Design of running water lamp based on digital integrated circuit

Today is an era of endless new technologies, especially in the field of electronic automation and intelligent control, which adds color to people's lives and brings convenience and comfort. For example, colorful advertising running lights decorate the doors of many entertainment venues, barber shops, hotels, restaurants, and companies every night. There are many ways to make running lights, including traditional discrete components, control systems composed of digital logic circuits, and single-chip intelligent control systems. For the first method, although the circuit is simple, the performance is unstable and the working reliability is poor; and the third method is difficult for beginners of electronic technology. Therefore, the following is an introduction to the design method of running lights controlled by digital integrated circuits based on many years of teaching experience.

1 Design Task Requirements
Use edge JK trigger (74LS112), D trigger (74LS74) and 3-8 line decoder (74LS138) to form a running light circuit. The system is required to have a total of 8 lights, and the effect is always 7 bright and 1 dark, and the 1 dark light moves down or up in a cycle.

2 Design ideas and circuit composition block diagram
First, use the three triggers in 74LS112 and 74LS74 to form an asynchronous octal addition or subtraction counter; then connect the output terminals Q2Q1Q0 to the address code input terminals A2A1A0 of 74LS138 (3-8 decoder) respectively, so that the decoder can decode successively. The circuit composition block diagram is shown in Figure 1.

3 Circuit working principle
3.1
Circuit principle The circuit principle diagram is shown in Figure 2. The system is an 8-way water light controller. Its control form is 7 bright and 1 dark, and this 1 dark always moves from top to bottom, and repeats this cycle to form a flow effect. The circuit consists of five parts: power supply, clock pulse generation circuit, adder counter, decoding and LED display system.

3.2 Functions of each circuit
3.2.1 Power
supply circuit In the power supply circuit, the 220 V mains voltage is stepped down to 12 V AC through a transformer, then rectified by a bridge rectifier circuit composed of VD1~VD4 and filtered by Ct, and then stabilized by 7805 to 5 V DC voltage as the power supply for the clock pulse generation circuit, counter and decoding display circuit.
3.2.2 Clock pulse generation
circuit The clock pulse generation circuit mainly provides clock pulses for the counter. It is a multivibrator composed of a 555 timer, as shown in Figure 2.
Among them, R1, R2 and C3 are external timing components. When the power is turned on, the capacitor C3 has no time to charge, resulting in a low voltage across the capacitor C3, that is, the potential of the 2nd and 6th pins of the 555 is less than 1/3 of the power supply voltage, so the output of the 3rd pin of the 555 is high, and the discharge tube VT is cut off. At this time, the power supply charges the capacitor C3 through R1 and R2, so that the potential of the 2nd and 6th pins rises exponentially. When the potential of the 2nd and 6th pins rises to 2/3 of the power supply voltage, the output of the 3rd pin turns to a low level, and the discharge tube VT is turned on. At this time, the charging of the capacitor C3 is completed and it begins to discharge through R2 and the discharge tube. As the discharge of the capacitor C2 continues, the voltage across it gradually decreases. When it drops to 1/3 of the power supply voltage, the output becomes a high level again, the discharge tube VT is cut off, and the power supply charges the capacitor C3 again. In this way, after the continuous charging and discharging of the capacitor C3, a series of rectangular pulses can be obtained at the output end. From the above analysis, it can be seen that the charging process of the capacitor is essentially the period of time when the voltage across the capacitor rises from 1/3Vcc to 2/3Vcc, so the charging time is: Tcharging = 0.7(R1+R2)C3, and the discharge of the capacitor refers to the period of time when the voltage across the capacitor drops from 2/3Vcc to 1/3Vcc, and the discharge time is: Tdischarging = 0.7R2C3. It can be seen that once the circuit starts to oscillate, the voltage across the capacitor always changes between (1/3~2/3)Vcc, and its oscillation period is: T=Tcharge+Tdischarge=0.7(R1+2R2)

3.2.3 Adder counter circuit
Figure 3 shows an asynchronous 3-bit binary adder counter composed of two falling edge JK flip-flops and one rising edge D flip-flop. Each flip-flop is connected to the counting state, which is implemented by 74LS112 dual JK flip-flops and 74LS74 dual D flip-flops. Assume that before counting, a negative pulse is given to the reset terminal RD to make each flip-flop in the 0 state. Starting from the initial state 000, each time a counting pulse is input, the state of the counter increases in binary (adds 1). After the 8th counting pulse is input, the counter returns to the 000 state. The counting rules are shown in Table 1.

3.2.4 Decoding and display circuit
As shown in Figure 2, the decoding and display circuit is composed of a 3-8 decoder 74LS138 and 8-way light-emitting diodes. Before starting to count, first press the reset button S to make the counter output 000, which is decoded into a decimal number "0" by the 3-8 decoder, so the corresponding output is low level, LED0 is not lit, and the other 7 output terminals are all high level, so LED1 to LED7 are all lit. Then, when the first CP is sent to the counter, the counter output is 001, which is decoded into a decimal number "1" by the 3-8 decoder, so the corresponding output is low level, LED1 is not lit, and the other 7 output terminals are all high level, so LED0, LED2 to LED7 are all lit. If the second and third pulses are sent, LED2 and LED3 are not lit in turn, and when the seventh pulse is sent, LED 7 is not lit. This completes the movement of the dark from top to bottom, and when the eighth pulse comes again, the next cycle begins. Since the clock generation cycle of this circuit is 0.1 s, the running light can complete 5 cycles within 4 s. The effect of repeated cycles is like the flow of water. Therefore, it is called "advertising running light".

4 Component selection
Since the power consumption of this circuit is relatively low, the power transformer uses a small transformer with a secondary voltage of 12 V and 3 to 5 W. VD1 to VD4 all use 1N4007 silicon rectifier diodes. LED0 to LED7 use φ5 mm high-brightness red light-emitting diodes. C1 can use an aluminum electrolytic capacitor with a withstand voltage of 25 V, C2 and C3 can use aluminum electrolytic capacitors with a withstand voltage of 10 V, and C4 is a disc capacitor. 5.1 kΩ, 220 Ω resistors use 1/8 W metal film or carbon film resistors. The reset switch S uses a 6×6 touch switch. The models of the remaining integrated circuits are selected according to the markings in the circuit diagram.

5 Function expansion
(1) If the counter in Figure 3 is designed as a subtraction counter, the 7 bright and 1 dark can be moved from bottom to top in a cycle. In addition, if the 8-way LEDs are connected in reverse, 7 are dark and 1 is bright.
(2) If several LEDs are connected in parallel in each way, a simple Chinese sentence or English sentence can be formed.

6 Conclusion
This design has the advantages of stable performance, long life, low cost, simple circuit, and strong practicality. It introduces the production method of common running lights in a relatively complete way. It is a classic, interesting and reproducible design process in the process of learning digital circuits. It can effectively improve the learning interest, hands-on ability and ability to solve practical problems of digital circuit beginners.