555 Timer IC Tutorial : Specification and Pin Diagram

What is 555 timer and how will it works ?

So, today we will discuss about the 555 IC. What is 555 timer and how will it works.            555 Timer IC

What is 555 timer IC?

555 timer is introduced in 1972 by signatics corporation.it is usually described as a highly stabled device for generation of accurate delay and oscillation. This is very cheap and popular timing device.
Let’s start a discussion about 555 timer. This is normally an IC (integrated circuit) and also a type of timer. Have you thought about why it’s named as 555 time? The answer is, it made up with THREE 5k ohm RESISTOR in series connection. Let we see how it is working, construction and application.

555 Timer Pin Diagram


Construction of 555 timer:
Once you start studying about IC you don’t go with an IC. First you learn internal diagram of IC. It will helpful for you know where will get an output in which condition and input? Let starts discuss about internal connection.
In introduction itself, I told, 555 timer made of THREE 5k ohm RESISTOR in series connection. Why they would connected in series connection? Because in series connection voltage should be divided.
Based on this three resistors has been connected in series, so voltage divided into 3 parts. The 1st part is VCC/3, 2nd part is 2VCC/3 and last part is 3VCC/3 (refer Fig 1).


Then this two voltages (2VCC/3 and VCC/3 ) is connected to the two separate comparator. One is called as a lower comparator and another one is upper comparator. This two comparators will have two terminals are inverting (-) and non- inverting (+). The VCC/3 is connected to non-inverting terminal which in lower comparator. 2VCC/3 is connected to inverting terminal in upper comparator (refer fig 1.1).

Next the lower comparator inverted pin goes outside which is the trigger pin 2 of the IC. Upper comparator non-inverting terminal is connected to the Threshold pin 6 of the IC.(refer Fig 1.2)

The output of two comparators is giving to the flip flop which is SR FLIP FLOP. On that flip flop have 4 terminals, which are the S,R,Q,Q’. Here we consider Q is an output of SR flip flop and Q’ is an output of 555 timer. So Q’ is connected to the output of IC pin 3. Then Q is connected through the resistor to the PN transistor, the transistor’s VCC pin is a discharge pin.(refer fig 1.3).

The reset pin is connected to the SR flip flop. In 2VCC/3 through that control pin has to be connected. (ref Fig 1.4)

Working of 555 timer IC:

The 555 timer shown above is configured as an astable circuit. This means that the output voltage is a periodic pulse that alternates between the VCC value and 0 volts.

The frequency is the number of pulses per second. The formula to calculate the frequency of the output voltage is:


The period is the time covered for one pulse. This is just the reciprocal of the frequency:


The high time (T1) and low time (T0) can be calculated using the formulas below. Note that the period is the sum of the high time and the low time.



The mark space ratio is the ratio between the high time and the low time or:

Mark Space Ratio=T1T0

The duty cycle is more commonly used than the mark space ratio. The formula for the duty cycle is:

Duty Cycle=T1T x 100

A 50% duty cycle means the high time is equal to the low time. If an LED is placed at the output of this astable circuit, it will turn on at the same span of time as it is turned off. Note that getting an exact 50% duty cycle is impossible with this circuit.


  • Increase C to increase the period (reduce the frequency).
  • Increase R1 to increase High Time (T1), without affecting the Low Time (T0).
  • Increase R2 to increase High Time (T1), increase Low Time (T0) and decrease the duty cycle.

Final result:

Light dimmer using NE555:

Light Dimmer


These are the final image of 555 timer circuit astable circuit connection and graphical representation of frequency. From this we learned about 555 timer working, construction, and how to calculate the 50% of frequency.