555 timer IC

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555 timer
Signetics NE555N.JPG
Signetics NE555 in 8-pin DIP package
Type Active, Integrated circuit
Invented Hans Camenzind
First production1971
Electronic symbol
NE555 Bloc Diagram.svg
Internal block diagram [1]

The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation, and oscillator applications. The 555 can be used to provide time delays, as an oscillator, and as a flip-flop element. Derivatives provide two (556) or four (558) timing circuits in one package. [2]

Integrated circuit electronic circuit manufactured by lithography; set of electronic circuits on one small flat piece (or "chip") of semiconductor material, normally silicon

An integrated circuit or monolithic integrated circuit is a set of electronic circuits on one small flat piece of semiconductor material that is normally silicon. The integration of large numbers of tiny MOS transistors into a small chip results in circuits that are orders of magnitude smaller, faster, and less expensive than those constructed of discrete electronic components. The IC's mass production capability, reliability, and building-block approach to circuit design has ensured the rapid adoption of standardized ICs in place of designs using discrete transistors. ICs are now used in virtually all electronic equipment and have revolutionized the world of electronics. Computers, mobile phones, and other digital home appliances are now inextricable parts of the structure of modern societies, made possible by the small size and low cost of ICs.

Timer device that automatically times a process or event or activates an operation or another device at a preset time or times

A timer is a specialized type of clock used for measuring specific time intervals. Timers can be categorized into two main types. A timer which counts upwards from zero for measuring elapsed time is often called a stopwatch, while a device which counts down from a specified time interval is more usually called a timer. A simple example of this type is an hourglass. Working method timers have two main groups: Hardware and Software timers.

Electronic oscillator electronic circuit that produces a repetitive, oscillating electronic signal, often a sine wave or a square wave

An electronic oscillator is an electronic circuit that produces a periodic, oscillating electronic signal, often a sine wave or a square wave. Oscillators convert direct current (DC) from a power supply to an alternating current (AC) signal. They are widely used in many electronic devices ranging from simplest clock generators to digital instruments and complex computers and peripherals etc. Common examples of signals generated by oscillators include signals broadcast by radio and television transmitters, clock signals that regulate computers and quartz clocks, and the sounds produced by electronic beepers and video games.


Introduced in 1972 [3] by Signetics, [4] the 555 is still in widespread use due to its low price, ease of use, and stability. It is now made by many companies in the original bipolar and in low-power CMOS technologies. As of 2003, it was estimated that 1 billion units were manufactured every year. [5] The 555 is the most popular integrated circuit ever manufactured. [6] [7]

Signetics integrated circuits manufacturer

Signetics was an American electronics manufacturer specifically established to make integrated circuits. Founded in 1961, they went on to develop a number of early microprocessors and support chips, as well as the widely used 555 timer chip. They were bought by Philips in 1975 and incorporated in Philips Semiconductors.

Bipolar junction transistor transistor that uses both electron and hole charge carriers.In contrast,unipolar transistors such as field-effect transistors,only use one kind of charge carrier.For their operation,BJTs use 2 junctions between 2 semiconductor types,n-type and p-type

A bipolar junction transistor is a type of transistor that uses both electrons and holes as charge carriers.

CMOS Technology for constructing integrated circuits

Complementary metal–oxide–semiconductor (CMOS), also known as complementary-symmetry metal–oxide–semiconductor (COS-MOS), is a type of MOSFET fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFETs for logic functions. CMOS technology is used for constructing integrated circuits (ICs), including microprocessors, microcontrollers, memory chips, and other digital logic circuits. CMOS technology is also used for analog circuits such as image sensors, data converters, RF circuits, and highly integrated transceivers for many types of communication.


Die of the first 555 chip (1971) Die of the first 555 chip.jpg
Die of the first 555 chip (1971)

The IC was designed in 1971 by Hans R. Camenzind under contract to Signetics, later acquired by Philips Semiconductors, now NXP. [3]

In 1962, Camenzind joined PR Mallory's Laboratory for Physical Science in Burlington, Massachusetts. [5] He designed a pulse-width modulation (PWM) amplifier for audio applications; [8] however, it was not successful in the market because there was no power transistor included. He became interested in tuners such as a gyrator and a phase-locked loop (PLL). He was hired by Signetics to develop a PLL IC in 1968. He designed an oscillator for PLLs such that the frequency did not depend on the power supply voltage or temperature. Signetics subsequently laid off half of its employees due to a recession; development on the PLL was thus frozen. [9]

Burlington, Massachusetts Town in Massachusetts, United States

Burlington is a town in Middlesex County, Massachusetts, United States. The population was 24,498 at the 2010 census..

Pulse-width modulation modulation technique

Pulse width modulation (PWM), or pulse-duration modulation (PDM), is a method of reducing the average power delivered by an electrical signal, by effectively chopping it up into discrete parts. The average value of voltage fed to the load is controlled by turning the switch between supply and load on and off at a fast rate. The longer the switch is on compared to the off periods, the higher the total power supplied to the load. Along with MPPT maximum power point tracking, it is one of the primary methods of reducing the output of solar panels to that which can be utilized by a battery. PWM is particularly suited for running inertial loads such as motors, which are not as easily affected by this discrete switching, because they have inertia to react slow. The PWM switching frequency has to be high enough not to affect the load, which is to say that the resultant waveform perceived by the load must be as smooth as possible.

Gyrator analog circuit

A gyrator is a passive, linear, lossless, two-port electrical network element proposed in 1948 by Bernard D. H. Tellegen as a hypothetical fifth linear element after the resistor, capacitor, inductor and ideal transformer. Unlike the four conventional elements, the gyrator is non-reciprocal. Gyrators permit network realizations of two-(or-more)-port devices which cannot be realized with just the conventional four elements. In particular, gyrators make possible network realizations of isolators and circulators. Gyrators do not however change the range of one-port devices that can be realized. Although the gyrator was conceived as a fifth linear element, its adoption makes both the ideal transformer and either the capacitor or inductor redundant. Thus the number of necessary linear elements is in fact reduced to three. Circuits that function as gyrators can be built with transistors and op-amps using feedback.

Camenzind proposed the development of a universal circuit based on the oscillator for PLLs and asked that he develop it alone, borrowing equipment from Signetics instead of having his pay cut in half. Other engineers argued the product could be built from existing parts; however, the marketing manager approved the idea. Among 5xx numbers that were assigned for analog ICs, the part number "555" was chosen. [5] [9]

Camenzind also taught circuit design at Northeastern University in the morning attending the university himself at night working toward a Master's degree in Business Administration. [10] The first design for the 555 was reviewed in the summer of 1971. Assessed to be without error, it proceeded to layout design. A few days later, Camenzind got the idea of using a direct resistance instead of a constant current source finding later that it worked. The change decreased the required 9 pins to 8 so the IC could be fit in an 8-pin package instead of a 14-pin package. This revised design passed a second design review with the prototype completed in October 1971. The 9-pin copy had been already released by another company founded by an engineer who attended the first review and retired from Signetics; that firm withdrew its version soon after the 555 was released. The 555 timer was manufactured by 12 companies in 1972 and it became a best selling product. [9]

Northeastern University Private university in Boston, Massachusetts, USA

Northeastern University is a private research university in Boston, Massachusetts, established in 1898. It is categorized as an R1 institution by the Carnegie Classification of Institutions of Higher Education. The university offers undergraduate and graduate programs on its main campus in Boston. The university has satellite campuses in Charlotte, North Carolina; Seattle, Washington; San Jose, California; and Toronto, Canada, that exclusively offer graduate degrees. Northeastern recently purchased the New College of the Humanities in London and plans to open an additional campus in Vancouver, Canada. The university's enrollment is approximately 18,000 undergraduate students and 8,000 graduate students.

Part name

It has been falsely hypothesized that the 555 got its name from the three 5  resistors within the bipolar IC. [11] Hans Camenzind has stated that the part number was arbitrary, [5] thus it was simply a coincidence they matched. The "NE" and "SE" prefix letters of the original Signetic parts numbers, NE555 and SE555, were temperature designations for analog chips, where "NE" was commercial temperature family and "SE" was military temperature family.


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 (DIP-8). [12] Variants available include the 556 (a DIP-14 combining two complete 555s on one chip), [13] and 558 / 559 (both a DIP-16 combining four reduced-functionality timers on one chip). [2]

The NE555 parts were commercial temperature range, 0 °C to +70 °C, and the SE555 part number designated the military temperature range, −55 °C to +125 °C. These were available in both high-reliability metal can (T package) and inexpensive epoxy plastic (V package) packages. Thus the full part numbers were NE555V, NE555T, SE555V, and SE555T.

Low-power CMOS versions of the 555 are also available, such as the Intersil ICM7555 and Texas Instruments LMC555, TLC555, TLC551. [14] [15] [16] [17] CMOS timers use significantly less power than bipolar timers; CMOS timers also cause less supply noise than bipolar version when the output switches states. The ICM7555 datasheet claims that it usually doesn't require a "control" capacitor and in many cases does not require a decoupling capacitor across the power supply pins. A decoupling capacitor reduces noise or variation in power supply voltage that may affect other parts of a circuit or affect the timer's threshold voltages.

Internal schematic

The internal block diagram and schematic of the 555 timer are highlighted with the same color across all three drawings to clarify how the chip is implemented: [2]


The typical pinout of the 555 and 556 IC packages are as follows: [2] [1] [18]

555 Pin#556 Pin#Pin namePin directionPin purpose [2]
17GNDPowerGround supply: this pin is the ground reference voltage (zero volts).
26, 8TRIGInputTrigger: when the voltage at this pin falls below 12 of CONT pin voltage (13VCC except when CONT is driven by an external signal), the OUT pin goes high and a timing interval starts. As long as this pin continues to be kept at a low voltage, the OUT pin will remain high.
35,9OUTOutputOutput: this is a push-pull (P.P.) output that is driven to either a low state (ground supply at GND pin) or a high state (positive supply at VCC pin minus approximately 1.7 Volts). (Note: For CMOS timers, the high state is driven to VCC.) When bipolar timers are used in applications where the output drives a TTL input, a 100 to 1000 pF decoupling capacitor may need to be added to prevent double triggering. [2]
44,10RESETInputReset: a timing interval may be reset by driving this pin to GND, but the timing does not begin again until this pin rises above approximately 0.7 Volts. This pin overrides TRIG (trigger), which overrides THRES (threshold). In most applications this pin is not used, thus it should be connected to VCC to prevent electrical noise causing a reset.
53,11CONTInputControl (or Control Voltage): this pin provides access to the internal voltage divider (23VCC by default). By applying a voltage to the CONT input one can alter the timing characteristics of the device. In most applications this pin is not used, thus a 10 nF decoupling capacitor (film or C0G) should be connected between this pin and GND to ensure electrical noise doesn't affect the internal voltage divider. [2] This control pin input can be used to build an astable multivibrator with a frequency-modulated output.
62,12THRESInputThreshold: when the voltage at this pin is greater than the voltage at CONT pin (23VCC except when CONT is driven by an external signal), then the timing (OUT high) interval ends.
71,13DISCHOutputDischarge: this is an open-collector (O.C.) output (CMOS timers are open-drain), which can be used to discharge a capacitor between intervals, in phase with output.
814VCCPowerPositive supply: the guaranteed voltage range of bipolar timers is typically 4.5 to 15 Volts (some timers are spec'ed for up to 16 Volts or 18 Volts), though most will operate as low as 3 Volts. (Note: CMOS timers have a lower minimum voltage rating, which varies depending on the part number.) See the supply min and max columns in the derivatives table. For bipolar timers, a decoupling capacitor is required because of current surges during output switching. [2]


The IC 555 has three operating modes:

  1. Astable (free-running) mode – the 555 can operate as an electronic oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation and so on. The 555 can be used as a simple ADC, converting an analog value to a pulse length (e.g., selecting a thermistor as timing resistor allows the use of the 555 in a temperature sensor and the period of the output pulse is determined by the temperature). The use of a microprocessor-based circuit can then convert the pulse period to temperature, linearize it and even provide calibration means.
  2. Monostable mode – in this mode, the 555 functions as a "one-shot" pulse generator. Applications include timers, missing pulse detection, bounce-free switches, touch switches, frequency divider, capacitance measurement, pulse-width modulation (PWM) and so on.
  3. Bistable mode – the 555 can operate as a flip-flop or schmitt trigger, if the DIS pin is not connected and no capacitor is used. Uses include bounce-free latched switches.


Schematic of a 555 in astable mode 555 Astable Diagram.svg
Schematic of a 555 in astable mode
Waveform in astable mode (french) NE555 Astable Waveforms fr.png
Waveform in astable mode (french)

In astable configuration, the 555 timer puts out a continuous stream of rectangular pulses having a specific frequency. The astable configuration is implemented using two resistors, and , and one capacitor . In this configuration, the control pin is not used, thus it is connected to ground through a 10 nF decoupling capacitor to shunt electrical noise. The threshold and trigger pins are connected to the capacitor , thus they have the same voltage. Initially, the capacitor is not charged, thus the trigger pin receives zero voltage which is less than third of the supply voltage. Consequently, the trigger pin causes the output to go high and the internal discharge transistor to go to cut-off mode. Since the discharge pin is no longer short circuited to ground, the current flows through the two resistors, and , to the capacitor charging it. The capacitor starts charging till the voltage becomes two-thirds of the supply voltage. At this instance, the threshold pin causes the output to go low and the internal discharge transistor to go into saturation mode. Consequently, the capacitor starts discharging through till it becomes less than third of the supply voltage, in which case, the trigger pin causes the output to go high and the internal discharge transistor to go to cut-off mode once again. And the cycle repeats.

In the first pulse, the capacitor charges from zero to two-thirds of the supply voltage, however, in later pulses, it only charges from one-third to two-thirds of the supply voltage. Consequently, the first pulse have a longer high time interval compared to later pulses. Moreover, the capacitor charges through both resistors but only discharges through , thus the high interval is longer than the low interval. This is shown in the following equations.

The high time interval of each pulse is given by:

The low time interval of each pulse is given by:

Hence, the frequency of the pulse is given by:


where is in seconds (time), is in ohms (resistance), is in farads (capacitance), is the natural log of 2 multiplier that might be shown as a rounded constant of 0.7 or 0.69 or 0.693 in some 555 documents.

power capability of must be greater than , per Ohm's law.

Particularly with bipolar 555s, low values of must be avoided so that the output stays saturated near zero volts during discharge, as assumed by the above equation. Otherwise the output low time will be greater than calculated above. The first cycle will take appreciably longer than the calculated time, as the capacitor must charge from 0V to 23 of VCC from power-up, but only from 13 of VCC to 23 of VCC on subsequent cycles.

To have an output high time shorter than the low time (i.e., a duty cycle less than 50%) a fast diode (i.e. 1N4148 signal diode) can be placed in parallel with R2, with the cathode on the capacitor side. This bypasses R2 during the high part of the cycle so that the high interval depends only on R1 and C, with an adjustment based the voltage drop across the diode. The voltage drop across the diode slows charging on the capacitor so that the high time is a longer than the expected and often-cited ln(2)*R1C = 0.693 R1C. The low time will be the same as above, 0.693 R2C. With the bypass diode, the high time is

where Vdiode is when the diode's "on" current is 12 of Vcc/R1 which can be determined from its datasheet or by testing. As an extreme example, when Vcc= 5 and Vdiode= 0.7, high time = 1.00 R1C which is 45% longer than the "expected" 0.693 R1C. At the other extreme, when Vcc= 15 and Vdiode= 0.3, the high time = 0.725 R1C which is closer to the expected 0.693 R1C. The equation reduces to the expected 0.693 R1C if Vdiode= 0.

The operation of RESET in this mode is not well-defined. Some manufacturers' parts will hold the output state to what it was when RESET is taken low, others will send the output either high or low.

The astable configuration, with two resistors, cannot produce a 50% duty cycle. To produce a 50% duty cycle, eliminate R1, disconnect pin 7 and connect the supply end of R2 to pin 3, the output pin. This circuit is similar to using an inverter gate as an oscillator, but with fewer components than the astable configuration, and a much higher power output than a TTL or CMOS gate. The duty cycle for either the 555 or inverter-gate timer will not be precisely 50% and will change depending on the load that the output is also driving while high (longer duty cycles for greater loads) due to the fact the timing network is supplied from the device's output pin, which has different internal resistances depending on whether it is in the high or low state (high side drivers tend to be more resistive). An alternate method to set the duty cycle practically, is to connect a diode parallel to pin 6 & 7. The operation of the diode when connected is explained above. The resultant duty cycle is given as D=R2/(R1+R2). If a potentiometer is used to supply R1 and R2, R1 + R2 is constant. The duty cycle then varies with the potentiometer at a constant frequency. A series resistor of 100 ohms must be added to each R1 and R2 to limit peak current of the transistor(within) when R1 and R2 are at minimum level. This method of adding a diode has a restriction of choosing R1 and R2 values. An alternate way is to add a JK flip-flop to the output of non-symmetrical square wave generator. But, with this the output frequency is one half of the timer.


Schematic of a 555 in monostable mode 555 Monostable.svg
Schematic of a 555 in monostable mode
Waveform in monostable mode NE555 Monotable Waveforms (English).png
Waveform in monostable mode

In monostable mode, the output pulse ends when the voltage on the capacitor equals 23 of the supply voltage. The output pulse width can be lengthened or shortened to the need of the specific application by adjusting the values of R and C. [20]

Assume initially the output of the monostable is zero, the output of flip-flop(Q bar) is 1 so that the discharging transistor is on and voltage across capacitor is zero. One of the input of upper comparator is at 2/3 of supply voltage and other is connected to capacitor. For lower comparator, one of the input is trigger pulse and other is connected at 1/3 of supply voltage. Now the capacitor charges towards supply voltage(Vcc). When the trigger input is applied at trigger pin the output of lower comparator is 0 and upper comparator is 0. The output of flip-flop remains unchanged therefore the output is 0. when the voltage across capacitor crosses the 1/3 of the vcc the output of lower comparator changes from 0 to 1. Therefore, the output of monostable is one and the discharging transistor is still off and voltage across capacitor charges towards vcc from 1/3 of vcc,

When the voltage across capacitor crosses 2/3 of VCC, the output of upper comparator changes from 0 to 1, therefore the output of monostable is 0 and the discharging transistor is on and capacitor discharges through this transistor as it offers low resistance path. The cycle repeats continuously. The charging and discharging of capacitor depends on the time constant RC.

The voltage across capacitor is given by vc = Vcc(1-e^(-t/RC)) at t=T. Since vc =(2/3)Vcc, therefore 2/3Vcc = Vcc(1-e^(-T/RC)), thus reduced to T = RC ln(3) seconds.

The output pulse width of time t, which is the time it takes to charge C to 23 of the supply voltage, is given by

where is in seconds (time), is in ohms (resistance), is in farads (capacitance), is the natural log of 3 multiplier that might be shown as a rounded constant of 1.1 or 1.099 in some 555 documents.

While using the timer IC in monostable mode, the main disadvantage is that the time span between any two triggering pulses must be greater than the RC time constant. [21] Conversely, ignoring closely spaced pulses is done by setting the RC time constant to be larger than the span between spurious triggers. (Example: ignoring switch contact bouncing.)



Schematic of a 555 in bistable flip-flop mode. High-value pull-up resistors should be added to the two inputs. 555 Bistabiel digitaal.svg
Schematic of a 555 in bistable flip-flop mode. High-value pull-up resistors should be added to the two inputs.
Inverted SR flip-flop symbol is similar to this circuit, though /Q should be removed Inverted SR Flip-flop.svg
Inverted SR flip-flop symbol is similar to this circuit, though /Q should be removed

In bistable mode, the 555 timer acts as a basic SR flip-flop. The trigger and reset inputs (pins 2 and 4 respectively on a 555) are held high via pull-up resistors while the threshold input (pin 6) is grounded. Thus configured, pulling the trigger momentarily to ground acts as a 'set' and transitions the output pin (pin 3) to VCC (high state). Pulling the reset input to ground acts as a 'reset' and transitions the output pin to ground (low state). No timing capacitors are required in a bistable configuration. Pin 7 (discharge) is left unconnected, or may be used as an open-collector output. [22]

Schmitt trigger

Schematic of a 555 in bistable schmitt trigger mode 555 Bistabiel analoog.svg
Schematic of a 555 in bistable schmitt trigger mode
Inverted schmitt trigger lower symbol is similar to this cricuit Schmitt trigger inverted symbol.svg
Inverted schmitt trigger lower symbol is similar to this cricuit

A 555 timer can be used to create a basic schmitt trigger which converts a noisy input into a clean digital output. The input signal should be connected through a series capacitor which then connects to the trigger and threshold pins. A resistor divider, from VCC to GND, is connected to the previous tied pins. The reset pin is tied to VCC.


Texas Instruments NE555 in DIP-8 and SO-8 packages NE555 DIP & SOIC.jpg
Texas Instruments NE555 in DIP-8 and SO-8 packages

These specifications apply to the bipolar NE555. Other 555 timers can have different specifications depending on the grade (military, medical, etc.). These values should be considered "ball park" values; the current official datasheet from the exact manufacturer of each chip should be consulted instead for parameter limitation recommendations.

Supply voltage (VCC)4.5 to 15 V
Supply current (VCC = +5 V)3 to 6 mA
Supply current (VCC = +15 V)10 to 15 mA
Output current (maximum)200 mA
Maximum Power dissipation600 mW
Power consumption (minimum operating)30 mW@5V, 225 mW@15V
Operating temperature 0 to 75 °C


In 1972, Signetics originally released the 555 timer in DIP-8 and TO5-8 metal can packages, and the 556 timer was released in DIP-14 package. [4]

Currently, the 555 is available in through-hole packages as DIP-8 and SIP-8 (both 2.54mm pitch), [23] and surface-mount packages as SO-8 (1.27mm pitch), SSOP-8 / TSSOP-8 / VSSOP-8 (0.65mm pitch), BGA (0.5mm pitch). [1] The Microchip Technology MIC1555 is a 555 CMOS timer with 3 fewer pins available in SOT23-5 (0.95mm pitch) surface mount package. [24]

The dual 556 timer is available in through hole packages as DIP-14 (2.54mm pitch), [18] and surface-mount packages as SO-14 (1.27mm pitch) and SSOP-14 (0.65mm pitch).


Numerous companies have manufactured one or more variants of the 555, 556, 558 timers over the past decades as many different part numbers. The following is a partial list: AMD, California Eastern Labs, CEMI, Custom Silicon Solutions, Diodes Inc, ECG Philips, Estek, Exar, Fairchild, Gemini, GoldStar, Harris, HFO, Hitachi, IK Semicon, Intersil, JRC, Lithic Systems, Maxim, Micrel, MOS, Motorola, ON, Microchip, National, NEC, NTE Sylvania, NXP, Philips, Raytheon, RCA, Renesas, Sanyo, Signetics, Silicon General, Solid State Scientific, STMicroelectronics, Teledyne, TI, Unisonic, Wing Shing, X-REL, Zetex.

Min (Volt)
Max (Volt)
5V Supply
Iq (μA)
Max (MHz)
Custom Silicon SolutionsCSS555YesCMOS11. Voltage, Lowest Current
Internal EEPROM configuration

[25] [26]

Diodes Incorporated ZSCT155NoCMOS10.961500.33Lowest supply voltage


Intersil ICM7555YesCMOS1218401.0Lowest current of common parts


IntersilICM7556YesCMOS2218801.0Lowest current of common parts


Japan Radio Company NJM555YesBipolar14.51630000.1* SIP-8 package


Microchip Technology MIC1555YesCMOS1*2.7182405.0* SOT23-5 package


ON Semiconductor LM555YesBipolar14.51630000.1*


Signetics NE555NoBipolar14.51630000.1*First 555 timer
DIP-8 and TO5-8 package

[4] [13] [29] [2]

SigneticsNE556NoBipolar24.51660000.1*First 556 timer
DIP-14 package

[13] [2]

SigneticsNE558NoBipolar4*4.5184800*0.1*First 558 timer
DIP-16 package


Texas Instruments LM555YesBipolar14.51830000.1*


Texas InstrumentsLM556YesBipolar24.51660000.1*


Texas InstrumentsLMC555YesLinCMOS11.5151003.0 DSBGA-8 package (smallest 555)


Texas InstrumentsNE555YesBipolar14.51630000.1*Similar to Signetics NE555


Texas InstrumentsNE556YesBipolar24.51660000.1*Similar to Signetics NE556


Texas InstrumentsTLC551YesLinCMOS11151701.8Lowest voltage of active parts


Texas InstrumentsTLC552YesLinCMOS21153401.8Lowest voltage of active parts


Texas InstrumentsTLC555YesLinCMOS12151702.1


Texas InstrumentsTLC556YesLinCMOS22153402.1


X-REL SemiconductorXTR655Yes SOI 12.85.51704.0Extreme temperature (-60°C to +230°C),
ceramic DIP-8 package


Die of a NE556 dual timer manufactured by STMicroelectronics. STM-NE556-HD.jpg
Die of a NE556 dual timer manufactured by STMicroelectronics.
Die of a NE558D quad timer manufactured by Signetics. Signetics Corporation NE558D 0136O07 9331KK KOREA.jpg
Die of a NE558D quad timer manufactured by Signetics.
Table notes
Table manufacturer notes

Over the years, numerous IC companies have merged. The new parent company inherits everything from the previous company then datasheets and chip logos are changed over a period of time to the new company. This information is useful when tracking down datasheets for older parts. Instead of including every related company in the above table, only one name is listed, and the following list can be used to determine the relationship.

556 dual timer

The dual version is called 556. It features two complete 555s in a 14 pin package. Only the two power supply pins are shared between the two timers. [13] Bipolar version are currently available, such as the NE556 and LM556. [18] [30] CMOS versions are currently available, such as the Intersil ICM7556 and Texas Instruments TLC556 and TLC552, see derivatives table. [14] [32] [31]

558 quad timer

Pinout of 558 quad timer (16 pins). The 558 timers are different than 555 timer (obsolete part) NE558 pennen.svg
Pinout of 558 quad timer (16 pins). The 558 timers are different than 555 timer (obsolete part)

The quad version is called 558. It has four reduced-functionality timers in a 16 pin package (four complete 555 timer circuits would have required 26 pins). [2] Since the 558 is uniquely different than the 555 and 556, the 558 was not as popular. Currently the 558 is not manufactured by any major chip companies (possibly not by any companies), thus the 558 should be treated as obsolete. Parts are still available from a limited number of sellers as "new old stock" (N.O.S.). [34]

Partial list of differences between 558 and 555 chips: [2]

Example applications

Stepped tone generator

This circuit requires two 555 or one 556 to generate a variety of sounds.

Joystick and game paddles

IBM PC Game Control Adapter
(8-bit ISA card) IBM PC Original Game Control Adapter.jpg
IBM PC Game Control Adapter
(8-bit ISA card)

The Apple II microcomputer used a quad timer 558 in monostable (or "one-shot") mode to interface up to four "game paddles" or two joysticks to the host computer. [36] It also used a single 555 for flashing the display cursor. [37]

The original IBM PC used a similar circuit for the game port on the "Game Control Adapter" 8-bit ISA card (IBM part number 1501300). [35] [38] In this joystick interface circuit, the capacitor of the RC network (see Monostable Mode above) was generally a 10 nF capacitor to ground with a series 2.2 KΩ resistor to the game port connector. [35] The external joystick was plugged into the adapter card. Internally it had two potentiometers (100 to 150 KΩ each), one for X and other for Y direction. The center wiper pin of the potentiometer was connected to an Axis wire in the cord and one end of the potentiometer was connected to the 5 Volt wire in the cord. The joystick potentiometer acted as a variable resistor in the RC network. [38] By moving the joystick, the resistance of the joystick increased from a small value up to about 100 kΩ. [38] [38]

Software running in the IBM PC computer started the process of determining the joystick position by writing to a special address (ISA bus I/O address 201h). [35] [38] [39] This would result in a trigger signal to the quad timer, which would cause the capacitor of the RC network to begin charging and cause the quad timer to output a pulse. The width of the pulse was determined by how long it took the capacitor to charge up to 23 of 5 V (or about 3.33 V), which was in turn determined by the joystick position. [38] [39] The software then measured the pulse width to determine the joystick position. A wide pulse represented the full-right joystick position, for example, while a narrow pulse represented the full-left joystick position. [38]

See also

Related Research Articles


A multivibrator is an electronic circuit used to implement a variety of simple two-state devices such as relaxation oscillators, timers and flip-flops. It consists of two amplifying devices cross-coupled by resistors or capacitors. The first multivibrator circuit, the astable multivibrator oscillator, was invented by Henri Abraham and Eugene Bloch during World War I. They called their circuit a "multivibrator" because its output waveform was rich in harmonics.

Operational amplifier electronic circuit or component serving as a high-gain voltage amplifier with a differential input and a single-ended output

An operational amplifier is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op-amp produces an output potential that is typically hundreds of thousands of times larger than the potential difference between its input terminals. Operational amplifiers had their origins in analog computers, where they were used to perform mathematical operations in many linear, non-linear, and frequency-dependent circuits.

Comparator device that compares two voltages or currents

In electronics, a comparator is a device that compares two voltages or currents and outputs a digital signal indicating which is larger. It has two analog input terminals and and one binary digital output . The output is ideally


A monostable multivibrator, also called a one shot, is an electronic circuit that generates an output pulse. When triggered, a pulse of pre-defined duration is produced. The circuit then returns to its stable state and produces no more output until triggered again.

Rectifier AC-DC conversion device; electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction.

Relaxation oscillator

In electronics a relaxation oscillator is a nonlinear electronic oscillator circuit that produces a nonsinusoidal repetitive output signal, such as a triangle wave or square wave. The circuit consists of a feedback loop containing a switching device such as a transistor, comparator, relay, op amp, or a negative resistance device like a tunnel diode, that repetitively charges a capacitor or inductor through a resistance until it reaches a threshold level, then discharges it again. The period of the oscillator depends on the time constant of the capacitor or inductor circuit. The active device switches abruptly between charging and discharging modes, and thus produces a discontinuously changing repetitive waveform. This contrasts with the other type of electronic oscillator, the harmonic or linear oscillator, which uses an amplifier with feedback to excite resonant oscillations in a resonator, producing a sine wave. Relaxation oscillators are used to produce low frequency signals for applications such as blinking lights and electronic beepers and in voltage controlled oscillators (VCOs), inverters and switching power supplies, dual-slope analog to digital converters, and function generators.

In electronics, a linear regulator is a system used to maintain a steady voltage. The resistance of the regulator varies in accordance with the load resulting in a constant voltage output. The regulating device is made to act like a variable resistor, continuously adjusting a voltage divider network to maintain a constant output voltage and continually dissipating the difference between the input and regulated voltages as waste heat. By contrast, a switching regulator uses an active device that switches on and off to maintain an average value of output. Because the regulated voltage of a linear regulator must always be lower than input voltage, efficiency is limited and the input voltage must be high enough to always allow the active device to drop some voltage.

Envelope detector

An envelope detector is an electronic circuit that takes a (relatively) high-frequency amplitude modulated signal as input and provides an output which is the envelope of the original signal.

Schmitt trigger

In electronics, a Schmitt trigger is a comparator circuit with hysteresis implemented by applying positive feedback to the noninverting input of a comparator or differential amplifier. It is an active circuit which converts an analog input signal to a digital output signal. The circuit is named a "trigger" because the output retains its value until the input changes sufficiently to trigger a change. In the non-inverting configuration, when the input is higher than a chosen threshold, the output is high. When the input is below a different (lower) chosen threshold the output is low, and when the input is between the two levels the output retains its value. This dual threshold action is called hysteresis and implies that the Schmitt trigger possesses memory and can act as a bistable multivibrator. There is a close relation between the two kinds of circuits: a Schmitt trigger can be converted into a latch and a latch can be converted into a Schmitt trigger.

Resistor–transistor logic (RTL) is a class of digital circuits built using resistors as the input network and bipolar junction transistors (BJTs) as switching devices. RTL is the earliest class of transistorized digital logic circuit used; other classes include diode–transistor logic (DTL) and transistor–transistor logic (TTL). RTL circuits were first constructed with discrete components, but in 1961 it became the first digital logic family to be produced as a monolithic integrated circuit. RTL integrated circuits were used in the Apollo Guidance Computer, whose design was begun in 1961 and which first flew in 1966.

Current source electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it; dual of a voltage source

A current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it.

Electronic component basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields

An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not to be confused with electrical elements, which are conceptual abstractions representing idealized electronic components.

Widlar current source

A Widlar current source is a modification of the basic two-transistor current mirror that incorporates an emitter degeneration resistor for only the output transistor, enabling the current source to generate low currents using only moderate resistor values.

This article illustrates some typical operational amplifier applications. A non-ideal operational amplifier's equivalent circuit has a finite input impedance, a non-zero output impedance, and a finite gain. A real op-amp has a number of non-ideal features as shown in the diagram, but here a simplified schematic notation is used, many details such as device selection and power supply connections are not shown. Operational amplifiers are optimised for use with negative feedback, and this article discusses only negative-feedback applications. When positive feedback is required, a comparator is usually more appropriate. See Comparator applications for further information.

An analog chip is a set of miniature electronic analog circuits formed on a single piece of semiconductor material.

LED circuit Electrical circuit involving a light emitting diode

In electronics, an LED circuit or LED driver is an electrical circuit used to power a light-emitting diode (LED). The circuit must provide sufficient current supply to light the LED at the required brightness, but must limit the current to prevent damaging the LED. The voltage drop across an LED is approximately constant over a wide range of operating current; therefore, a small increase in applied voltage greatly increases the current. Very simple circuits are used for low-power indicator LEDs. More complex, current source circuits are required when driving high-power LEDs for illumination to achieve correct current regulation.

Clamper (electronics)

A clamper is an electronic circuit that fixes either the positive or the negative peak excursions of a signal to a defined value by shifting its DC value. The clamper does not restrict the peak-to-peak excursion of the signal, it moves the whole signal up or down so as to place the peaks at the reference level. A diode clamp consists of a diode, which conducts electric current in only one direction and prevents the signal exceeding the reference value; and a capacitor, which provides a DC offset from the stored charge. The capacitor forms a time constant with the resistor load, which determines the range of frequencies over which the clamper will be effective.

Bipolar transistor biasing process necessary for BJT amplifiers to work correctly

Bipolar transistors must be properly biased to operate correctly. In circuits made with individual devices, biasing networks consisting of resistors are commonly employed. Much more elaborate biasing arrangements are used in integrated circuits, for example, bandgap voltage references and current mirrors. The voltage divider configuration achieves the correct voltages by the use of resistors in certain patterns. By selecting the proper resistor values, stable current levels can be achieved that vary only little over temperature and with transistor properties such as β.

Intersil ICL8038

The ICL8038 waveform generator was an Integrated circuit by Intersil designed to generate sine, square and triangular waveforms, based on bipolar monolithic technology involving Schottky barrier diodes. ICL8038 was a voltage-controlled oscillator capable of producing frequencies between a millihertz and 100kHz., some specimens capable of reaching 300kHz. The device has been discontinued by Intersil in 2002.

Capacitive power supply type of power supply using a capacitor

A capacitive power supply, also called a capacitive dropper, is a type of power supply that uses the capacitive reactance of a capacitor to reduce the mains voltage to a lower voltage. There are two important limitations: First, the high withstanding voltage required of the capacitor, along with the high-capacitance required for a given output current, mean that this type of supply is only practical for low-power applications. The second is that due to the absence of electrical isolation between input and output, anything connected to the power supply must be reliably insulated so that it is not possible for a person to come into electrical contact with it. By the equation of state for capacitance, where , the current is limited to: 1 amp, per farad, per volt-rms, per radian. Or amps, per farad, per volt-rms, per hertz.


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Further reading

Books with timer chapters