Understanding 555 Timer


As 555 timer is robust, stable and most commonly used IC in the area of electronics and also use in many electronic circuits. IC 555 is a square wave generator and its duty cycle range from 50% to 100%. The time delay in the circuit is provided by an oscillator. 555 timer IC got its name from the three 5 kilo-ohm resistor attached as a pattern of voltage divider as shown in the below figure. While in the full circuit 555 timer IC consists of many other components via 16 resistors, 20 transistors and 2 diodes also included flip-flop.

Related Post: Working of 555Timer IC Testing Circuit

Before knowing more about IC first have a glance at the pin diagram.

555 Timer Pin Diagram

Pin Diagram of 555 Timer
Image Source: ElectronicsClub.info/555timer.htm

IC 555 timer is a renowned device in the field of electronics, although the majority of the people does not have any information regarding the internal circuitry as well as about the pins present on the IC.  So here I have tried to explain the working of this IC so that you can build many projects around it and do your experiments.

The internal circuitry of the IC in the simplest form is shown below:

Internal Arrangement of 555 Timer

555 Timer – Internal Arrangement

Important Note: The 555 can be used with a supply voltage (Vs) in the range 4.5 to 15V (18V absolute maximum).

Functional Parts of 555 Timer IC:

555 Timer IC has basically three functional parts. They are as follows:

  1. Comparator: It is used to compare two voltages at the input level which is inverting (-) one and non inverting (+). If the voltage at the non-inverting is higher than other than the output is high. For ideal comparator  input resistance is infinite.
  2. Voltage Divider: As the input resistance is infinite in the comparator so the voltage among all three resistors is being divided equally. Value across each resistor is Vin/3.
  3. Flip/Flop: Digital electronic device with memory. If the input is high while low at R then the output at Q is high. It means when S is high Q output is high and if R is high Q output is low.

To get more knowledge about 555 timer, one should need to get aware about the pins. Description of the 555 Timer pins is given below.

Pin Description of 555 Timer:

  1. Trigger Input: If the input <1/3Vs than at that condition output is high. It is useful in examining  the timing capacitor discharging in a stable circuit.
  2. Threshold Input: This pin is attached to the first comparator at the non-inverting input terminal. If the threshold voltage is higher than (2/3) Vin than the output is high at the comparator thus reset the output from high to low in the flip flop.
  3. Reset Input: As based on the internal condition of the flip-flop this pin reset the output of the flip-flop. To get rid  of any noise interference an active low pin is attached to high state till any reset operator is required. So for most of the time it is attached to the supply voltage as in the figure.
  4. Control Input: An external voltage applied to this terminal changes the threshold as well as trigger voltage. Thus by imposing a voltage on this pin or by connecting a pot between this pin and ground, the pulse width of the output wave can be varied. when not used, the control pin should be bypassed to ground with a 0.01uF capacitor to prevent any noise problem.
  5. Discharge Pin: When the timer output is low this pin is connected to 0 volts. It is used to discharge the timing capacitor in astable and monostable mode.
  6. Ground: The reference point or ground point of IC555 is connected to the ground terminal of the dc voltage source, Voltages of the entire circuit are measured with respect to this terminal.
  7. Output: The output of timer and the load is connected to this pin.
  8. Supply: This voltage must be a pure ripple free DC voltage which is obtained by connecting a positive regulated power supply.

Modes of 555 Timer:

The 555 Timer has three operating modes and the details are explained below.

1. Monostable Mode: In this mode, the 555 functions as a “one-shot” pulse generator. Applications comprise many things viz timers, missing pulse detection also included bounce free switches, touch switches as well as frequency divider, capacitance measurement and pulse-width modulation (PWM) and many more.

2. Astable Mode: In this mode, the 555 work as a free running mode. The output of this astable multivibrator toggle between low and high continuously there by generating a train of pulse, that is why it is known as pulse generator.   They are used as an inverter and also used in many of the internal part of the radio. Selecting a thermistor as a timing resistor allows the use of the 555 in a temperature sensor.

Block diagram of 555 timer in Astable mode is shown below.

Block Diagram of 555 Timer in Astable Mode

Block Diagram of 555 Timer in Astable Mode

An astable circuit produces a ‘square wave’ and is shown below.

555 Astable Output - A Square Wave3. Bistable Mode or Schmitt Trigger: If there is no capacitor as well as there is no DIS pin used, then 555 timer can work as a flip flop. Its uses Include bounce free latched switches.

555 Timer as Astable Multivibrator:

As shown in the diagram below timer 555 works in astable mode along with the internal circuit described in the block. There are three resistors named R inside it and all have equal values. These form a voltage divider having reference voltages of 1/3 and 2/3 of Vcc (the power supply). The logic state of the flip flop is controlled by the reference voltage which is given to one of the inputs of both two comparators.

Pin 2 of the 555 is the trigger input. If the voltage at pin 2 is<1/3 of Vcc, the flip flop switch to a low state of the output of the lower comparator. The output stage has an inverting action. In other words, output at 555 high when flip-flop output is low.

Now imagine if the power supply is first connected to the astable circuit. Timing capacitor is discharged at the starting. The output in 555 is high and voltage is 0V at pin2. With the help of resistor R1 and R2 and capacitor C starts charging. Note that C is also connected to pin 6, which is the threshold input of 555 timer.

555 Timer in Astable Multivibrator Mode - Circuit Diagram

555 Timer in Astable Multivibrator Mode – Circuit Diagram

The output of the lower comparator is reached at the next level when the voltage across C is more than 1/3of Vcc. The output of flip flop doesn’t change and remain at low state. The inputs to the second comparator are the voltage at pin 6, the threshold input, and 2/3 VCC from the internal voltage divider. The output of the second comparator reaches for next level when the voltage at C moves more than 2/3 of Vcc. Now the flip flop switch from low to high state and output of 555 also switch from high to low.

Flip-flop is attached with NPN transistor internally and pin 7which is discharge pin of the IC is connected to the collector of the transistor. The transistor gets into On mode when the flip-flop is in high state giving a low resistance path to 0V from the discharging pin. The voltage decreases as the capacitor C starts for vacant via a resistor R2.

Note capacitor discharges only via R2 while charging via R1 and R2 both. The lower comparator moves to the next level when the voltage across C <1/3 of Vcc, state of flip-flop switches and 555 reaches to high.

The graph below shows how the voltage across the timing capacitor, Vc changes with the output voltage of the 555, Vout.

Voltage Across Timing Capacitor

Voltage Across Timing Capacitor

In short, you can see the above phenomenon in the following pictures.

555 Timer as Astable Multivibrator - Figure 1

555 Timer as Astable Multivibrator – Figure 1

555 Timer as Astable Multivibrator - Figure 2

555 Timer as Astable Multivibrator – Figure 2

555 Timer as Astable Multivibrator - Figure 3

555 Timer as Astable Multivibrator – Figure 3

This cycle repeats continuously.

Above phenomena can be represented below:

555 Timer as Astable Multivibrator - Graphical Representation

555 Timer as Astable Multivibrator – Graphical Representation

  • Time for which Output is High     = 0.693 (R1+ R2) C
  • Time for which Output is Low Ts = 0.693 R2 C
  • Period of Output T = Tm + Ts       = 0.693(R1+ 2R2)C
  • PRF of Output                                     = 1.44 / (R1+ 2R2)C
  • Duty Cycle Tm/T                              = (R1 + R2)/(R1 + 2R2)‏

Calculation of Tm, Ts and PRFMathematical Calculation of Tm, Ts and PRF

Selection  R1, R2 and C1 for different ferquency range are as follow:

R1 and R2 should be in the range 1k to 1M . It is best to Choose C1 first (because capacitors are available in just a few values)  as per the frequency range  from the following table.

Choose R2 to give the frequency (f) you require.
R2 = 0.7 /(f × C1)‏
Choose R1 to be about a tenth of R2 (1k min.)

Frequency Range Values for 555 Timer in Astable Mode

555 Timer IC as Monostable Multivibrator:

Circuit Diagram:

555 Timer as Monostable Multivibrator - Circuit Diagram

555 Timer as Monostable Multivibrator Circuit Diagram – ElectronicsHub.Org

As you can see, the trigger input is kept at HIGH by the 10 kΩ pull up resistor and is pulsed LOW when the trigger switch is pressed. The circuit is triggered by a falling edge, that is, by a sudden transition from HIGH to LOW.

The trigger pulse, produced by pressing the button, must be of shorter duration than the intended output pulse.

The period, t, of the output pulse can be calculated from the design equation:


Working of 555 Timer in Monostable mode:

555 timer in Monostable mode - case 1

555 timer in Monostable mode – case 1

555 timer in Monostable mode - case 2

555 timer in Monostable mode – case 2

555 timer in Monostable mode - case 3

555 timer in Monostable mode – case 3


Above phenomenon can be represented as below:

555 timer in Monostable mode - Graphical Presentation

555 timer in Monostable mode – Graphical Presentation

2 /3 Vs  = Vs ( 1 – e -T/RC )               or   T= RC ln3  = 1.1 RC

Brief Description:

  • Monostable  circuit  provides  a single pulse  when   triggered.
  • Monostable  means  one  stable that is  ‘output low’.
  • The ‘output high’  on triggered   is  a temporary  state.
  • The duration of the pulse is called the time period (T) and this is determined by resistor R and capacitor C:

Time period, T = 1.1 × R × C

T = time period in seconds (s)‏
R = resistance in ohms
C = Capacitance in farads (F)‏

Choosing of R and C:

  • Choose C first (there are relatively few values available).
  • Choose R to give the time period required.
  • R should be in the range 1k to 1M.

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  1. kennedy sunday says:

    how can a 555 timer be use to build an alarm system

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