The 555 Timer IC

The 555 timer IC is an integrated circuit used in a variety of timer, delay, pulse generation, and oscillator applications. Derivatives provide two (556) or four (558) timing circuits in one package. 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 dual in-line package

The 555 timer IC (integrated circuit) is a versatile and widely used component in electronics that can function as an oscillator, timer, pulse generator, and more. Here's a brief explanation of how the internals of a 555 timer IC work:

Pin Description:

Pin 8: Voltage Regulator (Vcc): The 555 timer IC typically requires a supply voltage (Vcc) ranging from 4.5V to 15V. This voltage powers the internal circuitry of the IC.

Pin 1: GND (Ground): The ground pin serves as the reference point for the IC's voltage and signals.

Pin 2: Trigger: The Trigger pin is connected to the inverting input of an internal comparator. When the voltage at this pin drops below 1/3 of Vcc, it triggers the internal flip-flop and initiates various functions depending on the IC's configuration.

Pin 6: Threshold: The Threshold pin is connected to the non-inverting input of a second internal comparator. When the voltage at this pin rises above 2/3 of Vcc, it resets the internal flip-flop.

Pin 5: Control Voltage: The Control Voltage pin allows an external voltage to be applied to control the timing characteristics of the IC.

Pin 7: Discharge: The Discharge pin is connected to the open collector of an internal NPN transistor. It is used to discharge the external timing capacitor during certain parts of the timing cycle.

Pin 4: Reset: The Reset pin is used to reset the internal flip-flop, stopping the timer and forcing the output to go low.

Pin 3: Output: The Output pin is the main output of the 555 timer. It provides a square wave or pulse output depending on the configuration of the IC.

Internal Circuitry:

In summary, the 555 timer's internal components work together to create versatile timing and pulse generation functions, making it a fundamental building block in electronic circuits. Its simplicity, reliability, and flexibility have contributed to its enduring popularity in a wide range of applications.

SR Flip-Flop

In a 555 timer, the SR (Set-Reset) flip-flop is an essential internal component responsible for controlling the timing and output of the IC. The SR flip-flop has two states: Set and Reset. Here's how it functions:

Set State:

Reset State:

The Transition between Set and Reset:

Output Control:

Timing Functions:

In summary, the SR flip-flop in a 555 timer is a fundamental building block that controls the output state based on external triggers and threshold conditions. It is the core component that enables the 555 timer to function as a versatile timer, oscillator, or pulse generator in a variety of electronic applications.

Modes of the 555

The 555 IC has the following operating modes:


In astable mode, the 555 timer operates as an oscillator, generating a continuous square wave output. The circuit configuration allows the timer to continuously switch between high and low states, producing a continuous oscillating waveform. This makes astable mode particularly useful for generating clock pulses, tone generation, or other timing applications where a continuous square wave output is needed.

Here's a brief overview of the key components and connections in a 555 timer circuit operating in astable mode:


Resistors (R1 and R2):

Astable 555 timer

Capacitor (C):

The timing components (R1, R2, and C) determine the frequency and duty cycle of the output waveform. The charging and discharging of the capacitor create the oscillations.

In astable mode, the 555 timer continually triggers itself, causing the output to oscillate between high and low states. The frequency of oscillation (f) and the duty cycle (the ratio of time the output is high to the total period) can be calculated using the following formulas:

By adjusting the values of the resistors and capacitor, you can control the frequency and duty cycle of the output waveform generated by the 555 timer in astable mode, making it a versatile component in electronic circuits.

Basic Astable Circuit

In this configuration, pins 2 and 6 are linked, enabling the circuit to autonomously trigger itself during each timing cycle, effectively operating as an oscillator. Capacitor C1 undergoes charging via resistors R1 and R2, but its discharge occurs through R2. The voltage across C1 fluctuates between 1/3 Vcc and 2/3 Vcc. Notably, the oscillation frequency remains unaffected by variations in Vcc.

Monostable 555 timer


In monostable mode, the 555 timer acts as a one-shot pulse generator. It produces a single, fixed-duration pulse (output goes high) in response to an external trigger pulse. After the pulse is generated, the 555 timer returns to its stable state (output goes low) until it is triggered again.

Here's a basic overview of the key components and connections in a 555 timer circuit operating in monostable mode:


Resistors (R1 and R2):

Capacitor (C):

The timing components (R1, R2, and C) determine the duration of the output pulse. The external trigger pulse applied to Pin 2 initiates the generation of a pulse whose duration is given by the formula:

After the pulse is generated, the output returns to its low state until another trigger pulse is applied.

Monostable mode is commonly used in applications where a single, precisely timed pulse is required, such as in delay circuits, pulse-width modulation, or in situations where a stable output is needed in response to an external event. Adjusting the values of the resistors and capacitor allows control over the pulse duration.

Basic Monostable Circuit

An initial negative trigger pulse applied to pin 2 deactivates a transistor responsible for shorting C1 to ground. Subsequently, the output signal rises as capacitor C1 charges via resistor R1. Upon reaching a charge of 2/3 Vcc, the 555 timer discharges C1 to ground, causing the output to transition to a low state.

Bistable (flip-flop)

In bistable mode, the 555 timer functions as a flip-flop, meaning it can be used to store a binary state or toggle between two stable states. The bistable mode is less common than the monostable and astable modes for the 555 timer, but it can be useful in certain applications.

Here's a basic overview of the key components and connections in a 555 timer circuit operating in bistable mode:


Resistors (R1 and R2):

In bistable mode, the external control voltage applied to Pins 2 and 6 determines the output state of the 555 timer. When the control voltage is above a certain threshold, the output is in one stable state. When the control voltage is below a different threshold, the output is in the other stable state. The bistable mode essentially turns the 555 timer into a set-reset (SR) flip-flop.

The transition between the two states can be controlled by manipulating the external control voltage. This makes bistable mode suitable for applications where you need a stable output that can be toggled between two states based on an external signal.

It's important to note that while bistable mode can be implemented with the 555 timer, dedicated flip-flop ICs are more commonly used for flip-flop applications due to their specific design for sequential logic circuits. The 555 timer's bistable mode is more of an additional feature rather than its primary use.

Schmitt Trigger (inverter) mode

The Schmitt Trigger mode is a unique configuration of the 555 timer IC that allows it to function as a Schmitt trigger inverter. In this mode, the 555 timer can be used to convert an analog input signal into a digital output signal with hysteresis.

Here's a basic overview of the key components and connections in a 555 timer circuit operating in Schmitt Trigger mode:


Resistors (R1 and R2):

In Schmitt Trigger mode, the external voltage connected to Pins 2 and 6 sets the thresholds for the digital output. The 555 timer acts as a Schmitt trigger, which is a type of comparator with hysteresis. Hysteresis means that the input voltage must cross a higher threshold to switch from a low to a high output state and a lower threshold to switch from high to low.

The formula for the upper and lower thresholds is as follows:

When the external voltage exceeds the upper threshold, the output goes low. When the external voltage falls below the lower threshold, the output goes high. This behavior creates a digital waveform at the output, and the Schmitt Trigger mode is particularly useful in applications where noise or signal fluctuations could cause false triggering.

Applications for the Schmitt Trigger mode include signal conditioning, debouncing switches, and shaping analog signals into clean digital signals with noise immunity.

Note: Debouncing switches refers to the process of removing or reducing the unwanted noise or "bouncing" that occurs when a mechanical switch is pressed or released. 

Clean Signals

A "clean signal" typically refers to an electronic signal that exhibits minimal distortion, noise, or unwanted variations. In the context of electronic circuits, a clean signal is one that accurately represents the intended information or waveform without unwanted artifacts. Achieving a clean signal is essential in various electronic applications, such as communications, data transmission, audio systems, and sensor readings. Inaccuracies, noise, or distortions in a signal can lead to errors, misinterpretation of information, or degraded performance in electronic systems. Designers use techniques such as filtering, shielding, proper grounding, and high-quality components to ensure signals remain as clean as possible in electronic circuits.


While the 555 timer is a versatile and widely used integrated circuit, the signals it generates may not always be perfectly clean, especially in certain operating conditions or configurations. Here are some factors that can affect the signal quality produced by a 555 timer:

To enhance the signal quality and reliability of a 555-timer circuit, designers often implement proper filtering, shielding, and power supply regulation techniques. Additionally, precision components and careful attention to circuit layout can contribute to achieving cleaner signals.

It's worth noting that while the 555 timer is a versatile IC, for applications requiring extremely precise and clean signals, more specialized components or dedicated signal generators may be preferred.