circuits. The internal control of 380V high voltage in the external factory is through 5V. The overall design is high security, easy protection, low power consumption and high voltage. Stable and easy to control. The LM555 timer can be used to form a multivibrator, also known as a square wave generator (the circuit has no stable state, only two transient states, which can produce rectangular wave signals of a certain amplitude and frequency, and “1” and “0” are used to represent two different states respectively). As shown in Figure 3 below, the basic circuit diagram of multi-resonance oscillation shows that the time constant of R1C1 is much larger than that of (R2 R3) C1 [2]. Before switching on, the voltage of C1 and C2 terminals is 0V, and the diode VD2 is cut off. The unsteady oscillator composed of 555 time-base circuit and R2, R3 and C2 can start normally.
The starting process is: 555 timer is set at the beginning, 3 pins output high level, C2 starts charging and 555 reset at 2/3 VDD; 3 and 7 pins output low level, C2 starts discharging, which reduces the voltage of both ends to 1/3 VDD. The setting and reset cycle causes circuit oscillation.
The oscillation frequency is F=1.44/(R2 2R3)*C2. LS148 is one of the 8-3-line priority coders.
Its main function is to encode 8 data lines (0-7) in 3-bit (octal) priority, decode the highest bit data lines, and expand them in octal by using input gates (EI) and output gates (E0).
This system uses two 74LS148 pieces to complete 16-4 line priority coding, parallel connection, 8 data lines connected with a pull-up resistance, U 3 EO port to U 4 EI, common ground line and power line, three output data lines are input to four high and low priority and non-gate (7400N), compiled low four-bit (hexadecimal) timing/counting initial value, completed from. Dynamic cycle operation. The digital logic circuit will not “run away” under high voltage and high counting accuracy. Timing/counting function is realized by 74LS161. 74LS161 is a commonly used four-bit binary preset synchronous adder counter. It can be flexibly used in various digital logic circuits and single-chip computer systems to realize frequency divider functions. 7400N outputs four-digit hexadecimal number and inputs it to the initial value of 74LS161 preset. On this basis, add one count (hexadecimal addition).
The counter overflows high/low level signals to the trigger (74ALS20AN). The trigger flips over high and low levels and latches the clock pin of 74LS161, so that the counting cycle reaches the preset value [3]. In the relay, the whole conduction and disconnection are completed mainly by the gain and loss of coils and the suction and disconnection of contacts. The relay coil is composed of iron core, coil, armature, return spring, dynamic and static contacts, etc.
Current flowing through coils (1 and 2 pins) produces specific magnetic flux. Under the action of electromagnetic suction, armature (moving iron core) overcomes the pulling force of return spring to normal open contact (4 pins), and drives the closure or disconnection of contact and static contact. When relay coil is disconnected, the action of connecting iron and connecting point in return spring pressure when there is no electromagnetic suction. Return to the original position (5 pins), at this time, the static and dynamic contact restores the original position. Relay belongs to electronic control device, which is usually used in automatic control circuit. It is a kind of “automatic switch” component. It is widely used in large current circuit controlled by small current, and plays the role of automatic adjustment, conversion circuit and safety protection in the circuit. This system completes a set of automatic control timing circuit based on digital logic circuit through the above modules and auxiliary modules. The system input 380V high voltage and strong electric current to 12A weak electric current internally. The system is started or shut down when the same value as the preset number is achieved by controlling the count with the coded logic circuit, so that the control purpose can be achieved. The timing control circuit is mainly used in the timing oxygen supply system of fish farm in fish pond. After a large number of repeated tests and laboratory tests with different loads in the field, the system shows the advantages of reliability, high accuracy and good stability under different environmental conditions, and has a wide application prospect. This system only has two states: start and stop. Preset starting value, in the stop state, the system counts overflow relay suction drive load automatically start; in the start process, when the system counts reach the preset value, the relay contacts are disconnected and the load stops working.
The maximum working time of load start or stop is 4 hours, and the working time of security load is between 1 and 3 hours [4]. When the system starts or stops, the maximum time is usually 4 hours. Two knob switches are set with different adjustable timing and running time. Timing time is 0-4 hours and running time is 0-60 minutes per cycle. Users can set different state time according to their needs. The external input current is 20A and the voltage is 380V. The control circuit converts the external high voltage, current and power into low voltage 12A, low current 2A and driving load power of 5.5-10kW three-phase motor.
It is safe, thermostatic element simple and convenient. It is widely used in high voltage control.