Application of multivibrator based on 555 timer

1. Introduction

In a bustling city, when night falls, colorful lights light up, lighting up this dark world and adding a little interest to people's lives, and running lights are one of the roles. With the continuous development of technology, the circuits for controlling colored lights are constantly updated. Here we mainly introduce the running light control circuit composed of 555 timer.

2.555 Timer

2.1 The internal structure of the 555 timer (as shown in Figure 1):

(1) Voltage divider

①When pin 5 is suspended.

②When pin 5 is connected to external control voltage.

(2) Voltage comparator

The voltage comparators C1 and C2 are two ideal operational amplifiers with the same structure. The comparator has two input terminals, 1 and 0 represent the voltage applied to the corresponding input terminals, and represents the comparison result of the comparator (1 represents a high level, 0 represents a low level).

(3) Basic RS flip-flop

(4) Discharge transistor

V1 is a discharge transistor with an open collector. When , V1 is turned on; when , V1 is turned off.

2.2 555 timer function table (as shown in Figure 2) The 555 timer function table basically explains the functions of the 555 timer.

2.3 Multivibrator composed of 555 timer

The multivibrator composed of 555 timer is shown in Figure 3. RA, RB and C are external timing components. In the circuit, the high level trigger terminal (pin 6) and the low level trigger terminal (pin 2) are connected in parallel and then connected to the connection between RB and C, and the discharge terminal (pin 7) is connected to the connection between RA and RB. Since the capacitor C has no time to charge at the moment of power on, the voltage across the capacitor is low, less than (1/3) Vcc, so the high level trigger terminal and the low level trigger terminal are both low, the output is high, and the discharge tube V1 is cut off. At this time, the power supply charges the capacitor C through RA and RB, causing the voltage to rise exponentially. When it rises to (2/3)Vcc, the output is low, and the discharge tube V1 is turned on. The voltage rises from (1/3)Vcc to (2/3)Vcc. Since the discharge tube V1 is turned on, the capacitor C discharges through the resistor RB and the discharge tube, and the circuit enters the second quasi-stable state. The duration of its maintenance is related to the discharge time of the capacitor. As C discharges, it decreases. When it decreases to (1/3)Vcc, the output is high, the discharge tube V1 is turned off, and Vcc charges the capacitor C again, and the circuit flips to the first quasi-stable state.

3.CD4017 logic function

The CD4017 chip is a decade counter/timing decoder with extremely wide applications. Figure 4 shows the pinout of the CD4017, which is packaged in a 16-pin dual in-line plastic package.

The CL terminal of CD4017 counts on the rising edge of the input clock pulse. When the clock enable terminal EN is "0", the clock pulse input is allowed, and when it is "1", the clock pulse input is prohibited.

Under the action of the input clock pulse, the 10 decoded outputs Y0~Y9 are high level in turn. R is the reset terminal. When R=1, the counter is cleared, Y0 is "1", and the rest Y1~Y9 are "0". CO is the carry output terminal. After CD4017 records 10 numbers, CO outputs a positive carry pulse.

4. Circuit Implementation

4.1 Schematic diagram of the running light circuit (as shown in Figure 5):

4.2 Principle of the Flowing Light Circuit

The circuit is composed of a multivibrator composed of a time base integrated circuit NE555 and a decimal counting/decoding circuit of CD4017. After the power is turned on, the capacitor C1 is charged through R1 and R2, so that it gradually increases. At that time, the output of pin 3 (Q end) is high. When it rises to more than, the output of pin 3 is still high. When it continues to rise to slightly more than, the state of the RS trigger is reversed, and the output of pin 3 is low. At the same time, C1 is discharged to the ground through R2 and the discharge tube VT turned on in pin 7, and it drops rapidly. When it drops to slightly less than, the trigger state is reversed again, and the output of pin 3 becomes high. At the same time, the discharge tube VT turned on in pin 7 is cut off, and the capacitor C1 is charged again, and its potential rises again, and the cycle continues. According to, it can be seen that by changing the resistance value of potentiometer R2, the oscillation period of the oscillator can be changed, thereby changing the conversion time of the high and low levels of the output of pin 3, and then changing the speed of the running water lamp.

For CD4017, its pin 14 receives the output pulse from the pin 3 of the 555 timer. When the output level state of the pin 3 of the 555 timer flips, the pin 14 receives the change of high and low levels, triggering the ten output pins to alternately output high levels, lighting up the LED lights of the corresponding pins. As time goes by, the ten LED lights are lit one after another, forming a running light.

5. Conclusion

The 555 timer is an integrated device that combines analog and digital functions. Through the simple combination of peripheral components, many basic and practical circuits can be formed. The most basic and most widely used are monostable triggers, Schmitt triggers, and multivibrators. Based on these basic circuits, they can be combined with other circuits to form a variety of practical electronic circuits to achieve a variety of functions, such as timers, frequency dividers, pulse signal generators, toy game circuits, sound alarm circuits, automatic control circuits, power switching circuits, frequency conversion circuit element parameters and circuit detection circuits. I believe that in the near future, the 555 timer will be used in more fields, adding more fresh elements to our lives and making our lives more colorful.