Summary

The NE555 timer integrated circuit (IC) is a highly versatile and widely used electronic component known for its simplicity and adaptability in generating precise time delays and oscillations. Developed in the early 1970s by Swiss engineer Hans Camenzind for Signetics, the NE555 quickly became an essential tool in both hobbyist projects and complex industrial applications. Since its release, the 555 timer has remained one of the best-selling ICs, maintaining its relevance and popularity for over three decades due to its robust design and wide range of functionalities. The NE555’s ability to operate in various modes, including monostable, astable, and bistable, allows it to serve multiple purposes, from pulse generation to oscillator functions and flip-flop applications. Its simple pin configuration and internal structure, which includes key components like comparators, flip-flops, and transistors, enable it to perform complex timing and waveform generation tasks with high reliability. The IC’s design allows it to drive a broad range of loads directly from its output pin, making it suitable for diverse applications such as experimental physics, data acquisition, and communication devices. Despite its many advantages, including robustness against manufacturing variations and a wide operating range, the NE555 also has some limitations. For instance, the frequency stability can be affected by the tolerance levels of the external resistor-capacitor (RC) components, which may introduce variability in timing-critical applications. Additionally, its inverting output nature sometimes requires additional circuitry to achieve desired logic levels. However, these drawbacks have not significantly impeded its widespread adoption and continued use in the electronics field. Over the years, the NE555 has seen various iterations, including CMOS and bipolar versions, each tailored to specific power and performance requirements. The 556 variant, which houses two 555 timers in a single package, further extends its utility. The enduring legacy of the NE555 timer IC underscores its importance in electronics, from its historical significance to its ongoing application in modern technology, solidifying its status as a cornerstone component in the industry.

History

The 555 timer integrated circuit (IC) is one of the most important and enduring components in electronics history. Its development began in the late 1960s when Swiss-born engineer Hans Camenzind moved to the United States and started working at P. R. Mallory, a Massachusetts company known for its dry cell batteries. After earning his Master’s degree from Northeastern University, Camenzind joined Signetics in 1968, a company formed in 1961 by ex-Fairchild engineers who believed the future lay in integrated circuits rather than transistors. The early 1970s marked a significant period in the semiconductor industry, with advancements such as the planar process allowing for the construction of complex circuits on a single piece of silicon. It was during this innovative period that Camenzind conceptualized and designed the 555 timer IC. Despite some initial reluctance from Signetics to invest in the project, the IC was designed to be flexible and found a wide range of applications beyond its original purpose as a timer and oscillator. By the second year of its release, it had become the highest-selling IC and maintained that status for over 30 years. The first design of the 555 timer was reviewed in the summer of 1971. Camenzind’s decision to use a direct resistance instead of a constant current source enabled the reduction of the required external pins from 9 to 8, allowing the IC to fit in a smaller 8-pin package. This revised version, known as the NE555V (plastic DIP) and SE555T (metal TO-5), was completed in October 1971. Although a competitor had released a 9-pin version, they withdrew it soon after the 555’s release. By 1972, twelve companies were manufacturing the 555 timer, which became a best-selling product. The 555 timer’s long-lasting popularity can be attributed to its versatility, with applications ranging from simple hobby projects to sophisticated industrial uses. Its ability to generate accurate time delays and produce controlled pulses made it an indispensable component in various fields such as experimental physics, data acquisition, and communication devices. The IC’s stable and adjustable operation ensured its continued relevance and widespread adoption in numerous electronic systems.

Features and Specifications

The NE555 timer IC is a versatile component used in various electronic applications due to its simplicity and effectiveness. One of the key features of the NE555 is its ability to operate in different modes such as monostable, astable, and bistable, providing flexibility for various circuit designs.

Pin Configuration

The NE555 is typically available in both 8-pin and 14-pin DIP packages.

Pin 1 (Ground): Connects to the 0V power supply.
Pin 2 (Trigger): Detects 1/3 of rail voltage to make the output HIGH. It has control over Pin 6; if Pin 2 is LOW and Pin 6 is LOW, the output goes HIGH. If Pin 6 is HIGH and Pin 2 goes LOW, the output goes LOW.
Pin 3 (Output): The output terminal where the output signal is available.
Pin 4 (Reset): Resets or disables the timer irrespective of the trigger input. When not in use, it is connected to the high voltage supply.
Pin 5 (Control Voltage): Changes the threshold voltage level, affecting the timing characteristics. When not used, it is connected to ground via a 0.01µF capacitor to filter out external noise.
Pin 6 (Threshold): The positive pin of the comparator; its amplitude is responsible for setting the flip-flop state.
Pin 7 (Discharge): Used to discharge the capacitor in the timing circuit when the transistor saturates.
Pin 8 (VCC): Connects to the positive voltage supply.

Internal Circuit

The NE555 consists of a comparator, RS flip-flop, and a transistor. The timing circuit, which determines the width of the output pulse, is composed of resistors (R1 and R2) and a capacitor (C).

Applications

The NE555 can perform multiple functions such as:

Timers: Providing precise time delays.
Pulse Generation: Creating accurate pulses of specified widths.
Oscillators: Used in applications that require oscillating signals.
Toggle Functions: Switching between high and low states.

Historical Significance

Designed by Hans Camenzind for Signetics in 1971, the NE555 has evolved to include CMOS and bipolar versions, demonstrating its enduring utility and versatility in electronics. The 556 variant includes two 555 timers in a single package, further extending its application potential.

Internal Structure

The internal structure of the NE555 timer chip consists of several key components arranged in a precise layout to achieve its versatile functionality. These components include transistors, resistors, comparators, a flip-flop, a discharge transistor, and an output stage.

Voltage Divider

The voltage divider within the NE555 timer consists of three 5kΩ resistors connected in series between the supply voltage (Vcc) and ground. This arrangement creates reference voltages at 1/3 and 2/3 of Vcc, which are used by the comparators. When the control voltage pin is not used, the divider sets upper and lower reference voltages of 2/3 Vcc and 1/3 Vcc, respectively. If an external control voltage is applied, these reference points shift accordingly.

Comparators

The NE555 contains two comparators that compare the voltages at their input terminals. The negative input of the first comparator is connected to the 2/3 reference voltage, while its positive input is connected to the threshold pin. The second comparator’s positive input is connected to the 1/3 reference voltage, and its negative input is connected to the trigger pin. The comparators’ outputs feed into a flip-flop, which determines the final output state of the timer.

Output Stage

The final output from the flip-flop is fed into a series of drivers capable of handling up to 200mA of current. This ensures that the NE555 can drive a wide range of loads directly from its output pin. The robust design of the output stage makes the NE555 suitable for various applications, including pulse generation, timers, and oscillators. The integration of these components within the NE555 chip allows it to perform complex timing and waveform generation tasks with high reliability and versatility.

Transistors and Doping

Transistors are fundamental elements in the NE555 chip. The 555 timer uses both NPN and PNP bipolar transistors. In contrast to traditional textbook diagrams of NPN transistors that show a simplistic N-P-N layer arrangement, the actual transistors on the chip appear more complex. The chip’s silicon regions are doped with impurities to form N-type and P-type silicon, which show up as different tints under microscopy. The metal layer on top of the silicon forms the connections to the collector, emitter, and base. The emitter is identifiable by its “bullseye” structure, and the base by the surrounding rectangle.

Flip-Flop and Discharge Transistor

The flip-flop within the NE555 toggles between two states based on the outputs of the comparators. This flip-flop can also be reset externally via the reset pin. When the flip-flop’s output is low, the discharge transistor (connected to pin 7) is turned on, pulling the discharge pin to ground. This mechanism is crucial for the timer’s operation in astable and monostable modes, where the discharge transistor helps to manage the timing intervals by discharging the external timing capacitor.

Operating Modes

The NE555 timer IC is renowned for its versatility and can operate in three primary modes: Astable, Monostable, and Bistable.

Common Applications

The NE555 timer integrated circuit (IC) is renowned for its versatility and robustness in a broad array of applications. Its functionality is typically classified into three primary modes of operation: monostable (one-shot), astable (oscillator), and bistable (flip-flop).

Monostable Mode

In monostable mode, the NE555 functions as a one-shot timer that stays on for a predetermined duration before turning off. This mode is triggered by an external pulse and is commonly used in timing applications, such as delay timers and pulse generation. For instance, it is used in the creation of a calibrated pulse generator . The configuration in this mode involves utilizing Pin 2 as the trigger terminal to initiate the timing process .

Astable Mode

Astable mode enables the NE555 to operate as an oscillator, generating a continuous square wave output without requiring any external triggering. This makes it suitable for use in applications such as pulse-width modulation, pulse-position modulation, and frequency shift keying. Additionally, it can be used in digital object counters, anti-theft alarms, and automatic lighting systems .

Bistable Mode

In bistable mode, the NE555 operates as a flip-flop circuit, toggling between two stable states in response to external inputs. This mode is used in applications requiring a toggle switch function, such as a switch debouncing circuit, which helps eliminate false triggers due to the mechanical bouncing of physical switches.

Other Applications

The NE555’s versatility extends beyond the basic timing functions.

Pulse amplitude modulation
Digital object counters
Security systems
Automatic electrical equipment operating systems Furthermore, the NE555 can be implemented in more complex designs such as infrared remote-controlled timer circuits, programmable industrial on/off timers with RF remote, and even inductor-less DC-DC converters . The NE555’s adaptability, combined with the simplicity of using just a few external components, has made it a staple in both commercial products and hobbyist projects since its inception .

Circuit Examples

Frequency Counter

One common application of the NE555 timer IC is in the design of a frequency counter. In this setup, the 555 timer operates as a pulse generator, producing a square wave output. The frequency of this square wave can be calculated using the equation ( f = \frac{1.44}{(R_1+2R_2)C} ) and the duty cycle using ( D = \frac{R_1+R_2}{R_1+2R_2} \times 100% ). For instance, with a duty cycle ((D)) of 90% and (T_H) of 0.1s, the relationship ( R_1+2R_2 = 1.6 \times 10^5 ) is derived.

Operating Modes

The NE555 timer IC can operate in three main modes: Monostable, Astable, and Bistable.

Practical Applications

Light Sensor Circuit

A practical application of the 555 timer involves using it in conjunction with a Light Dependent Resistor (LDR) to create a light sensor circuit. The timer is connected to transistors that drive a relay, allowing the circuit to switch based on the light intensity. This setup can be used in automated lighting systems.

Seven-Segment Counter Display

Another example is an astable multivibrator circuit used to trigger a counter IC (CD 4033), which in turn advances each count on a seven-segment LED display (LT543). This application is often used in digital counters and displays to make circuits more interactive and informative.

LED Flasher Circuit

A 555 LED flasher circuit employs the timer in astable mode to produce pulses for flashing a lamp. The flashing rate is controlled by two resistors, and the circuit includes components such as a half-wave rectifier, transistor, and TRIAC to drive the load. This type of circuit is commonly used in visual signaling applications. These examples highlight the versatility of the NE555 timer IC in various timing and control applications, making it a fundamental component in electronics.

Advantages and Disadvantages

The NE555 timer’s lasting popularity is a testament to its remarkable versatility. It is used extensively in both simple hobby projects and sophisticated industrial applications, showcasing its wide range of functionalities and adaptability. One significant advantage of the NE555 is its robust output driving capability, allowing it to drive loads up to 200mA directly from the output pin. This means external components, such as transistors, are often unnecessary for driving moderate loads, simplifying the overall circuit design. Another advantage is the 555 timer’s resilience to manufacturing variations. During its development, the design had to account for high component tolerances and variations in manufacturing processes, as well as changes in temperature and supply voltage. This inherent robustness makes the NE555 reliable in diverse operational environments. Additionally, its operation modes—astable, monostable, and bistable—provide users with versatile timing and pulse generation options, catering to a broad spectrum of applications. However, the NE555 is not without its disadvantages. The reliance on a resistor-capacitor (RC) circuit to set its frequency can introduce variability due to the tolerance levels of these external components. This can affect precision in timing-critical applications. Moreover, the inverting nature of its output driving circuit can complicate designs where a non-inverted output is desired, necessitating additional components to achieve the required logic levels.

Variants and Alternatives

The NE555 timer IC, known for its versatility in producing accurate time delays and oscillations, has various variants and alternatives designed to cater to different requirements in electronic circuits. These include both bipolar and CMOS versions, each offering unique advantages and characteristics.

Bipolar Versions

The classic bipolar version of the 555 IC, such as the NE555, utilizes bipolar transistors. While robust, these ICs dissipate significant power and produce high current spikes, which limits their suitability for low-power applications. Over 12 independent companies have manufactured the original design, leading to widespread adoption and numerous applications.

CMOS Versions

To address the power dissipation and current spike issues associated with bipolar versions, complementary metal-oxide semiconductor (CMOS) versions were developed. CMOS versions, like the LMC555 produced by Texas Instruments, utilize both n-type MOSFET (NMOS) and p-type MOSFET (PMOS) transistors in enhancement mode. This configuration reduces power dissipation and lowers current spikes, making these ICs suitable for low-power applications. Motorola’s MC1455 is a popular CMOS variant that can be directly substituted for the original NE555 IC. It retains the functionality of the NE555 while offering improved power efficiency, making it a cost-effective option at around $0.28 USD.

Historical Context and Improvements

The original NE555 design had some limitations, including unbalanced comparators and sensitivity to temperature variations. Hans R. Camenzind redesigned the IC to address these flaws, resulting in a more balanced and reliable version. Although this improved IC, sold as ZSCTI555, did not gain as much popularity as the original NE555, it demonstrated the potential for further innovation and refinement in timer ICs.

Applications of Derivatives

Various circuits utilize 555 timers, including temperature measurement, moisture measurement, waveform generators, and different timer circuits. The NE555P, a specific variant of the NE555 family, is celebrated for its precision in timing and pulse generation applications. Its high output capacity of up to ±200 mA and TTL compatibility make it a versatile component in both analog and digital circuits.

Popularity and Legacy

The 555 timer’s lasting popularity is a testament to its remarkable versatility. From simple hobby projects to sophisticated industrial applications, its presence is ubiquitous in the electronics world. When the 555 hit the market in 1971, it was a sensation. By 1975, Signetics, the original manufacturer, was absorbed by Philips Semiconductors (now NXP), which reported that many billions of units had been sold. Engineers still use the 555 to create useful electronic modules as well as less useful things, like “Knight Rider”–style lights for car grilles. The invention of the 555 timer IC can be credited to Swiss-born engineer Hans Camenzind, who designed it while consulting for Signetics in the early 1970s. His oscillator design, initially proposed as a standalone product, became a foundational building block in electronics due to its ability to function both as a timer and an oscillator. The marketing push from Art Fury, a marketing manager at Signetics who was also adept at building circuits, was instrumental in bringing the 555 timer to market despite some internal hesitation. The 555 timer continues to be a popular choice for electronics enthusiasts and professionals alike due to its simplicity and reliability. It can be found in a variety of form factors and is available from numerous sources, underscoring its broad acceptance and enduring relevance in the field of electronics. For die-hard fans, there are even kits available to build a drop-in “macrocircuit” replica of the 555 using discrete transistors. Understanding the inner workings and functionalities of the 555 timer is not only beneficial for electronic hobbyists but also instrumental for professionals aiming for adept circuit designs. This continued usage and study highlight the 555 timer’s significant legacy in electronics, making it a cornerstone component even decades after its initial release.