# 555 timer IC

Last updated
Type Signetics NE555 in 8-pin DIP package Active, Integrated circuit Hans Camenzind 1971 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]

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 transistors into a small chip results in circuits that are orders of magnitude smaller, cheaper, and faster 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.

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.

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. 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.

## Contents

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 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.

A bipolar junction transistor is a type of 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 two junctions between two semiconductor types, n-type and p-type.

Complementary metal–oxide–semiconductor (CMOS) is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. CMOS technology is also used for several analog circuits such as image sensors, data converters, and highly integrated transceivers for many types of communication. Frank Wanlass patented CMOS in 1963 while working for Fairchild Semiconductor.

## History

The IC was designed in 1971 by Hans R. Camenzind under contract to Signetics (later acquired by Philips Semiconductors, and 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] but 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. However, Signetics laid off half of its employees, and the development was frozen due to a recession. [9]

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

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 they react slower. 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.

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 would develop it alone, borrowing their equipment instead of having his pay cut in half. Other engineers argued the product could be built from existing parts, but the marketing manager bought the idea. Among 5xx numbers that were assigned for analogue ICs, the special number "555" was chosen. [5] [9]

Camenzind also taught circuit design at Northeastern University in the morning, and went to the same university at night to get a master's degree in Business Administration. [10] The first design was reviewed in the summer of 1971. There was no problem, so it proceeded to layout design. A few days later, he got the idea of using a direct resistance instead of a constant current source, and found 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 design passed the second design review, and the prototype was completed in October 1971. Its 9-pin copy had been already released by another company founded by an engineer who attended the first review and retired from Signetics, but they withdrew it soon after the 555 was released. The 555 timer was manufactured by 12 companies in 1972 and it became the best selling product. [9]

Northeastern University (NU) 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 the Fenway-Kenmore, Roxbury, South End, and Back Bay neighborhoods of Boston. The university has satellite campuses in Charlotte, North Carolina; Seattle, Washington; and San Jose, California, that exclusively offer graduate degrees. An additional satellite campus opened in Toronto, Ontario, Canada, in late 2016. 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 used within, [11] but Hans Camenzind has stated that the part number was arbitrary, [5] thus it's just a coincidence they matched. The "NE" and "SE" letters of the original parts numbers (NE555 and SE555) were temperature designations for analog chips from Signetics, where "NE" was commercial temperature family and "SE" was military temperature family.

## Design

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, also CMOS timers 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. However, for good design practices, a decoupling capacitor should be included, because noise produced by the timer or variation in power supply voltage might interfere with other parts of a circuit or influence its 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]

• Green: Between the positive supply voltage VCC and the ground GND is a voltage divider consisting of three identical resistors, which create two reference voltages at 13 VCC and 23 VCC. The latter is connected to the "Control Voltage" pin. All three resistors have the same resistance, 5 for bipolar timers, 100 kΩ (or higher resistance values) for CMOS timers. It is a false myth that the 555 IC got its name from these three 5 kΩ resistors. [5]
• Yellow: The comparator negative input is connected to the higher-reference voltage divider of 23 VCC (and "Control" pin), and comparator positive input is connected to the "Threshold" pin.
• Red: The comparator positive input is connected to the lower-reference voltage divider of 13 VCC, and comparator negative input is connected to the "Trigger" pin.
• Purple: An SR flip-flop stores the state of the timer and is controlled by the two comparators. The "Reset" pin overrides the other two inputs, thus the flip-flop (and therefore the entire timer) can be reset at any time.
• Pink: The output of the flip-flop is followed by an output stage with push-pull (P.P.) output drivers that can load the "Output" pin with up to 200 mA (varies by device).
• Cyan: Also, the output of the flip-flop turns on a transistor that connects the "Discharge" pin to ground.

### Pinout

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]

## Modes

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 (schmitt trigger) mode – the 555 can operate as a flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bounce-free latched switches.

### Astable

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, ${\displaystyle R_{1}}$ and ${\displaystyle R_{2}}$, and one capacitor ${\displaystyle C}$. 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 ${\displaystyle C}$, thus they have the same voltage. Initially, the capacitor ${\displaystyle C}$ is not charged, thus the trigger pin receive 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, ${\displaystyle R_{1}}$ and ${\displaystyle R_{2}}$, to the capacitor charging it. The capacitor ${\displaystyle C}$ 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 ${\displaystyle R_{2}}$ 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.

It should be noted that 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 ${\displaystyle R_{2}}$, thus the high interval is longer than the low interval. This is shown in the following equations. Where the high interval of each pulse is given by:

${\displaystyle \mathrm {high} =\ln(2)\cdot C\cdot (R_{1}+R_{2})}$

And the low interval of each pulse is given by:

${\displaystyle \mathrm {low} =\ln(2)\cdot C\cdot R_{2}}$

Hence, the frequency ${\displaystyle f}$ of the pulse is given by:

${\displaystyle f={\frac {1}{\ln(2)\cdot C\cdot (R_{1}+2R_{2})}}}$ [19]

The power capability of R1 must be greater than ${\displaystyle {\frac {V_{cc}^{2}}{R_{1}}}}$.

Particularly with bipolar 555s, low values of ${\displaystyle R_{1}}$ 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

${\displaystyle \mathrm {high} =R_{1}\cdot C\cdot \ln \left({\frac {2V_{\textrm {cc}}-3V_{\textrm {diode}}}{V_{\textrm {cc}}-3V_{\textrm {diode}}}}\right)}$

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.

### Monostable

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, vc =(2/3)Vcc therefore, 2/3Vcc=Vcc(1-e^(-T/RC)), T=RC ln(3), T=1.1 RC (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

${\displaystyle t=\ln(3)\cdot RC\approx 1.1RC}$

where t is in seconds, R is in ohms (resistance) and C is in farads (capacitance).

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.)

### Bistable

In bistable mode, the 555 timer acts as a basic 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]

A 555 timer can be used to create a 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.

## Specifications

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, instead the current official datasheet from the exact manufacturer of each chip should be consulted 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 dissipation 600 mW Power consumption (minimum operating) 30 mW@5V, 225 mW@15V Operating temperature 0 to 75 °C

## Packages

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).

## Derivatives

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.

ManufacturerPart
Number
Production
Active
IC
Process
Timer
Total
Supply
Min (Volt)
Supply
Max (Volt)
5V Supply
Iq (μA)
Frequency
Max (MHz)
RemarkDatasheet
Custom Silicon SolutionsCSS555YesCMOS11.25.54.31.0Low Voltage, Lowest Current
Internal EEPROM configuration
Diodes Incorporated ZSCT155NoCMOS10.961500.33Lowest supply voltage

[27]

Intersil ICM7555YesCMOS1218401.0Lowest current of common parts

[14]

IntersilICM7556YesCMOS2218801.0Lowest current of common parts

[14]

Japan Radio Company NJM555YesBipolar14.51630000.1* SIP-8 package

[23]

Microchip Technology MIC1555YesCMOS1*2.7182405.0* SOT23-5 package

[24]

ON Semiconductor LM555YesBipolar14.51630000.1*

[28]

Signetics NE555NoBipolar14.51630000.1*First 555 timer
DIP-8 and TO5-8 package
SigneticsNE556NoBipolar24.51660000.1*First 556 timer
DIP-14 package
SigneticsNE558NoBipolar4*4.5184800*0.1*First 558 timer
DIP-16 package

[2]

Texas Instruments LM555YesBipolar14.51830000.1*

[21]

Texas InstrumentsLM556YesBipolar24.51660000.1*

[30]

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

[15]

Texas InstrumentsNE555YesBipolar14.51630000.1*Similar to Signetic NE555

[1]

Texas InstrumentsNE556YesBipolar24.51660000.1*Similar to Signetic NE556

[18]

Texas InstrumentsTLC551YesLinCMOS11151701.8Lowest voltage of active parts

[17]

Texas InstrumentsTLC552YesLinCMOS21183402.8Lowest voltage of active parts

[31]

Texas InstrumentsTLC555YesLinCMOS12151702.1

[16]

Texas InstrumentsTLC556YesLinCMOS22153402.1

[32]

X-REL SemiconductorXTR655Yes12.85.51704.0Extreme temp (-60°C to +230°C)

[33]

Table notes
• All information in the above table was pulled from references in the datasheet column, except where denoted below.
• For "Timer Total" column, a "*" denotes parts that are missing 555 timer features.
• For "Iq" column, a 5 volt supply was chosen as a common voltage to make it easier to compare. The value for Signetics NE558 is an estimate, because NE558 datasheets don't state Iq at 5V. [2] The value listed in this table was estimated by comparing the 5V to 15V ratio of other bipolar datasheets, then derating the 15V parameter for the NE558 part, which is denoted by the "*".
• For "Frequency Max" column, a "*" denotes values that may not be the actual maximum frequency limit of the part. The MIC1555 datasheet discusses limitations from 1 to 5 MHz. [24] Though most bipolar timers don't state the maximum frequency in their datasheets, they all have a maximum frequency limitation of hundreds of kHz across their full temperature range. Section 8.1 of the Texas Instruments NE555 datasheet [1] states a value of 100 kHz, and their website shows a value of 100 kHz in timer comparison tables, which is overly conservative. In Signetics App Note 170, states that most devices will oscillate up to 1 MHz, however when considering temperature stability it should be limited to about 500 kHz. [2]
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]

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]

• One VCC and one GND, similar to 556 chip.
• Four "Reset" are tied together internally to one external pin (558).
• Four "Control Voltage" are tied together internally to one external pin (558).
• Four "Triggers" are falling-edge sensitive (558), instead of level sensitive (555).
• Two resistors in the voltage divider (558), instead of three resistors (555).
• One comparator (558), instead of two comparators (555).
• Four "Output" are open-collector (O.C.) type (558), instead of push-pull (P.P.) type (555). Since the 558 outputs are open-collector, pull-up resistors are required to "pull up" the output to the positive voltage rail when the output is in a high state. This means the high state only sources a small amount of current through the pull-up resistor.

## Example applications

### Stepped tone generator

This example uses one 556 or two 555 chips.

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]

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Books
Books with timer chapters
Datasheets
• see links in "Derivatives" section and "References" section above