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Aug 20 2019

Constitution and Applications of 555 Timer

I Introduction

555 timer is a medium-sized integrated circuit that combines analog functions and logic functions. For there are three resistors of 5 kΩ in the integrated circuit, it is named as 555 timer. 555 timer has low cost and reliable performance. It only needs several external resistors and capacitors to form pulse generation and conversion circuits such as multivibrator, monostable trigger and Schmitt trigger. It is also widely used as a timer in instrument, apparatus, household appliances, electronic measurements and automatic control.

555 Timer


I Introduction

II Constitutions of 555 Timer Circuit

2.1 555 Timer Circuit Diagram

2.2 Function of Pins

III Working Principle of 555 Timer

IV Applications of 555 timer

4.1 Schmitt Trigger

4.2 Multivibrator

4.3 Monostable Trigger

V Conclusion

II Constitution of 555 Timer Circuit

2.1 555 Timer Circuit Diagram

The circuit of 555 timer consists of two voltage comparators, three equivalent series resistors, an RS flip-flop, a discharge tube. It has two reference voltages-- 1/3Vcc and 2/3Vcc 


Voltage comparators C1 and C2 are two ideal operational amplifiers of identical structure. The comparators has two input terminals, U+ and U- respectively indicates the voltage applied to the corresponding input terminal, and u c denotes the comparison result of the comparator. When U+>U-, u c= u h; when U+<U-, Uc=u l.

(2)Voltage Divider

Three resistors with a resistance of 5kΩ connected in series form a voltage divider, which provides a reference voltage for comparators C1 and C2. When the terminal CO is not used in operation, it is generally grounded through a capacitor of 0.01μF to bypass high-frequency interference.

(3)Basic RS Flip-Flop

It consists of two NAND gates, which is a direct-reset terminal that can be set to 0 from outside. When R=0, Q= 0; when S=1, Q=1.

(4)Transistor Switch (discharge tube)

The transistor TD constitutes a switch whose state is controlled by the terminals. When Q=1, the transistor is turned off; and when Q = 0, the transistor is turned on.

(5)Output Buffer

The output buffer is the inverter G3 connected to the output terminal, which is used to improve the load-carrying capacity of the timer and the effects of the isolated load on the timer.

555 Internal Circuit Diagram

555 Internal Circuit Diagram

2.2 Functions of Pins

Pin 1 Ground end. To connect the external power supply’s negative terminal VSS or ground, generally connecting the ground.

Pin 2 Trigger(TR)

Pin 3 Output terminal Vo(OUT)

Pin 4 Reset end. When the terminal is connected to a low level, the time-base circuit does not work. At this time, regardless of the level of TH, the output of the time-base circuit is “0”. The terminal should be connected to high level when it is not in use.

Pin 5 Voltage control terminal. If an external voltage is connected to this terminal, the reference voltage of the two internal comparators can be changed. When the terminal is not used, the terminal should be connected to a capacitor of 0.01μF in series to the ground to prevent interference.

Pin 6 Threshold(TH).

Pin 7 Discharge end(DIS). This terminal is connected to the electrode of the discharge tube set and is used as a discharge of the capacitor when the timer is used.

Pin 8 To connect the external power supply Vcc. The voltage range of the bipolar time-base circuit Vcc is 4.5 ~ 16V, and the voltage range of the CMOS type time-base circuit Vcc is 3 ~ 18V. 5V is generally used.

Pins Arrangement

 Pins Arrangement 

III Working Principle of 555 Timer

The function of the 555 timer is mainly determined by the two comparators. The output voltages of the two comparators control the state of the RS flip-flop and the discharge tube. When apply a voltage between the power supply and ground, and pin 5 is floating, the voltage at the in-phase input end of the voltage comparator C1 is 2/3Vcc , and the voltage at the reversed-phase input of comparator C2 is 1/3Vcc. If the voltage triggering the input end TR is less than 1/3Vcc , the output of comparator C2 is 0. In this case, the RS flip-flop can be set at 1 so that the output OUT = 1. If the voltage at the threshold input end TH is greater than 2/3Vcc and the voltage at the TR end is greater than1/3 Vcc , the output of C1 is 0, and the output of C2 is 1. Under this circumstance, the RS flip-flop can be set to 0 to make the output 0 level.

 1  When RD =0, Q=0, and the output voltage Uo=UOL is a low level, and TD is saturated and turned on;

 2  When RD =1, U TH> 2/3Vcc, and UTR> 1/3Vcc, the voltage comparator C1 outputs an active low level, C2 outputs a high level, and the terminal R of the basic RS flip-flop inputs an active level, so Q= 0, Uo=UOL, TD is saturatied and turned on;

 3  When RD =1, UTH< 2/3Vcc, UTR> 1/3Vcc, both C1 and C2 output high level (all are invalid levels), and the basic RS flip-flop remains unchanged, so U o and TD also Keep the original state;

 4  When RD =1, UTH< 2/3Vcc, UTR < 1/3Vcc, C1 outputs a high level, C2 outputs an active low level, and the terminal S of the basic RS flip-flop inputs an active level, so Q =1, Uo =UoL, TD is cut off;

 5  When RD =1, U TH> 2/3 Vcc, UTR < 1/3Vcc, both C1 and C2 output low level. For basic RS flip-flops, its input is restrained. At this time Q = 1, Uo= Uoh, and TD is cut off. But when U TH and UTR become invalid at the same time, its state can not be determined.








State of TD





Turn on


> 1/3Vcc



Turn on

< 2/3Vcc




Cut off

< 2/3Vcc

> 1/3Vcc




How a 555 Timer IC Works

 IV Application of 555 timer

4.1 Schmitt Trigger

What Is Schmitt Trigger and How It Works

 (1) Analysis

Schmitt triggers are one of the commonly used circuits in digital systems. They can transform a slowly changing pulse waveform into a rectangular pulse required by a digital circuit.

Schmitt circuit has two stable states, but its difference from the general trigger is that the conversion of the two stable states and the maintenance of the steady state require an external trigger signal, so its trigger method is level trigger.

The circuit diagram and waveform diagram of schmitt trigger are shown in Figure4, and the hysteresis voltage is 1/3Vcc. If the adjustable voltage Vco (1.5~5V) is externally connected to the voltage control terminal⑤, the backlash voltage can be changed, and the Schmitt trigger can conveniently convert the triangular wave into a square wave.

When the signal Ui<1/3Vcc is input, the basic RS flip-flop is set to 1, ie, image=0, Q=1, the output Uo is a high level; if Ui is increased, and 1/3Vcc<Ui<2/3Vcc, the circuit remains unchanged, the output Uo is still a high level; if the input signal increases to Ui≧2/3Vcc, and the RS flip-flop is set to 0, that is, Q=0, image=1, the output Uo is a low level; if Ui is increased again, as long as Ui≧2/3Vcc, the circuit maintains the state. If Ui falls, as long as 1/3Vcc<Ui<2/3Vcc, the circuit state remains unchanged; when Ui=1/3Vcc, the trigger will be set again and the circuit's output flips back to the high level.  

Schmitt Trigger Circuit & Work Waveform

Schmitt Trigger Circuit & Work Waveform

(2) Applications of Schmitt Trigger

 1  Waveform transformation: It can transform triangular waves and sinusoidal waves  into rectangular waves.

 2  Pulse wave shaping: In digital systems, rectangular pulses often have waveform distortion during transmission, and the rising edge and falling edge are not ideal. However, an ideal rectangular pulse can be obtained after being shaping by the Schmitt trigger.

 3  Pulse discrimination: When the irregularity pulse signals with different amplitude are added to the input end of the Schmitt trigger, the amplitude of the output pulse signal should be larger than the preset value.

 4.2 Multivibrator

Brief Concept of Multivibrator


Multivibrator is the basic circuit of 555-timer application. It means that the circuit has no steady state (that is, square wave generator) but only two transient steady states, whose function is to generate a rectangular wave signal with a certain frequency and amplitude. Its output state constantly switch between "1" and "0".

In the voltage-on state, since the voltage on the capacitor C cannot be changed abruptly, the 555 chip is in the set state,Uo=1, discharge tube TD is cutoff (Pin7 disconnected from ground), and Vcc charges capacitor C through R1, R2. When Uc rises, Uo=0 and TD is turned on, the voltage of capacitor’s terminal C passes R2 and the discharge tube TD to discharge the ground and then Uc drops. When Uc falls, Uo changes from 0 to 1, TD is turned off, and Vcc charges C again through R1 and R2. After the above process is repeated for several times, a continuous rectangular pulse will be generated at the output terminal Uo. Among all the electric components, R1, R2 and C are timing elements which determine the charging and discharging time of the circuit. Tl ≈ 0.7 (Rl + R2) C, T2 ≈ 0.7R2C.

 After the power is turned on, assuming it is a high level, T will be turned off and the capacitor C will be charged. The charging circuit is Vcc—R1—R2—C—ground, which rises exponentially. When it rises to 2/3Vcc (TH, the terminal level is greater than), the output flips to a low level. If V0 is a low level, T is turned on, C is discharged, and the discharge circuit is C-R2—T—ground, which decreases exponentially. When it falls to 1/3Vcc (TH, the terminal level is less than), the output is turned to a high level, and the discharge tube T is turned off, the capacitor is recharged again. Therefore this process repeat again and again and generates oscillation.

After analysis, the duty cycle of the output high-level time, the output low-level time, and the out put square of the oscillation period duty cycle are as follows. D=tPH/T=(R1+R2)/(R1+2R2). If R1>>R2, Uc is approximately sawtooth wave.

The multivibrator that constituted by the 555 is reliable in operation and convenient to adjust. It has been widely used in industrial control, timing, sound imitation and other related fields. but its oscillation frequency cannot be too high, generally not exceeding several hundred kilohertz. And its frequency stability is poor, and it’s susceptible to power-supply fluctuations and temperature variation.

Multivibrator Circuit& Work Waveform.png

Multivibrator Circuit & Work Waveform

4.3 Monostable Trigger

The 555 timer forms a monostable trigger for timing delay, shaping and some time switches. When the power is turned on, Vcc charges capacitor C through resistor R. When the voltage on the capacitor rises to VREF1, the comparator C1 inside the 555 timer outputs a high level. Since there is no trigger pulse at this time, the comparator C2 outputs low level, ie, RS=10. The basic RS flip-flop is reset, V0 outputs a low level, and the discharge tube T28 is turned on, quickly releasing the charge on the capacitor C , so that the comparator C1 outputs a low level. At this time, RS =00, the basic RS flip-flop is in the freeze mode, so the output is no longer changed and the circuit enters into a steady state.

If a trigger pulse is applied to the input terminal V1 of the monostable trigger, when the falling edge of the trigger pulse comes, since the electric potential of the 2 pin is lower than VREF2, the comparator C2 outputs a high level. At this time, RS=01, the basic RS flip-flop is Set. Q outputs a low level, the circuit begins to enter the transient steady state, outputting a high level V0 , and the discharge tube T28 is cut off. Vcc starts to charge capacitor C through resistor R. The voltage Vc on the capacitor rises exponentially. When Vc rises to VREF1, comparator C1 outputs a high level. Since the external trigger pulse has been undid, comparator C2 outputs a low level,that is, RS=10. The basic RS flip-flop is reset, ending the transient steady state, the circuit automatically returns to the initial steady state. V0 becomes a low level, and the discharge tube T28 is turned on. This circuit produces pulse with widths from a few microseconds to minutes, and its accuracy is 1℅. The input terminal of the control voltage (Pin 5) is grounded through through a capacitor of 0.01μF to prevent pulse interference.

555 Timer in Monostable mode


The 555 application circuit can use these three methods or one of the methods to form various practical electronic circuits, such as timers, frequency dividers, pulse signal generators, component parameters, and circuit detection circuit, toy game machine circuits. audio alarm circuit power exchange circuit, frequency conversion circuit and automatic control circuit, etc.

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pinglun 1 comment

    • pingluntus
    • Ranji on 2019/9/26 10:26:25

    EXCELLENT explanation! Clear, precise, includes an illustrative example of calculating relevant quantities, and schematics to make the application of 555 Timer clear to anyone with a basic understanding of electronics.

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