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18 March 2009
Author: Giorgos Lazaridis
555 Theory


The story began around 1971, when Signetics Corporation first introduced the SE555-NE555, "The IC Time Machine". By that time, it was the first and only integrated circuit with timer functions for commercial use. the original design has change many times since it's birth, but the pin-out is still compatible with the original. New devices with modern technologies and higher speed have been introduced since then, but the old good 555 is still a popular cheap and powerful solution for amateur and professional use.


The 555 came in two packages, the standard DIP package and the metal round package, also called the 'T' package, which was prety much the standard package the first decades that the 555 was first produced. The most popular packages today are the 555 and 556 DIP packages. The 'T' package is no longer produced and the 558 is about to be discontinued due to minimum usage. The pin-outs are shown bellow. The 556 is a Dual 555 and the 558 is a Quad 555:

Pin definitions
Ground: This pin is connected to the common (or negative pole of power supply). No need for further explanation

Vcc: AKA Powr Supply. On this pin the power supply for the operation of the 555 is connected. The power supply can be from 5V to 15V (4.5 - 16) and for some military designed packages could go up to 18V. There is not a big difference in timing operation of the 555 by changing the supply voltage, not more than 0.1% per volts which is considered to be stable enough. Actually, the only thing that significantly changes is the output supply capability in terms of voltage and current.

Output: This is the primary output of the 555. it is able to provide up to 1.7V lower than Vcc, about 3.3Volts for 5Vcc and 13.3 for 15Vcc. The output saturation levels depends on the Vcc. Typically, at Vcc=5V the low state is 0.25V at 5mA and could sink up to 200mA when Vcc=15V and an output low voltage of 2V is allowable.
The output is comes from Darlington transistors, providing high state output voltages with good noise margin, able to interface directly with logic circuits. Rise and fall times are typically as fast as 100nSec.

Trigger: This pin is the input to the lower comparator. It is used to control the latch that will set the output to high state. This triggering is done when the pin voltage is taken from above to below the one third (1/3) of the voltage level, that could be 1/2 of the voltage appeared at pin 5. A trigger could be accomplished from a slow ROC (rate of changing) waveform or even from pulses, due to the fact that the input is level sensitive. The allowable voltage range for triggering is between +V and ground. The current needed is typically 500nA.

Two precautions should be taken in account. First, the period of the trigger input signal should not remain lower than 1/3 of the Vcc for longer than the time cycle. In this case, the timer will re-trigger upon termination of the first output pulse. In monostable mode, the input trigger should be effectively shortened by differention. The minimum allowable pulse with for triggering is somewhat dependent upon pulse level, but in general, greater than 1μSec is reliable.
A second precaution that should be taken into account is the storage time in the lower comparator. This portion of the circuit can exhibit normal turn-off delays of several μSecs after triggering. The latch may still have a trigger input for this period of time after the trigger pulse. In this case, the minimum monostable output pulse width should be in the order of 10μSec to prevent possible re-triggering.

Control voltage: By this pin someone can gain access to the 2/3 of the Vcc on the voltage-divider point. That is the reference point of the upper comparator, and an indirect access to the lower comparator reference. When this pin is connected to an external voltage, the 555 operates in voltage-controlled. When in this mode, the voltage control ranges from 1V bellow the Vcc down to 2Volts above ground. Voltages outside those limits can be safely applied but with not a reliable operation.
This pin expands the uses of the chip. In monostable mode, When external power is connected, the timing of the device can be altered with no respect to the RC timing circuit, and the control voltage may vary from 45% to 90% of Vcc. In astable mode, applying voltage to that pin will make it act as a Frequency Modulator (FM).
This pin in basic wiring is not needed to be connected, instead a capacitor around 10nF is connected from this pin to the ground to reduce any parasitic noise. The capacitor can be omitted but is highly recommended to avoid false triggers.

Reset: This pin is used to set the output to the low state by resetting the latch circuit, regardless the states of any other input. A voltage of 0.5Volts and above with more than 0.1mA current is needed to accomplish a successful latch reset (Flip-flop output), and it will be accomplished with a delay of about 0.45μSec. The pulse with should be more than 0.5μSec. Those numbers are not guaranteed and may vary from manufacturer to manufacturer.
If this pin is not used, it is highly recommended to be connected to the Vcc to avoid any false reset.

Threshold: This is the input to the upper comparator. When the voltage at this pin is taken from below to above 2/3 of Vcc, it will reset the latch circuit and set the output to low state. Same as the trigger input, this pin is level sensitive and will permit slow ROC waveforms to perform a reset. The voltage that can be applied at this pin is between Vcc and ground, with a typical current of about 0.1μA.

Discharge: This pin will discharge the timing capacitor which is usually connected between pin 7 and ground. A voltage of about 100mV and a current of about 5mA is usually needed. Neither of those numbers is guaranteed by the manufacturers.
In certain cases, this output can be also used as an auxiliary output same as pin3.

Manufacturer's numbers
Manufacturer Number Number
(Military applications)
Philips ECG955M
Exar XR-555
Fairchild NE555
Harris HA555
Intersil NE555 SE555
Lithic Systems LC555
Maxim ICM7555
Motorola MC1555 MC1455
National LM555C LM1455
NTE Sylvania NTE955M
Raytheon RC555 RM555
RCA CA555C CA555
Sanyo LC7555
Texas Instruments SN72555 SN52555

Inside the 555

Following you can see the schematic diagram of a 555. Note that not all manufacturers follow the same drawings, just the same principal of operation. Depending to the manufacturer, a 555 could have around 20 transistors, 15 resistors and a couple of diodes.

The 555 block diagram

The 555 internal
schematic diagram

And the clue of the day: The 555 took it's name from the three 5K resistors performing the voltage divider !

555 Operation theory

The 555 operation is very simple. It uses a capacitor and one or two resistors to generate the pulses in 4 steps. To describe those steps, first you should take a closer look to the following drawing, demonstrating the 555 exposed and the minimum external parts required so that the 555 will generate pulses. Those pulses are generated on the pin number 3 (right side of the drawing). The three basic parts needed are connected on the left side of the 555 drawing. Those parts are the R1, the R2 and the capacitor C, and they define the so called 'RC network':

Up and down in 4 steps!
Step 1: An empty capacitor

By this time, the capacitor is considered to be empty. Because of that, the input - of the lower comparator will be more negative than the input + of it. This will generate a high (1) signal at the output of this comparator that will set the flip flop. The flip flop will finally set high the Q output.

Step 2: Filling the capacitor

This is the first critical time of the operation cycle. The capacitor will start increasing it's load from within the resistors R1 + R2. During all this time, the Q output will remain high due to the latch of the flip flop, however the output of the lower comparator will have become low as the - input will have become more positive than the + input of the comparator.

Step 3: A filled capacitor

Well, not quite filled, but almost filled. Step 3 will occur when the capacitor has equal to the 2/3 of the Vcc voltage charge. When it does, the input + of the upper comparator will become more positive than it's input -. This will cause the upper comparator's output to become high state and reset the flip flop. Reseting the flip flop will cause the output of the 555 to return to low state.

Step 4: Emptying the capacitor

When the previous step occurred, the upper comparator reseted the flip flop and the output Q of the flip flop became low. the reverse Q output will therefore become high. This signal is applied to the base of the transistor that it's colloector is the input 7 of the 555. The transistor's CE will become conductive, and the capacitor will start discharging from within R2 and the transistor.

When the capacitor is fully discharge, step 1 will occur again and so on and so on...

Pulse time calculations

As described before, the two stated of the output, the high and the low state, depends on the values of the R1, R2 and the capacitor C. Moreover, the high state depends on the R1+R2 and the capacitor C, and the low state depends on the R2 and the C. The formulas for calculating the oscillation frequency and the duty cycle are as follows:

The High output state that depends on both R1 + R2, is:

THIGH = 0.67 x (R1 + R2) x C (result in Seconds)

The Low output state depends only on R2 and is:

TLOW = 0.67 x R2 x C (result in Seconds)

To calculate the oscillation frequency, we add THIGH and TLOW.

TTOTAL = T1 + T2 = 0.67 x (R1 + R2) x C + 0.67 x R2 x C =>
TTOTAL = 0.67 (R1 + 2 x R2) x C

The TTOTAL is actually the period of the oscillation measured in seconds. Therefore, the frequency is:

F = 1 / TTOTAL => F = 1
0.67 (R1 + 2 x R2) x C


Although there is not a standard way to wire the 555, there are actually two standard ways that are most used. The first way is the Monostable Mode connection and the second the Astable Mode connection.

The 555 as Monostable Multivibrator

The wiring of 555 as a monostable multivibrator is shown in the following circuit:

When in input 2 of the 555 (trigger) is sent a low pulse, the output will go high. The output will remain high for a period of:

THIGH = 1.1 x R x C (Seconds)

There are several applications that a monostable circuit as the above is used. With very short periods, it could be used as a switch debouncer or a logic driver from fast acting sensors. Medium periods, like 1-10 seconds could be used for indication sound signals or any other general delay like car cabin lights turn off delay. Bigger periods can be used for delay timers.

The 555 as Astable Multivibrator

An astable multivibrator is often called also oscillator. A 555 can be used to generate clock pulses in a wide range of frequencies with enough output power to drive several ICs. The circuit is shown bellow:

The oscillation frequency is calculated with the following formula:

F = 1
0.67 (R1 + 2 x R2) x C

The same circuit can be used to control DC loads such as LEDs, lamps and DC motors. The idea is to use this circuit as a PWM signal generator. To do this, you need to replace R2 with a potentiometer. By altering the potentiometer's value, this results in changing the duty cycle output. The duty cycle is calculated as follows:

D = R1 + R2
R1 + 2 x R2

In a first glance, someone can understand that the minimum duty cycle could be no less than 50%. This is true. check the tips and tricks bellow to find out how can you achieve duty cycle less than 50%.

Relative pages
  • 555 timer basic circuits
  • The transistor theory of operation
  • Dr.Calculus: 555 Astable multivibrator calculator
  • Dr.Calculus: 555 Monostable calculator
  • International unit converter

  • Comments


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  • At 11 March 2015, 10:50:36 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @pantelis It is called "on delay"

  • At 11 March 2015, 0:44:38 user pantelis wrote:   [reply @ pantelis]
    • Hi George, i need an opposite circuit with 555 timer. I want to give a positive pulse and after 0-2 seconds the output to be positive. I want to adjust 0-2 seconds. It is not called nomostable ,do you have any circuit ,do you know how its called?

  • At 10 June 2013, 17:40:29 user Stevve wrote:   [reply @ Stevve]
    • @Fung The 558 timer didn't discontinue. I just bought a handful of them.

  • At 4 May 2012, 20:57:57 user Giorgos Lazaridis wrote:   [reply @ Giorgos Lazaridis]
    • @IBYISHAKA Marie Merci You can consider them as being the same. SE555 was the metal package and NE555 is the plastic dip package. They were the original chips, and then others came out from other companies with different letters (like LM555 from national semiconductors)

  • At 4 May 2012, 16:23:18 user IBYISHAKA Marie Merci wrote:   [reply @ IBYISHAKA Marie Merci]
    • Sir,i would like ask if 555 timer works the same as NE555. and if they're not i would like to have much explanations about NE555. it's urgent!!

  • At 18 January 2012, 22:17:44 user Tom G wrote:   [reply @ Tom G]
    • Thank you for your effort, very clear, excellent.

      i need an escalating oscillator, i thought i should connect two 555's but i dont see how, in other words i need a pwm that has a automated escalating 'RC network',
      any ideas?



  • At 6 December 2011, 6:10:35 user Chulaka Pitigala wrote:   [reply @ Chulaka Pitigala]
    • that informations are very useful but even if you can enter the derivation part of the equations actually it may be better than this

      however thank your work done

      nice future

  • At 20 November 2011, 17:21:19 user nadesh wrote:   [reply @ nadesh]
    • excellent,really usefull,thanx..

  • At 30 July 2011, 10:18:13 user peter petress wrote:   [reply @ peter petress]
    • Thanxs, Sir, for the genuine information which is precise, this is among the best sites i've visited, will always keep up to ur posts..........

  • At 25 June 2011, 6:22:19 user Fung wrote:   [reply @ Fung]
    • TS555 or 7555 is the CMOS version of 555 timer, after viewing the datasheet of TS555 in ST (the only manufacturer of this part), its minimum supply voltage is 2V.

      But I found some circuits which use these CMOS timers, but the supply voltage is 1.5V only, would it be unable to operate?

  • At 18 June 2011, 9:11:02 user Kammenos wrote:   [reply @ Kammenos]
    • @Fung 555 is not precise. If you plan to make a clock then better use a crustal oscillator with multiple frequency dividers.

  • At 18 June 2011, 5:42:38 user Fung wrote:   [reply @ Fung]
    • Concerning the 556 timer circuit I mentioned in the previous comment, I found that it does not work if a smoothing capacitor is added.

      The another question is, is it possible to make a oscillator circuit with exactly 1 Hz and 50% duty cycle by using 555 astable circuit?

  • At 14 June 2011, 18:58:57 user Kammenos wrote:   [reply @ Kammenos]
    • @Fung smoothing capacitors are always important if the power supply is not smooth, no matter what.

  • At 14 June 2011, 13:33:43 user Fung wrote:   [reply @ Fung]
    • This is because I have a circuit using 556 timer (but not designed by me), 2 astables, but 1 extra resistor is found in one of them. I found that it is connected in parallel with C1 but in series with R1 and R2.

      Use this connecting method gives a special effect on both outputs.

      Are smoothing capacitors (that one which connects directly to the power supply and 0V rail) not allowed in 555 timer circuits?

  • At 13 June 2011, 17:08:17 user Kammenos wrote:   [reply @ Kammenos]
    • @Fung i do not really know when it discontinued.
      If you connect a resistor parallel to the C (as you describe it), it will change the charge time of the capacitor. This resistor must have a large value, otherwise the capacitor will never reach the proper value to trigger the internal lower comparator. Actually, i do not find any reason to connect such a resistor to the circuit. I would never do it.

  • At 13 June 2011, 15:29:54 user Fung wrote:   [reply @ Fung]
    • When did the 558 quad timer discountinue?

      Also, when a resistor is added between the "R2" and "C" and it is connected to 0V, what effects will the timer give out?

  • At 11 June 2011, 17:22:49 user Sir Magneto wrote:   [reply @ Sir Magneto]
    • Fantastic clear and concise tutorial on 555 basics. Thanks for your efforts, very much appreciated!

  • At 17 February 2011, 15:18:30 user jack wrote:   [reply @ jack]
    • no. i already set up the timer 50% duty cycle...i have a project with using the sensor..if the sensor give the input the duty cycle timer back to the origin..example i set up the timer 1 minute, if have the input from signal the timer set again 1 minute and so on

  • At 17 February 2011, 6:43:10 user Kammenos wrote:   [reply @ Kammenos]
    • jack, i did not really understand your question. You want to make the duty cycle 0%? Check out this circuit:

  • At 17 February 2011, 5:41:02 user jack wrote:   [reply @ jack]
    • i have a question..
      how to reset the duty cycle back to start when the 555 timer is running..

  • At 28 January 2011, 6:26:16 user Kammenos wrote:   [reply @ Kammenos]
    • there is no most correct. it only has to do with the frequency that you want

  • At 27 January 2011, 8:25:15 user Fung wrote:   [reply @ Fung]
    • For the value of R2 of the 555 internal schematic, some datasheets state that it is 830R, while the others state that it is 330R. Actually, which one is the most correct?

  • At 19 January 2011, 10:29:59 user Kammenos wrote:   [reply @ Kammenos]
    • harratz, i cannot follow what you ask. please rephrase.

  • At 18 January 2011, 23:29:18 user harratz wrote:   [reply @ harratz]
    • how to calculated ranges of capacitance using a NE555 astable climb .. if you want the pattern but I'm sending you please answered months
      Lire phonétiquement
      Dictionnaire - Afficher le dictionnaireadverbe0.how

  • At 31 December 2010, 5:32:39 user hanu wrote:   [reply @ hanu]
    • simple,short and sound explanation

  • At 12 December 2010, 6:42:05 user Prasanna wrote:   [reply @ Prasanna]
    • Nice . . Thoery and explaination .

  • At 10 December 2010, 15:00:22 user John wrote:   [reply @ John]
    • You webpage, sir, is probably one of the best on the subjects it covers. Thank you

  • At 12 October 2010, 5:27:51 user Anirudh Rai wrote:   [reply @ Anirudh Rai]
    • Hello,
      The information provided by you about is very useful. I would like to thank you from deep of my heart.

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