How IC 555 timer works? Fundamentals of IC 555 timer & its basic applications

Pin Configuration & Function of IC 555

Have you checked our most popular mini projects using IC555? Simple mini projects for your school & college level project submission.

Pin-1, GROUND: It is the GROUND PIN of the IC. The negative terminal of DC power supply or battery is connected to this pin. Here note that IC555 works always on single rail power supply and NEVER on dual power supply, unlike operational amplifiers. Also note that this pin should be connected directly to ground and NOT through any resistor or capacitor. If done so, the IC will not function properly and may heat up and get damaged. This happens because all the semiconductor blocks inside the IC will be raised by certain amount of stray voltage and will damage the IC.

Pin configuration of ic555

Pin configuration of ic555

Pin-2, TRIGGER: It is known as TRIGGER PIN. As the name suggests in triggers i.e. starts the timing cycle of the IC. It is connected to the inverting input terminal of trigger comparator inside the IC.

As this pin is connected to inverting input terminal, it accepts negative voltage pulse to trigger the timing cycle. So it triggers when the voltage at this pin LESS THAN 1/3 of the supply voltage (Vcc). In number of applications, the IC must be triggered by a pulse. The amplitude and minimum pulse width required for triggering depend on operating temperature and supply voltage of the IC. Generally the current required for triggering is about 0.5uA for a period of 0.1uS. The triggering voltage may be in a range from minimum 1.67V when Vcc = 5V to maximum 5V when Vcc = 15V. The triggering circuit inside the IC is very sensitive and may be accidentally activated due to surrounding noise. To avoid this, the pin is always connected to a pull-up resistor (10k-ohm), if this pin is used separately.

Pin-3, OUTPUT: This is the OUTPUT PIN of the IC. It can SINK or SOURCE a maximum current of 200mA. Sinking the current means, when the output of the IC is at logic-0 state i.e. LOW and so it can absorb current into its output. Similarly sourcing the current means, when the output of the IC is at logic-1 i.e. HIGH and so it can give out current from its output. Due to this property of the IC, we can use it in number of typical digital applications also. Also note that the output voltage of the IC is slightly greater than zero, when it is in logic-0 state. Similarly it is slightly less than supply voltage (Vcc), when output of the IC is in logic-1 state.

Pin-4, RESET: It is the RESET PIN of the IC. When it is connected to positive terminal of battery, the IC works normally. However, when it is grounded (either directly or through a maximum of 100k-ohm resistor), the IC stops its working completely and its timing cycle stops i.e. the charging or discharging of the external capacitor stops, so output of the IC is locked in logic-0 state. It is interesting to note that the reset voltage required by this pin is typically 0.7V at a reset current of 0.1mA. However in general applications, this pin is always connected to positive terminal so that the IC works normally.

Pin-5, C. VOLTAGE: This is known as the CONTROL VOLTAGE pin. The 2/3 of supply voltage point on the terminal voltage divider is brought out to pin-5, known as the control terminal of the IC. The timing cycle can be modified by applying external DC control voltage to this pin. This allows manual or electronic remote controlling of the time interval of the IC.

The control terminal is frequently used when the timer is operated in MMV mode. But if you are NOT using this pin for any such purpose, then this pin MUST BE GROUNDED THROUGH A CAPACITOR OF 0.01uF. This prevents the time interval from being affected by picking up of stray AC or RF noise from the surrounding. Also note that, when the IC is used as an oscillator, in AMV mode, we can modulate the output waveform of the IC by applying a variable DC control voltage to this pin, as shown below.

Application of ic555 to control pin-5 modulation

Application of ic555 to control pin-5 modulation

Pin-6, THRESHOLD: This is known as the THRESHOLD PIN. It finalises the timing cycle of the IC, when its voltage is equal to or greater than 2/3Vcc, the output is at logic-0 state. Since this pin is connected to non-inverting terminal of threshold comparator inside the IC, it accepts positive going pulse to end up the timing cycle, also. Note that the typical value of threshold current is 0.1mA, just like the RESET PIN. The time width of this pulse should be greater than or equal to 0.1uS.

Pin-7, DISCHARGE: It is known as DISCHARGE PIN. It discharges the external capacitor into itself, but when fully charged…! It is connected to the collector of an NPN transistor inside the IC. Due to this, the discharging current going into this pin MUST NOT EXCEED 50mA, otherwise the internal transistor may get damaged. It is interesting to note that this pin can also be used as output pin with open collector output. I am working on one such practical circuit and will publish the circuit very soon.

Pin-8, +Vcc: It is known as the +ve supply terminal of the IC. The battery voltage connected across this pin and ground pin SHOULD NOT EXCEED 18V. Generally the range of operating voltage of the IC is 3V–18V.

Fundamentals of IC555 – Details of working

Basically, 555 timer is a highly stable circuit capable of functioning as an accurate time-delay generator and as a free running multivibrator. When used as an oscillator the frequency and duty cycle are accurately controlled by only two external components, a resistor (R) and a capacitor (C). The circuit may be triggered and reset on falling wave forms. Its prominent features are summarised below:

  • Timing from micro seconds through hours
  • Monostable and Astable operation
  • Adjustable duty cycle
  • Ability to operate from a wide range of supply voltages
  • Output compatible with CMOS, DTL and TTL (when used with appropriate supply voltage)
  • High current output that can sink or source 200 mA
  • Trigger and reset inputs are logic compatible
  • Output can be operated normal ON and OFF
  • High temperature stability

Let us see the internal details and operation of IC555 and see how the various features can be developed into practical circuits. The SE and NE versions are similar except for maximum temperature ratings. The precision type SE maintains essential characteristics over a temperature range of -55°C to +125°C while the general purpose type NE operates reliably only over a range of 0°C to 70°C. Some manufactures use the suffix C to indicate the commercial version for general purpose applications. Both types have a maximum rating of 18 volts and can handle power dissipation of up to 600 mW.

A functional block diagram of 555 timer is given below. The device consists of two comparators two transistors, a flip-flop and buffered outputs stage. The reference voltages for the two comparators inside the 555 are produced across a voltage divider consisting of three equal resistors of 5K ohms each.

Detailed internal block diagram of IC555. Note the three series resistors of 5k each

Look at the above given block diagram of the IC. There are three resistors of 5kΩ each connected in series. These three resistors produce 1/3 and 2/3 voltage levels for controlling the action of trigger and threshold comparators inside the IC. Due to this arrangement of the three resistors, the IC has a typical code number as IC555.

The threshold comparator is referenced at 2/3 Vcc and the trigger comparator is referenced at 1/3Vcc. The two comparators control the flip-flop which, in turn, controls the state of the output i.e. either ON or OFF states.

When the timer is in the quiescent state, the internal transistor T1 is conducting and represents a short circuit across timing capacitor C. The level of the output terminals in this state is low.

In practical circuits voltage at pin-2 is kept above the trigger point by a resistor connected to Vcc. When a negative going trigger pulse on pin-2 is applied, it causes the potential at this point to fall below 1/3Vcc and thus the trigger comparator RESETs the flip-flop.

Now transistor T1 is cut-off and the thus the output level of the IC goes HIGH to a value slightly less than Vcc. Capacitor (C) now starts to charge and the voltage across it rises exponentially until it reaches 2/3Vcc. At this point, the threshold comparator resets the flip-flop and the output returns to its low state-just slightly above ground. Transistor if T1 is turned ON, discharging capacitor C so that it is ready for the next timing period.

Once triggered, the circuit cannot respond to additional triggering until the timed interval has elapsed.
The delay period, the time that the output is high, in seconds is given by –
1.1 x C x R, where R is in Mega ohm and C is in microfarads.

To obtain large delay in some practical applications, the value of timing resistor should not exceed 20 Mega ohm. If you use an electrolytic timing capacitor, select a unit for low leakage. The time delay may have to be adjusted by varying PT to compensate for the wide tolerance of electrolytic.

An important feature to be noted here is that 555, unlike many RC timers, provide a timed interval that is virtually independent of supply voltage Vcc. This is because the charge rate of C and the reference voltages to the threshold comparator and trigger comparator are all directly proportional to the supply Voltage. Operating voltage can range from 3V to 18V.

Important formulas for calculations

Frequency Calculations To calculate the output frequency of the circuit following formula is used. In this you have to put the values of R1, R2 and the value of timing capacitor C. Note that R1 and R2 are in Ohms and C is in Farad.

calculating the output frequency of ic555

Calculating the output frequency of ic555


This sounds good so far as theoretical calculations are concerned. But when you deal with practical circuits and want to use this formula, then what to do? The formula contains three unknowns…! So how to calculate the output frequency?

Timing Calculations The total time period, the On time and Off time period of the IC are given by the same formula. The timing calculations will give you time in seconds, if the values of R1 and R2 are in Ohms and the value of timing capacitor is in Farad.

Duty Cycle The duty cycle of the IC is actually a specific ratio of the two resistors used in AMV circuit. Thus the formula for duty cycle of the IC is given by the same formula. The duty cycle of the circuit is always calculated in terms of percentage. There are three main values of duty cycle of the IC.

  • When duty cycle = 50%, we get the perfect square wave at the output of the circuit.
  • When duty cycle > 50%, we get a rectangular wave, such that ON TIME of the circuit is greater than the OFF TIME.
  • When duty cycle < 50%, we get a rectangular wave, such that OFF TIME of the circuit is greater than the ON TIME.
  • Always remember that the value of duty cycle CANNOT BE equal to 100% and also it CANNOT BE equal to 0%.
  • This is because, the value of R1 cannot be zero in the circuit of AMV.

More about IC555

The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation and oscillator applications. The 555 can be used to provide time delays, as an oscillator, and as a flip-flop element. Derivatives provide up to four timing circuits in one package. Introduced in 1971 by Signetics, the 555 is still in widespread use, thanks to its ease of use, low price and good stability, and is now made by many companies in the original bipolar and also in low-power CMOS types. As of 2003, it was estimated that 1 billion units are manufactured every year.

Hans R. Camenzind (know more about him on Wikipedia) has designed this IC in 1971, under contract to Signetics, which was later acquired by Philips. Depending on the manufacturer, the standard 555 package includes over 20 transistors, 2 diodes and 15 resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8). Variants available include the 556 (a 14-pin DIP combining two 555s on one chip), and the 558 (a 16-pin DIP combining four slightly modified 555s with DIS & THR connected internally, and TR is falling edge sensitive instead of level sensitive).

The NE555 parts were commercial temperature range, 0 °C to +70 °C, and the SE555 part number designated the military temperature range, −55 °C to +125 °C. These were available in both high-reliability metal can (T package) and inexpensive epoxy plastic (V package) packages. Thus the full part numbers were NE 555V, NE 555T, SE 555V, and SE 555T. As per hypothesis the 555 got its name from the three 5kΩ resistors within, but Hans Camenzind has stated that the number was arbitrary.

Low-power versions of the 555 are also available, such as the 7555 and CMOS TLC555. The 7555 has less supply glitching than the classic 555 and the manufacturer claims that it usually does not require a “control” capacitor and in many cases does not require a decoupling capacitor on the power supply. One must avoid such practice, because noise produced by the timer or variation in power supply voltage might interfere with other parts of a circuit or influence its threshold voltages.

About the author: My fields of interest are applied electronics, robotics & web designing. I’m an Engineer by education and a sincere student of science & technology. People call me teacher!

Comments on this entry are closed.

  • Nityashree Bajaria

    August 26, 2015, 5:08 AM

    Thanx for complete information.
    I recommend this page on my social networks

  • Sameer

    August 27, 2015, 8:59 PM

    Thankx for complete and neatly written information

  • payal rathi

    August 28, 2015, 7:13 AM

    Can we use IC555 for PWM?

  • Hrushikesh

    September 1, 2015, 6:03 PM

    Thanks for nice information

  • Brian

    September 24, 2015, 3:22 AM

    Why IC 555 is named so?

  • D'souza Mary

    September 24, 2015, 8:29 PM

    Most informative article on IC555.
    Thanks for one stop info…

  • Bunty Sadanshiv

    September 25, 2015, 7:01 AM

    Very useful info

  • Harrison

    October 27, 2015, 5:28 PM

    Nice and clear explanation…

  • Pranjal Kokate

    November 2, 2015, 6:05 AM

    Nice explanation

  • Steffen

    November 26, 2015, 10:22 PM

    Excellent way of explanation

  • Jordan Bishop

    December 1, 2015, 7:51 AM

    Thank you so much for such a great and one stop article on IC 555
    Thanks again

  • VSAcademy

    December 10, 2016, 8:00 AM

    The formula of T=1.1RC is obtained using the basic theory of charging and discharging of a capacitor (C) through a resistor (R). Learn this theory so that you will understand the idea of deriving this formula.