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How 555 Timers Work? Circuit Modes Analysis

Author: Apogeeweb
Date: 21 May 2021
what is a 555 timer


For people who have been in touch with digital circuits or analog circuits, the 555 IC is definitely classic work. With its low cost and reliable performance, it is widely used in various electrical appliances, including instruments and meters, household appliances, electric toys, and automatic control. The 555 timer only needs a few external resistors and capacitors to realize pulse generation and conversion circuits, such as multiple oscillators, monostable triggers and schmitt triggers. So how does it work in the circuit? What the role of its circuit? Here gives several typical 555 circuit examples for specific analysis.

555 Timers Circuit Learning



Ⅰ Basic 555 Timer Circuit Analysis

Ⅱ 555 Multivibrator Circuit Analysis

Ⅲ 555 Timer Monostable Flip Flop Circuit Analysis

Ⅳ Classic 555 Timer Circuits Diagrams

Ⅴ 555 Timer IC Modes

Ⅰ Basic 555 Timer Circuit Analysis

555 Means What?
555 timer is a convenient and powerful IC, which is widely used in signal generation, conversion, control and detection. The origin of this name, because it is divided by three 5KΩ resistors. The 555 timer is a simple integrated circuit that can be used to make many different electronic circuits. With the following circuits analysis you will know how 555 IC works.

basic 555 timer circuit

Figure 1. Basic 555 Timer Circuit

✔️ Circuit Analysis
R is not the reset terminal, when set to 0, Q is 0, Q is 1, Uo outputs 0, and Q is 1 added to the base of the transistor T, the transistor is in the conducting state.
① When R=0, Q=1, uo=0, T is saturated and turned on.
② When R=1 (there is no reset function at this time):
UTH>2VCC/3, UTR>VCC/3, C1=0, C2=1, Q=1 or Q=0, uo=0, T is saturated and turned on. (Analysis: C1's positive input terminal is 2VCC/3, C1's negative input UTH terminal is greater than the positive input terminal, working in saturation, and output 0. C2's negative input terminal is 1VCC/3, which is smaller than the positive input Terminal UTH, and outputs 1. There is a horizontal line above RD and SD, which means low level, meaning is Reset. C1 outputs 0, RD is valid, then Q is 0, not 1, Uo outputs 0, and Qis not acting on the base of the triode.)
③ When R=1, UTH<2VCC/3, UTR>VCC/3, C1=1, C2=1, Q and Q remain unchanged, uo and T remain unchanged. (Analysis is the same as above)
④ When R=1, UTH<2VCC/3, UTR<VCC/3, C1=1, C2=0, Q=0, Q=1, uo=1, T is cut off. (Analysis is the same as above) 
Learn how the inputs interact with the supply voltage to trigger and reset the output high and low. Find out which pins can be used to adjust the threshold at which that change happens.

Ⅱ 555 Multivibrator Circuit Analysis

555 Multivibrator Circuit Analysis

Figure 2. 555 Multivibrator Circuit Analysis


555 Multivibrator Circuit example

Figure 3. 555 Multivibrator Circuit Example

✔️ Circuit Analysis 

First, the power supply VCC charges the capacitor C through R1 and R2, and the voltage of the capacitor must be relatively small, less than 1VCC/3. Similarly, the positive terminal of C1 is 2VCC/3, the negative terminal of C2 is 1VCC/3, and the TH and TR terminals are connected At the same time, it is less than 1VCC/3 at the beginning. At this time, C1 outputs 1, C2 outputs 0, and the set terminal is valid (with detailed confirmation): Q is 1, Qis not 0, and uo is 1, the transistor is cut off, and outputs high level. At this time, the power supply is still charging the capacitor. When the TH and TR terminals are connected together, the voltage is less than 2VCC/3 and greater than 1VCC/3; C1 outputs 1, C2 outputs 1, the transistor is cut off, and uo is 1. When the capacitor is greater than 2VCC/3, C1 outputs 0 and C2 outputs 1. At this time, Q is 0, Q is not 1, uo is 0, the output is low, and the transistor is turned on. The capacitor will be discharged through pin 7. After this, the voltage at the point where TH and TR connected will gradually decrease, less than 2VCC/3 and greater than 1VCC/3, and then it will be less than 1VCC/3, to form a harmonic oscillator.
The pulse width tp1 of the first transient state, that is, the time required for uc to rise from VCC/3 charging to 2VCC/3 (charged through two resistors):

pulse width tp1 formula

The second transient state pulse width tp2, that is, the time required for uc to discharge from 2VCC/3 to VCC/3:

pulse width tp2 formula

oscillation period T

Duty cycle: the time that the high level occupies the entire cycle.
duty cycle, it can be seen that its duty cycle is always greater than 50%.

Examples 1
Circuit with Adjustable Duty Cycle (add an adjustable resistor)

Circuit with Adjustable Duty Cycle (add an adjustable resistor)

Figure 4. Circuit with Adjustable Duty Cycle (add an adjustable resistor)

It can be calculated:
Where T1=0.7R1C (T1 is charging time), T2=0.7R2C (T2 is discharging time)
Total time T=T1+T2=0.7(R1+R2)C
So R1, R2, and C are determined, and the period T is also determined.

Duty Cycle Calculation

Duty Cycle Equations


Example 2
Circuit with Adjustable Duty Cycle (1KHz)

Circuit with Adjustable Duty Cycle (1KHz)

Figure 5. Circuit with Adjustable Duty Cycle (1KHz)

✔️ Circuit Analysis

T = 0.7(R1+R2)C, f = 1/T, the duty cycle circuit only needs to adjust the resistance value.


Ⅲ 555 Timer Monostable Flip Flop Circuit Analysis

Working Characteristics
① It has two different working states: steady state and transient state.
② Under the action of an external trigger pulse, it can switch from the steady state to the transient state. After the transient state is maintained for a period of time, the circuit can automatically return to the steady state.
③ The transient state cannot be maintained for a long time, and the duration of its sustaining time depends on the parameters of the circuit itself and has nothing to do with the trigger pulse.

555 timer transient state


So what is the principle of a monostable circuit?

555 Timer Monostable Circuit Analysis

Figure 6. 555 Timer Monostable Circuit Analysis


555 Timer Monostable Circuit Example

Figure 7. 555 Timer Monostable Circuit Example

✔️ Circuit Analysis

First, the TR terminal is at a high level ui, which must be greater than 1VCC/3. At this time, C2 outputs 1, and the power supply charges capacitor C through R. The charging voltage is less than 1VCC/3 (TH), CO voltage is equal to 2VCC/3, C1 outputs 1, and it is in the holding state at this time. Assuming that the non-reset terminal of R is reset before power on, the output of uo is 0, and then the previous state is still maintained and the output is 0 at this time. Qis 1, the transistor is turned on, the capacitor is discharged through pin 7, and uc is zero level. At a certain moment, ui is low, C1 still outputs 1, C2 outputs 0, Q is 1, Q is 0, uo outputs 1 (high level), and the transistor has been in the cut-off state. At this time, VCC can charge the capacitor (uc is getting larger). When uc is between 1VCC/3~2VCC/3, assuming that the TR terminal returns to the original state (high level), C1 outputs 1 , C2 outputs 1, at this time uo keeps in original state, it is still 1, and the transistor is in the cut-off state. When uc is greater than 2VCC/3, C2 is still 1, C1 output is 0, Q is 0, Q is 1, and uo is 0, the transistor is turned on and in a discharging state, at this time, uc is getting smaller and smaller.
1. As long as a low-level trigger signal is given, the temporary stable stay time is the charging time of voltage 0V~ 2Ucc/3 (the time represented by tp).
2. Charging time Tp=1.1RC
3. It can be used as a timing circuit, and the time can be determined by RC.

Example: Timing Circuit Design (1s delay time)

Figure 8. 555 Timer Delay Circuit Example

Ⅳ Classic 555 Timer Circuits Diagrams

There are A LOT of projects out there using the 555 in various ways and it’s easy to find schematics to make a project that has already been proven. Here lists some typical projects using 555 timer in circuits. Let’s have a look. 

Car Tachometer

🔺 Car Tachometer



Flashing Lights

🔺 Flashing Lights

Knight Rider Circuit

🔺 Knight Rider Circuit

Laser Ray

🔺 Laser Ray


🔺 Latch

LED Dimmer

🔺 LED Dimmer

555 Amplifier

🔺 555 Amplifier

Light Detector

🔺 Light Detector

Machine Gun

🔺 Machine Gun

Metal Detector

🔺 Metal Detector

Motor PWM

🔺 Motor PWM

Music Box

🔺 Music Box

Zener Diode Tester

🔺 Zener Diode Tester


Ⅴ 555 Timer IC Modes

555 timer will use different models in different circuits to meet circuit requirements. Therefore, it has many derivative models produced by different companies with different pin functions, and uses CMOS design. What;s more, some chips include several integrated 555 timers. Some common models of the 555 chip family are as follows:




Custom Silicon Solutions


CMOS chip, minimum working voltage 1.2V, IDD < 5µA




ECG Semiconductors


Timer Single Rc-type Oscillator



Highly stable controller



Time-delay or mono-stable




IK Semicon


CMOS chip, minimum working voltage 2V

Texas Instruments





CMOS RC timers

Lithic Systems


Available in Industry's Smallest 8-Bump DSBGA



CMOS RC timers, minimum working voltage 2V



Monolithic timer

National Semiconductor



National Semiconductor


CMOS chip, minimum working voltage 1.5V

NTE Sylvania


Accurate time delays








NE555N/ K3T647


Texas Instruments



Texas Instruments


CMOS chip, minimum working voltage 2V



Precision single cell timer




Hitachi Semiconductor


Accurate time delays or oscillations


Frequently Asked Questions about 555 Timer Circuit

1. What does a 555 timer do in a circuit?
The 555 timer IC is a very cheap, popular and useful precision timing device which can act as either a simple timer to generate single pulses or long time delays, or as a relaxation oscillator producing a string of stabilised waveforms of varying duty cycles from 50 to 100%.


2. How much voltage can a 555 timer take?
The standard TTL 555 can operate from a supply voltage between 4.5 volts and 18 volts, with its output voltage approximately 2 volts lower than its supply voltage VCC. The 555 can source or sink a maximum output current of 200mA, (but it may get hot at this level), so the circuit variations are unlimited.


3. What are the modes of operation of a timer?
The timer registers can be used in two modes. These modes areTimer mode and the Counter mode. The only difference between these two modes is the source for incrementing the timer registers.


4. What are the basic operation modes of the 555 timer?
The operating modes of a 555 timer are astable, bistable and monostable. Each mode of operation signifies with a circuit diagram and its output.


5. What is the maximum frequency of a 555 timer?
according to the website, the 555 timer has a maximum frequency of 2MHz.

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