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CHIP TECHNOLOGY World’s best-selling chip: 555 Timer IC

From Venus Kohli Reading Time: 5 min |

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555 Timer IC has continued to rule the world’s chip market since the seventies. Often addressed as among the “greatest chips”, the 555 Timer IC is a best-selling analog semiconductor. The Signetica-introduced chip is capable of producing desirable oscillations and output waveforms for an engineer.

The article starts to explain 555 Timer IC and its pin diagram in detail.
The article starts to explain 555 Timer IC and its pin diagram in detail.
(Source: Dan Warner -

What is a 555 Timer chip?

555 Timer is an integrated circuit or semiconductor chip that functions in various timing, pulse generation, and oscillatory applications. In simple words, 555 Timer IC is capable of generating highly stable time delays or oscillations, ranging from a few microseconds to hours. The output pulse generation and associated waveforms of the IC 555 Timer depend on the trigger pulse and mode of operation.

Why is IC 555 an engineer’s favorite?

(Source: Wikipedia)

In 1968, Signetics Corporation hired Hans Camenzind, a Swiss electrical engineer, to work with phase-locked loops (PLL). The limitations in existing chips, then, were supply voltage and temperature-dependent operation. However, the Swiss scientist invented the legendary “555 Timer IC”. After multiple rejections of his invention, the Signetics Corporation finally introduced the 555 Timer IC in 1972.

Indeed, more than billions of 555 Timer ICs have been sold worldwide. 555 Timer IC is often addressed as one of the “greatest chips” that has ever been made. The inventor Hans Camenzind redesigned the chip years later, but the new design was a huge failure as the 555 Timer continued to rule the analog semiconductor market.

The reason for the “technological hype” around the 555 Timer is its low cost, simple integration, easy operation, adjustable duty cycle, operating modes, stable output, and reliability. 555 Timer IC output can drive the transistor logic and produce high current output. Hobbyists, engineers, and enthusiasts can easily work with this beginner-friendly Timer chip.

IC 555: Pin diagram and functions

IC 555 Timer is available in three packages:

  • 8-pin MSOP
  • 8-pin metal can
  • 8-pin mini DIP
  • 14-pin DIP

The 8-pin mini DIP package of IC 555 Timer is used among wide applications. The 555 Timer IC was initially available initially as SE555 or NE555. The SE555 is designed for operating over a wide range of temperatures from -55 to 125 degrees Celsius. On the other hand, NE555 operates from 0 to 70 degrees Celsius. The structure of 555 IC Timer consists of:

Pin diagram of IC 555 Timer, courtesy component datasheet
Pin diagram of IC 555 Timer, courtesy component datasheet
(Source: Venus Kohli)

  • 8 pins
  • An SR flip-flop
  • Two comparators- upper comparator 1 and lower comparator 2
  • Internal “discharge transistor” to discharge the capacitor
  • Voltage divider
  • Voltage reference source
  • Various resistances and capacitors

In electronics, engineers often use the words “high” or “low” to describe the state of a pin, input, output, or any electronic device. Any electronic entity is ‘low’ or ‘off’ at ‘0’ and ‘high’ or ‘on’ at ‘1’.

Pin 1: Ground

The GND pin of 555 Timer serves as a reference to measure all the associated voltages with the IC. Pin 1 is connected to the reference (0 V) or ground terminal of the DC supply voltage source to the circuit.

Pin 2: Trigger

The trigger pin in the 555 Timer is where a triggering pulse is applied to drive the IC and begin its timing cycle. The magnitude of this triggering pulse influences the internal SR flip-flop to set or reset for determining the 555 Timer output.

When the voltage at pin 2 falls below ⅓ Vcc, the internal SR flip flop is set to switch the 555 Timer IC output to high (1).

If the voltage at pin 2 is above ⅔ Vcc, the SR flip flop is triggered to reset and the output of the 555 Timer IC is low (0).

Pin 3: Output

The output pin is in the 555 Timer IC to provide the output signal. The load is connected between pin 3 and pin 1 (Ground) or pin 3 and pin 8 (+Vcc).

Pin 3: Low (0)

When the output pin is low, the load current flows between pin 3 (Output) and pin 8 (+Vcc) towards the output terminal. The load connected between pin 3 (Output) and pin 8 (+Vcc) is normally on load and the current is called sink current (or take-in current).

Pin 3: High (1)

When the output pin is high, no current flows between pin 3 (Output) and pin 8 (+Vcc). Instead the output terminal supplies current into the load. The load connected between pin 3 (output) and pin 1 (ground) is known as normally off-load and the current is called source current (or give-out current).

Pin 4: Reset

Just as the name suggests, the reset pin “resets” the 555 Timer IC operation to a low state (0). When the reset pin goes low, the 555 Timer IC immediately overrides the operations, enters the reset state, and forces the output to go low.

Under normal 555 Timer operation, the reset pin is connected to +Vcc to avoid false triggering. A simple way to disable 555 Timer IC operation is to feed pin 4 with a negative pulse.

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Pin 4: Low (0)

555 Timer IC is reset to zero output.

Pin 4: High (1)

555 Timer IC is not reset and operates normally.

Pin 5: Control Voltage

An input to control voltage pin changes trigger and threshold voltage to vary output waveform. The DC voltage at pin 5 is ⅔ Vcc through an internal voltage divider. Normally, a 0.01 uF capacitor bypasses pin 5 to avoid noise, and faulty operation and stabilize the voltage.

Pin 6: Threshold

The threshold pin sets a maximum limit of voltage or “threshold” voltage level, beyond which the internal states of the 555 Timer IC change. Internally, the threshold pin is the non-inverting terminal of the upper comparator 1.

Pin 6 monitors the voltage across the external capacitor. When the voltage at pin 6 goes above ⅔ Vcc, the output of the 555 Timer IC goes low (0).

Pin 7: Discharge

The discharge pin is internally connected to the collector terminal of the discharging transistor (Q1).

Pin 7: Low (0)

When the discharge pin is low, the external capacitor is shorted to ground through an internal transistor (Q1). Q1 enters saturation and functions as a short circuit to the capacitor.

Pin 8: High (1)

Transistor Q1 turns off when the discharge pin is high. The transistor acts as an open switch to the external capacitor. As a result, the external capacitor is effectively disconnected from the ground.

Pin 8: +Vcc

The pin 8 is connected to the circuit's positive regulated supply voltage. 555 Timer IC operates on a positive ripple-free supply voltage from 5 V to 18 V.


555 Timer serves a wide range of applications including:

  • Multivibrators (Monostable, astable, and bistable)
  • Pulse generators
  • Pulse width modulator (PWM)
  • Oscillators
  • Frequency dividers
  • Burglar alarming systems
  • Power Electronics
  • DC-DC converters
  • Voltage-controlled oscillators (VCO)
  • Control devices

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