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How 555 Timers Work? Circuit Modes Analysis

Author: Apogeeweb Date: 21 May 2021  345

what is a 555 timer

Introduction

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

Catalog

Introduction

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.
Summery:
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

SIREN

🔺 SIREN

Flashing Lights

🔺 Flashing Lights

Knight Rider Circuit

🔺 Knight Rider Circuit

Laser Ray

🔺 Laser Ray

Latch

🔺 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:

Manufacturer

Model

Remarks

Custom Silicon Solutions

CSS555/CSS555C

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

CEMI

ULY7855

*

ECG Semiconductors

ECG955M

Timer Single Rc-type Oscillator

Exar

XR-555

Highly stable controller

Fairchild

NE555/KA555

Time-delay or mono-stable

Harris

HA555

*

IK Semicon

ILC555

CMOS chip, minimum working voltage 2V

Texas Instruments

SE555/NE555

*

Renesas

ICM7555

CMOS RC timers

Lithic Systems

LC555

Available in Industry's Smallest 8-Bump DSBGA

Maxim

ICM7555

CMOS RC timers, minimum working voltage 2V

Motorola

MC1455/MC1555

Monolithic timer

National Semiconductor

LM1455/LM555/LM555C

*

National Semiconductor

LMC555

CMOS chip, minimum working voltage 1.5V

NTE Sylvania

NTE955M

Accurate time delays

Raytheon

RM555/RC555

*

RCA

CA555/CA555C

*

STMicroelectronics

NE555N/ K3T647

*

Texas Instruments

SN52555/SN72555

*

Texas Instruments

TLC555

CMOS chip, minimum working voltage 2V

Zetex

ZSCT1555

Precision single cell timer

NXP

ICM7555

CMOS

Hitachi Semiconductor

HA17555

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

Ordering & Quality

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pinglun 3 comments

    • pingluntus
    • May on 2021/6/8 10:17:22

    Hello, I want a circuit powered by a 9 volt battery that when a momentary switch is pressed will light an led for 30minutes. Is a 555 capable of the 30minutes?

      • pingluntu
      • author on 2021/6/11 10:11:40
        author

      Re:

      Yes it can. In monostable mode if you use a capacitor and resistor of large enough value. The values that will give you 30 minutes high output should be about 100uf and 16Kohm. 😶

    • pingluntus
    • Mikeynia on 2021/6/15 11:08:42

    I'm having problems with the 555 timer model. I've used these devices for many uears, but can't get the 555 model to work in a simulation. The surrounding circuitry has been verifed OK (and is pretty standard anyway), but the trigger input insists on clamping at 1 volt, even when tied to the + rail by 100 ohms, and (predictably) the output won't go high when the trigger input is switched to 0 volts, presumably because that input hasn't come from a suitably positive voltage. It really looks like there is a fault with the model.

      • pingluntu
      • author on 2021/6/16 16:34:24
        author

      Re:

      Not sure quite what the problem is you're seeing. 🤔
      I think your problem is that you're running the 555 with only a 1V supply.
      Also, unless you intend that the Counter output be driven in phase with the Input then you might need to put a C in series between Q1 collector and the 555 Trigger input and a pullup resistor from the Trigger input to the 555 VCC supply to differentiate the pulses at Q1 collector and hence only trigger the 555 on the falling edges (i.e. the rising edge of the Input signal).
      BTW, if you run the 555 at anything but a 5V supply, you'll then see the behaviour I pointed out in the bug report. 

    • pingluntus
    • Jimmylee on 2021/6/18 10:55:25

    I am trying to teach beginners to electronics the concept of bread-boarding and how to use the 555 timer. Now I consider myself a pretty knowledgable person in electronics and this is why I'm kind of annoyed.

      • pingluntu
      • author on 2021/6/19 16:08:08
        author

      Re:

      Have you tried swapping parts out or even swapping out the breadboard? I know from my school days that students often put bad parts back in the parts drawers. It shouldn't take long to completely start over with new parts.

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