555 Timer Working: Specifications And Working Basics


The 555 timer IC is an integral part of electronics projects. Be it a simple project involving a single 8 bit micro-controller and some peripherals or a complex one involving system on chips (SoCs), 555 timer working is involved. These provide time delays, as an oscillator and as a flip-flop element among other applications.

Introduced in 1971 by the American company Signetics, the 555 is still in widespread use due to its low price, ease of use and stability. It is made by many companies in the original bipolar and low-power CMOS types. According to an estimate, a billion units were manufactured back in the year 2003 alone. (That time, only 555 i knew was a cough syrup).


Depending on the manufacturer, the standard 555 package includes 25 transistors, 2 diodes and 15 resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8). Variants consists of combining multiple chips on one board. However 555 is still the most popular. Let’s look at the pin diagram to have an idea about the timer IC before we talk about 555 timer working.

Pin diagram and description

1GNDGround reference voltage, low level (0 V)

The OUT pin goes high and a timing interval starts when this input falls below 1/2 of CTRL voltage (which is typically 1/3 Vcc, CTRL being 2/3 Vcc by default if CTRL is left open). In other words, OUT is high as long as the trigger low. Output of the timer totally depends upon the amplitude of the external trigger voltage applied to this pin.

3OUTThis output is driven to approximately 1.7 V below +Vcc, or to GND.

A timing interval may be reset by driving this input to GND, but the timing does not begin again until RESET rises above approximately 0.7 volts. Overrides TRIG which overrides threshold.


Provides “control” access to the internal voltage divider (by default, 2/3 Vcc).


The timing (OUT high) interval ends when the voltage at threshold is greater than that at CTRL (2/3 Vcc if CTRL is open).


Open collector output which may discharge a capacitor between intervals. In phase with output.


Positive supply voltage, which is usually between 3 and 15 V depending on the variation.

Some important features of the 555 timer:

555 is used in almost every electronic circuit today. For a 555 timer working as a flip flop or as a multi-vibrator, it has a particular set of configurations. Some of the major features of the 555 would be,

  • It operates from a wide range of power ranging from +5 Volts to +18 Volts supply voltage.
  • Sinking or sourcing 200 mA of load current.
  • The external components should be selected properly so that the timing intervals can be made into several minutes along with the frequencies exceeding several hundred kilo hertz.
  • The output of a 555 timer can drive a transistor-transistor logic (TTL) due to its high current output.
  • It has a temperature stability of 50 parts per million (ppm) per degree celsius change in temperature which is equivalent to 0.005 %/ °C.
  • The duty cycle of the timer is adjustable.
  • Also, the maximum power dissipation per package is 600 mW and its trigger and reset inputs has logic compatibility.

555 timer working

The 555 generally operates in 3 modes. A-stable, Mono-stable and Bi-stable modes.

A-stable mode

This means there will be no stable level at the output. So the output will be swinging between high and low. This character of unstable output is used as clock or square wave output for many applications.

Mono-stable mode

This configuration consists of one stable and one unstable state. The stable state can be chosen either high or low by the user. If the stable output is set at high(1), the output of the timer is high(1). At the application of an interrupt, the timer output turns low(0). Since the low state is unstable it goes to high(1) automatically after the interrupt passes. Similar is the case for a low stable mono-stable mode.

Bi-stable mode

In bi-stable mode, both the output states are stable. At each interrupt, the output changes from low(0) to high(1) and vice versa, and stays there. For example, if we have a high(1) output, it will go low(0) once it receives an interrupt and stay low(0) till the next interrupt changes the status.

This datasheet should provide an insight into the specifics: 555 Timer IC

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The below video from Skinny R&D gives an insight into 555 timer as well.

Also let us know if Jason explained the 555 timer working to your liking.