Editing 555 timer IC

{{Short description|Integrated circuit used for timer applications}}
{{Lead too short|date=January 2024}}
{{Infobox electronic component
|name           = 555 timer IC
|image          = Signetics NE555N.JPG
|image_size     = 250
|caption        = [[Signetics]] NE555 in 8-pin [[Dual in-line package|DIP package]]
|type           = [[Passivity (engineering)|Active]], [[integrated circuit]]
|invented       = [[Hans Camenzind]] (1971)
|first_produced = 1972
|symbol         = [[File:555 esquema.png|250px]]
|symbol_caption = Internal block diagram<ref name="TI_NE555_Datasheet"/>
}}

The '''555 timer IC''' is an [[integrated circuit]] used in a variety of [[timer]], delay, pulse generation, and [[Electronic oscillator|oscillator]] applications. It is one of the most popular timing ICs due to its flexibility and price. Derivatives provide two ([[#556 dual timer|556]]) or four ([[#558 quad timer|558]]) timing circuits in one package.<ref name="Signetics_1985_Linear_Databook"/> The design was first marketed in 1972 by [[Signetics]]<ref name="EEtimes_20120815">{{cite news|last1=Fuller|first1=Brian|date=15 August 2012|title=Hans Camenzind, 555 timer inventor, dies|work=EE Times|url=http://www.eetimes.com/document.asp?doc_id=1262353|access-date=27 December 2016}}</ref><ref name="Signetics_1972_Linear1_Databook" /> and used [[bipolar junction transistor]]s. Since then, numerous companies have made the original timers and later similar low-power [[CMOS]] timers. In 2017, it was said that over a billion 555 timers are produced annually by some estimates, and that the design was "probably the most popular integrated circuit ever made".<ref name="Dummies">{{Cite book|last=Lowe|first=Doug|url=https://books.google.com/books?id=Ivn-DQAAQBAJ&pg=PA339|title=Electronics All-in-One For Dummies|date=2017-02-06|publisher=Wiley |isbn=978-1-119-32079-1|pages=339|quote=The 555 timer chip, developed in 1970, is probably the most popular integrated circuit ever made. By some estimates, more than a billion of them are manufactured every year.}}</ref>

== History ==
[[File:Die of the first 555 chip.jpg|thumb|Silicon die of the first 555 chip (1971)]]
[[File:NXP-7555-HD.jpg|thumb|Die of a CMOS NXP ICM7555 chip]]
The timer IC was designed in 1971 by [[Hans Camenzind]] under contract to [[Signetics]].<ref name="EEtimes_20120815"/>  In 1968, he was hired by Signetics to develop a [[phase-locked loop]] (PLL) IC.  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 the [[Recession of 1969–70|1970 recession]], and development on the PLL was thus frozen.<ref name="Birth">{{Cite journal|last=Carmenzind|first=Hans|translator-last=三宅|translator-first=和司|year=2010|title=タイマIC 555 誕生秘話|trans-title=The birth of the 555 timer IC|journal=トランジスタ技術 (Transistor Technology)|language=ja|publisher=CQ出版|volume=47|issue=12|pages=73, 74|issn=0040-9413}}</ref>  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. Camenzind's idea was originally rejected, since other engineers argued the product could be built from existing parts sold by the company; however, the marketing manager approved the idea.<ref>{{Cite journal |first=Brian |last=Santo|url=https://spectrum.ieee.org/25-microchips-that-shook-the-world|title = 25 Microchips That Shook the World|date = May 2009 |journal=IEEE Spectrum|volume=46|issue=5|pages=34–43|doi=10.1109/MSPEC.2009.4907384|s2cid=20539726|url-access=subscription}}</ref>

The first design for the 555 was reviewed in the summer of 1971.<ref name="Redesigning" /> After this design was tested and found to be without errors, Camenzind got the idea of using a direct resistance instead of a constant current source, finding that it worked satisfactorily.<ref name="Redesigning" /> The design change decreased the required 9 external pins to 8, so the IC could be fit in an 8-pin package instead of a 14-pin package.<ref name="Redesigning" /> This revised version passed a second design review, and the prototypes were completed in October 1971 as the NE555V (plastic [[Dual in-line package|DIP]]) and SE555T (metal [[TO-5]]).<ref name="semiconductormuseum-interview">{{cite web |url=http://www.semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Index.htm |last=Ward |first=Jack |date=2004 |title=The 555 Timer IC – An Interview with Hans Camenzind |publisher=The Semiconductor Museum |access-date=2010-04-05}}</ref>  The 9-pin version had already been released by another company founded by an engineer who had attended the first review and had 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.<ref name="Birth"/>

The 555 found many applications beyond timers. Camenzind noted in 1997 that "nine out of 10 of its applications were in areas and ways I had never contemplated. For months I was inundated by phone calls from engineers who had new ideas for using the device."<ref name="Redesigning" />

=== Name ===
Several books report the name "555" timer IC derived from the three 5 kΩ resistors inside the chip.<ref>{{Cite book|last1=Scherz|first1=Paul|url=https://books.google.com/books?id=lhSCCwAAQBAJ|title=Practical Electronics for Inventors |edition=4th |last2=Monk|first2=Simon|date=2016 |publisher=McGraw Hill |isbn=978-1-259-58755-9|pages=687|quote=The 555 gets its name from the three 5-kW +VCC R1 discharging path 555 R 2 C 6 resistors shown in the block diagram. These resistors act as a three-step voltage.}}</ref><ref>{{Cite book|last=Kleitz|first=William|title=Digital electronics : a practical approach|publisher=Prentice Hall|year=1990|isbn=0-13-211657-X|edition=2nd|pages=401|oclc=20218185|quote=The 555 got its name from the three 5-kOhm resistors}}</ref><ref>{{Cite book|last=Simpson|first=Colin D. |title=Industrial electronics|publisher=Prentice Hall|year=1996|isbn=0-02-410622-4|pages=357|oclc=33014077|quote=The reference voltage for the comparators is established by a voltage divider consisting of three 5 - k2 resistors, which is where the name 555 is derived}}</ref> However, in a recorded interview with an online transistor museum curator,<ref>{{Cite journal |first=Harry |last=GoldStein |date=March 3, 2003|title=The Irresistible Transistor|url=https://spectrum.ieee.org/the-irresistible-transistor|access-date=2020-08-29|journal=IEEE Spectrum|volume=40 |issue=3 |pages=42–47 |doi=10.1109/MSPEC.2003.1184435 |url-access=subscription }}</ref> Hans Camenzind  said "It was just arbitrarily chosen. It was Art Fury (marketing manager) who thought the circuit was gonna sell big who picked the name '555' timer IC."<ref>{{Cite web|title=Oral History Hans Camenzind Historic 555 IC Page2|url=http://www.semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Page2.htm|access-date=2020-08-28|publisher=The Semiconductor Museum}}</ref>

==Design==
Depending on the manufacturer, the standard 555 package incorporated the equivalent of 25 [[transistor]]s, 2 [[diode]]s, and 15 [[resistor]]s on a [[silicon]] chip packaged into an 8-pin [[dual in-line package]] (DIP-8).<ref>{{cite web |title=Oral History Hans Camenzind Historic 555 Integrated Circuit Page6 |url=http://www.semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Page6.htm |website=Semiconductor Museum |access-date=2022-02-27}}</ref> Variants available included the 556 (a DIP-14 combining two complete 555s on one chip),<ref name="Signetics_1973_555-556_Databook"/> and 558 / 559 (both variants were a DIP-16 combining four reduced-functionality timers on one chip).<ref name="Signetics_1985_Linear_Databook"/>

The '''NE555''' parts were commercial temperature range, 0&nbsp;°C to +70&nbsp;°C, and the '''SE555''' part number designated the military temperature range, −55&nbsp;°C to +125&nbsp;°C. These chips were available in both high-reliability metal can (T package) and inexpensive epoxy plastic (V package) form factors. Thus, the full part numbers were NE555V, NE555T, SE555V, and SE555T.

Low-power CMOS versions of the 555 are now available, such as the Intersil ICM7555 and Texas Instruments LMC555, TLC555, TLC551.<ref name="Intersil_ICM7555-556_Datasheet"/><ref name="TI_LMC555_Datasheet"/><ref name="TI_TLC555_Datasheet"/><ref name="TI_TLC551_Datasheet"/>

=== 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:<ref name="Signetics_1985_Linear_Databook"/>
* {{font color||#D5FFD5|Voltage divider}}: Between the positive [[Voltage source|supply voltage]] V<sub>CC</sub> and the [[Ground (electricity)|ground]] GND is a [[voltage divider]] consisting of three identical [[resistor]]s (5{{nbsp}}[[ohm|kΩ]] for bipolar timers, 100{{nbsp}}kΩ or higher for CMOS) to create reference voltages for the [[analog comparator]]s. CONTROL is connected between the upper two resistors, allowing an external voltage to control the reference voltages:
** When CONTROL is not driven, this divider creates an upper reference voltage of {{frac|2|3}} V<sub>CC</sub> and a lower reference voltage of {{frac|1|3}} V<sub>CC</sub>.
** When CONTROL is driven, the upper reference voltage will instead be V<sub>CONTROL</sub> and the lower reference voltage will be {{frac|1|2}} V<sub>CONTROL</sub>.
* {{font color||#FFEEAA|Threshold comparator}}: The [[comparator]]'s negative input is connected to voltage divider's upper reference voltage, and the comparator's positive input is connected to THRESHOLD.
* {{font color||#FFAAAA|Trigger comparator}}: The [[comparator]]'s positive input is connected to voltage divider's lower reference, and the comparator's negative input is connected to TRIGGER.
* {{font color||#AAAAFF|Latch}}: A [[Set-Reset latch|set-reset latch]] stores the [[State (computer science)|state]] of the timer and is controlled by the two comparators. RESET overrides the other two inputs, thus the latch (and therefore the entire timer) can be reset at any time.
* {{font color||#FFAAEE|Output}}: The output of the latch is followed by an output stage with [[Push–pull output|push{{ndash}}pull]] output drivers that can supply up to 200{{nbsp}}mA for bipolar timers, lower for CMOS timers.
* {{font color||#AAEEFF|Discharge}}: Also, the output of the latch controls a [[transistor]] acting as an [[electronic switch]] that connects DISCHARGE to ground.

<gallery mode="packed" widths="200px" heights="200px">
File:NE555 Bloc Diagram.svg|555 internal block diagram<ref name="TI_NE555_Datasheet"/>
File:NE555 Internal Circuit.svg|555 internal schematic of bipolar version
File:C555 Internal Circuit.svg|555 internal schematic of CMOS version
</gallery>

===Pinout===
The [[pinout]] of the 8-pin 555 timer<ref name="TI_NE555_Datasheet"/> and 14-pin 556 dual timer<ref name="TI_NE556_Datasheet"/> are shown in the following table.  Since the 556 is conceptually two 555 timers that share power pins, the pin numbers for each half are split across two columns.<ref name="Signetics_1985_Linear_Databook"/>

{| class="wikitable"
! 555 pin# !! 556 {{Nowrap|(unit 1)}} !! 556 {{Nowrap|(unit 2)}} !! Pin name !! Pin direction !! Pin description<ref name="TI_NE555_Datasheet"/><ref name="TI_NE556_Datasheet"/><ref name="Signetics_1985_Linear_Databook"/>
|-
| {{yes|1}} || colspan="2" | {{center|7}} || {{center|GND}} || {{center|Power}}
|| Ground supply: this pin is the [[Ground (electricity)#Electronics|ground]] reference voltage (zero volts).<ref name="Book-IC-Timer-Cookbook"/>
|-
| {{yes|2}} || {{center|6}} || {{center|8}} || {{center|{{overline|TRIGGER}}}} || {{center|Input}}
|| Trigger: when ''V''<sub>TRIGGER</sub> falls below {{Frac|1|2}} ''V''<sub>CONTROL</sub> ({{Frac|1|3}} ''V''<sub>CC</sub>, except when CONTROL is driven by an external signal), OUTPUT goes to the high state and a timing interval starts.<ref name="Book-IC-Timer-Cookbook"/> As long as TRIGGER continues to be kept at a low voltage, OUTPUT will remain in the high state.
|-
| {{yes|3}} || {{center|5}} || {{center|9}} || {{center|OUTPUT}} || {{center|Output}}
|| Output: this pin is a [[push-pull output|push-pull]] (P.P.) output that is driven to either a low state (GND) or a high state (''V''<sub>CC</sub> minus approximately 1.7 volts for bipolar timers, or ''V''<sub>CC</sub> for CMOS timers).  
|-
| {{yes|4}} || {{center|4}} || {{center|10}} || {{center|{{overline|RESET}}}} || {{center|Input}}
|| Reset: 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 {{overline|TRIGGER}}, which in turn overrides THRESHOLD. If this pin is not used, it should be connected to ''V''<sub>CC</sub> to prevent electrical noise accidentally causing a reset.<ref name="Book-TTL-Cookbook">{{cite book |last1=Lancaster |first1=Don |author-link=Don Lancaster |title=TTL Cookbook |date=1974 |publisher=Sams |isbn=978-0672210358 }}</ref><ref name="Book-IC-Timer-Cookbook">{{cite book |last1=Jung |first1=Walt |author-link=Walt Jung |title=IC Timer Cookbook |date=1977 |publisher=Sams Publishing |isbn=978-0672219320 |edition=1}}</ref>
|-
| {{yes|5}} || {{center|3}} || {{center|11}} || {{center|CONTROL}} || {{center|Input}}
|| Control: this pin provides access to the internal [[voltage divider]] ({{Frac|2|3}} ''V''<sub>CC</sub> by default). By applying a voltage to this pin, the timing characteristics can be changed.  In astable mode, this pin can be used to frequency-modulate the OUTPUT state.<ref name="Signetics_1973_555-556_Databook"/>  If this pin is not used, it should be connected to a 10 [[Farad|nF]] [[decoupling capacitor]] (between this pin and GND) to ensure electrical noise doesn't affect the internal voltage divider.<ref name="Signetics_1985_Linear_Databook"/><ref name="Book-TTL-Cookbook"/><ref name="Book-IC-Timer-Cookbook"/>
|-
| {{yes|6}} || {{center|2}} || {{center|12}} || {{center|THRESHOLD}} || {{center|Input}}
|| Threshold: when the voltage at this pin is greater than ''V''<sub>CONTROL</sub> ({{Frac|2|3}} ''V''<sub>CC</sub> by default except when CONTROL is driven by an external signal), then the OUTPUT high state timing interval ends, causing OUTPUT to go to the low state.<ref name="Book-IC-Timer-Cookbook"/>
|-
| {{yes|7}} || {{center|1}} || {{center|13}} || {{center|DISCHARGE}} || {{center|Output}}
|| Discharge: This pin is an [[open-collector]] (O.C.) output for bipolar timers, or an open-drain (O.D.) output for CMOS timers. This pin can be used to discharge a [[capacitor]] when OUTPUT is low. In bistable latch and bistable inverter modes, this pin is unused, which allows it to be used as an alternate output.<ref name="Book-IC-Timer-Cookbook"/>
|-
| {{yes|8}} || colspan="2" | {{center|14}} || {{center|''V''<sub>CC</sub>}} || {{center|Power}}
|| Positive supply: For bipolar timers, the supply voltage range is typically 4.5 to 16 volts (some are spec'ed for up to 18 volts, though most will operate as low as 3 volts).  For CMOS timers, the supply voltage range is typically 2 to 15 volts (some are spec'ed for up to 18 volts, and some are spec'ed as low as 1 volt).  See the supply min and max columns in the [[#Derivatives|derivatives table]] in this article. [[Decoupling capacitor]](s) are generally applied (between this pin and GND) as a good practice.<ref>{{Cite book|last=Carr|first=Joseph|url=https://books.google.com/books?id=DyX_dKLyZVwC&q=555+%22decoupling+capacitor%22&pg=PA119|title=Linear IC Applications: A Designer's Handbook|date=1996-12-19|publisher=Newnes|isbn=978-0-7506-3370-3|pages=119|language=en}}</ref><ref name="Book-TTL-Cookbook"/>
|}

<gallery class="center" widths="200px" heights="200px">
File:555 Pinout.svg| Pinout of 555 single timer<ref name="TI_NE555_Datasheet"/><ref name="Signetics_1985_Linear_Databook"/>
File:NE556 pennen.svg| Pinout of 556 dual timer<ref name="TI_NE556_Datasheet"/><ref name="Signetics_1985_Linear_Databook"/>
</gallery>

== Modes ==
The 555 IC has the following operating modes:
# [[Multivibrator#Astable|Astable]] (free-running) mode – The 555 operates as an [[electronic oscillator]]. Applications include:
#* [[Light emitting diode]] and lamp flashers, [[Pulse generator|pulse generation]], [[pulse-width modulation]] (PWM), logic clocks, tone generation, security alarms, [[pulse-position modulation]], etc.
#* [[Analog-to-digital converter|Analog-to-digital conversion]] (ADC) from an analog value represented by a resistance or capacitance into a digital pulse length.
#** e.g., selecting a [[thermistor]] as timing resistor allows the use of the 555 in a [[temperature sensor]] with the period of the output pulse determined by the [[temperature]]. A microprocessor can then convert the pulse period to temperature, linearize it, and even provide calibration.
# [[Multivibrator#Monostable|Monostable]] (one-shot) mode – The 555 operates as a "one-shot" [[pulse generator]]. Applications include:
#* timers, missing pulse detection, bounce-free switches, touch switches, frequency dividers, triggered measurement of resistance or capacitance, PWM, etc.
# [[Multivibrator#Bistable|Bistable]] (latch) mode – The 555 operates as a [[Set-Reset latch|set-reset latch]]. Applications include:
#* [[Debouncing|switch debouncing]].
# [[Schmitt trigger]] (inverter) mode – the 555 operates as a Schmitt trigger [[Inverter (logic gate)|inverter gate]]. Application:
#* Converts a noisy input into a clean digital output.

=== Astable ===
{{stack|[[File:555 Astable Diagram.svg|thumb|Schematic of a 555 timer in astable mode]]}}
{{stack|[[File:NE555 Astable Waveforms.svg|thumb|Waveform in astable mode]]}}
{{see also|Electronic oscillator}}
{| class="wikitable floatright" style="text-align: right"
|-
|+ Astable mode examples with [[E series of preferred numbers|common values]]
! [[Frequency]] !! [[Capacitance|C]] !! [[Electrical resistance and conductance|R<sub>1</sub>]] !! R<sub>2</sub> !! [[Duty cycle]]
|-
| 0.1{{nbsp}}Hz (+0.048%) || 100{{nbsp}}μF || 8.2{{nbsp}}kΩ || 68{{nbsp}}kΩ || 52.8%
|-
| 1{{nbsp}}Hz (+0.048%) || 10{{nbsp}}μF || 8.2{{nbsp}}kΩ || 68{{nbsp}}kΩ || 52.8%
|-
| 10{{nbsp}}Hz (+0.048%) || 1{{nbsp}}μF || 8.2{{nbsp}}kΩ || 68{{nbsp}}kΩ || 52.8%
|-
| 100{{nbsp}}Hz (+0.048%) || 100{{nbsp}}nF || 8.2{{nbsp}}kΩ || 68{{nbsp}}kΩ || 52.8%
|-
| 1{{nbsp}}kHz (+0.048%) || 10{{nbsp}}nF || 8.2{{nbsp}}kΩ || 68{{nbsp}}kΩ || 52.8%
|-
| 10{{nbsp}}kHz (+0.048%) || 1{{nbsp}}nF || 8.2{{nbsp}}kΩ || 68{{nbsp}}kΩ || 52.8%
|-
| 100{{nbsp}}kHz (+0.048%) || 100{{nbsp}}pF || 8.2{{nbsp}}kΩ || 68{{nbsp}}kΩ || 52.8%
|}<!-- NOTE: Common preferred values are best for examples. Capacitors are E3 series values, resistors are E6 or E12 series values. user:sbmeirow -->

In the astable configuration, the 555 timer puts out a continuous stream of rectangular pulses having a specific period.

The astable configuration is implemented using two resistors, <math>R_1</math> and <math>R_2 ,</math> and one capacitor <math>C</math>. The threshold and trigger pins are both connected to the capacitor; thus they have the same voltage.

Its repeated operating cycle (starting with the capacitor uncharged) is:

# Since the capacitor's voltage will be below {{Frac|1|3}}&nbsp;''V''<sub>CC</sub>, the trigger pin causes the 555's internal latch to change state, causing OUT to go high and the internal discharge transistor to cut-off.
# Since the discharge pin is no longer short-circuited to ground, the capacitor starts charging via current from Vcc through the resistors <math>R_1</math> and <math>R_2</math>.
# Once the capacitor charge reaches {{Frac|2|3}}&nbsp;Vcc, the threshold pin causes the 555's internal latch to change state, causing OUT to go low and the internal discharge transistor to go into saturation (maximal-conductivity) mode.
# This discharge transistor provides a discharge path, so the capacitor starts discharging through <math>R_2</math>.
# Once the capacitor's voltage drops below {{Frac|1|3}}&nbsp;''V''<sub>CC</sub>, the cycle repeats from step 1.

During the first pulse, the capacitor charges from 0&nbsp;V to {{Frac|2|3}}&nbsp;''V''<sub>CC</sub>, however, in later pulses, it only charges from {{Frac|1|3}}&nbsp;''V''<sub>CC</sub> to {{Frac|2|3}}&nbsp;''V''<sub>CC</sub>. Consequently, the first pulse has a longer high time interval compared to later pulses. Moreover, the capacitor charges through both resistors but only discharges through <math>R_2</math>, thus the output high interval is longer than the low interval. This is shown in the following equations:

The output high time interval of each pulse is given by:<ref name="Signetics_1973_555-556_Databook"/>

: <math>t_\text{high} = \ln(2) \cdot (R_1 + R_2) \cdot C</math>

The output low time interval of each pulse is given by:<ref name="Signetics_1973_555-556_Databook"/>

: <math>t_\text{low} = \ln(2) \cdot R_2 \cdot C</math>

Hence, the [[frequency]] <math>f</math> of the pulse is given by:<ref name="Signetics_1973_555-556_Databook"/>

: <math>f = \frac{1}{t_\text{high} + t_\text{low}} = \frac{1}{\ln(2) \cdot (R_1 + 2 \, R_2) \cdot C}</math>

and the [[duty cycle]] <math>D</math> is given by:<ref name="Signetics_1973_555-556_Databook"/>

: <math>D~(\%) = \frac{t_\text{high}}{t_\text{high} + t_\text{low}} \cdot 100 = \frac{R_1 + R_2}{R_1 + 2 \, R_2} \cdot 100</math>

where <math>t</math> is the time in [[second]]s, <math>R</math> is the resistance in [[ohm]]s, <math>C</math> is the capacitance in [[farad]]s, and <math>\ln(2)</math> is the [[natural logarithm of 2|natural log of 2]] constant.{{Efn|[[natural logarithm of 2|ln(2)]] is a constant, approximately 0.693147 (rounded to 6 significant digits), or commonly rounded to fewer digits in 555 timer books and datasheets to 0.693, 0.69, or 0.7}}

[[File:555 Mk-sp Diagram.svg|thumb|right|Schematic of a 555 timer in astable mode with a 1N4148 diode to create a duty cycles less than 50%]]
Resistor <math>R_1</math> requirements:
* The maximum current through <math>R_1</math> must be lower than the maximum current rating of the internal transisor at the DISCHARGE pin, because this transistor "shorts" the DISCHARGE pin to the GND pin (per internal schematics above) to drain the capacitor.  This is the reason why <math>R_1</math> shouldn't be a very low resistance, such as when a variable [[Trimmer (electronics)|trimmer]] or [[potentiometer]] is used instead of a fixed value resistor.
* The maximum power rating of <math>R_1</math> must be greater than <math>\frac{{V_\text{CC}}^2}{R_1}</math>, per [[Ohm's law]].

==== Shorter duty cycle ====
To create 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 R<sub>2</sub>, with the cathode on the capacitor side.<ref name="Signetics_1973_555-556_Databook"/> This bypasses R<sub>2</sub> during the high part of the cycle, so that the high interval depends only on R<sub>1</sub> and C, with an adjustment based on the voltage drop across the diode. The low time is unaffected by the diode and so remains <math display="inline">\ln(2) \, R_2 \, C \, .</math> But the diode's forward [[voltage drop]] ''V''<sub>diode</sub> slows charging on the capacitor, so the high time is longer than the often-cited <math display="inline">\ln(2) \, R_1 \, C</math> to become:

: <math>t_\text{high} = \ln\left(\frac{2 \, V_\text{CC} - 3 \, V_\text{diode}}{V_\text{CC} - 3 \, V_\text{diode}}\right) \cdot R_1 \cdot C,</math>

where ''V''<sub>diode</sub> is when the diode's "on" current is {{Frac|1|2}} of ''V''<sub>CC</sub>/R<sub>1</sub> (which [[Diode forward voltage drop|depends on the type of diode]] and can be found in datasheets or measured). When V<sub>diode</sub> is small relative to ''V''<sub>cc</sub>, this charging is faster and approaches <math display="inline">\ln(2) \, R_1 \, C</math> but is slower the closer V<sub>diode</sub> is to ''V''<sub>cc</sub>:

<blockquote>As an extreme example, when ''V''<sub>CC</sub> = 5&nbsp;V, and V<sub>diode</sub> = 0.7&nbsp;V, high time is 1.00&nbsp;R<sub>1</sub>C, which is 45% longer than the "expected" 0.693&nbsp;R<sub>1</sub>C. At the other extreme, when ''V''<sub>cc</sub> = 15&nbsp;V, and V<sub>diode</sub> = 0.3&nbsp;V, the high time is 0.725&nbsp;R<sub>1</sub>C, which is closer to the expected 0.693&nbsp;R<sub>1</sub>C. The equation approaches 0.693&nbsp;R<sub>1</sub>C as ''V''<sub>diode</sub> approaches 0&nbsp;V.</blockquote>

==== Voltage-controlled pulse-width modulation ====
In the previous example schematics, the control pin was not used, thus it should connected to ground through a 10&nbsp;[[Farad|nF]] [[decoupling capacitor]] to shunt electrical noise. However, if a time-varying voltage source was applied to the control pin, then the pulse widths would be dependent on the control voltage.

=== Monostable ===
{{stack|[[File:555 Monostable.svg|thumb|Schematic of a 555 in monostable mode. Example values C = 100&nbsp;nF, R = 180&nbsp;kΩ to 220&nbsp;kΩ for debouncing a pulled-up pushbutton.]]}}
{{stack|[[File:NE555 Monotable Waveforms (English).png|thumb|Waveform in monostable mode]]}}
{{see also|RC circuit}}

Monostable mode produces an output pulse when the trigger signals drops below {{Frac|1|3}} ''V''<sub>CC</sub>. An [[RC circuit]] sets the output pulse's duration as the time <math>t</math> in [[second]]s it takes to charge C to {{Frac|2|3}} ''V''<sub>CC</sub>:<ref name="Signetics_1973_555-556_Databook" />

: <math>t = \ln(3) \cdot R \cdot C,</math>

where <math>R</math> is the resistance in [[ohm]]s, <math>C</math> is the capacitance in [[farad]]s, <math>\ln(3)</math> is the [[Natural logarithm|natural log]] of 3 constant.{{efn|[[Natural logarithm|ln(3)]] is a constant, approximately 1.098612 (rounded to 6 significant digits), or commonly rounded to fewer digits in 555 timer books and datasheets to 1.099 or 1.1}}  The output pulse duration can be lengthened or shortened as desired by adjusting the values of R and C. Subsequent triggering before the end of this timing interval won't affect the output pulse.<ref name="TI_LM555_Datasheet" />

==== Example values ====
{| class="wikitable floatright" style="text-align: right"
|-
|+ Monostable mode examples with [[E series of preferred numbers|common values]]
! [[Second|Time]] !! [[Farad|C]] !! [[Ohm|R]]
|-
| 100 μs (−0.026%) || 1 nF || 91 kΩ
|-
| 1 ms (−0.026%) || 10 nF || 91 kΩ
|-
| 10 ms (−0.026%) || 100 nF || 91 kΩ
|-
| 100 ms (−0.026%) || 1 μF || 91 kΩ
|-
| 1 s (−0.026%) || 10 μF || 91 kΩ
|-
| 10 s (−0.026%) || 100 μF || 91 kΩ
|}<!-- NOTE: Common preferred values are best for examples. Capacitors are E3 series values, resistors are E24 series values. user:sbmeirow -->

The timing table (right) shows common electronic component value solutions for various powers of 10 timings.

Scaling R and C by opposite powers of 10 will provide the same timing. For instance:<!-- NOTE: per [[Wikipedia:These are not original research#Simple calculations]], from user:sbmeirow -->
* 1{{nbsp}}ms [[≅]] 1{{nbsp}}nF and 910{{nbsp}}kΩ,
* '''1{{nbsp}}ms ≅ 10{{nbsp}}nF and 91{{nbsp}}kΩ (values from table)''',
* 1{{nbsp}}ms ≅ 100{{nbsp}}nF and 9.1{{nbsp}}kΩ.

For each row in the example table (right), additional timing values can easily be created by adding one to three of the same resistor value in parallel and/or series. A second resistor in parallel, the new timing is half the table time. A second resistor in series, the new timing is double the table time.<!-- NOTE: per [[Wikipedia:These are not original research#Simple calculations]], from user:sbmeirow -->
* 2.5{{nbsp}}ms (0.25x) [[≅]] 100{{nbsp}}nF and 22.75{{nbsp}}kΩ (four 91{{nbsp}}kΩ resistors in [[Series and parallel circuits|parallel]]),
* 5{{nbsp}}ms (0.5x)  ≅ 100{{nbsp}}nF and 45.5{{nbsp}}kΩ (two 91{{nbsp}}kΩ resistors in parallel),
* '''10{{nbsp}}ms (1x) ≅ 100{{nbsp}}nF and 91{{nbsp}}kΩ (values from table)''',
* 15{{nbsp}}ms (1.5x) ≅ 100{{nbsp}}nF and 136.5{{nbsp}}kΩ (one 91{{nbsp}}kΩ resistor in series with "two 91{{nbsp}}kΩ resistors in parallel"),
* 20{{nbsp}}ms (2x) ≅ 100{{nbsp}}nF and 182{{nbsp}}kΩ (two 91{{nbsp}}kΩ resistors in [[Series and parallel circuits|series]]),
* 25{{nbsp}}ms (2.5x) ≅ 100{{nbsp}}nF and 227.5{{nbsp}}kΩ ("two 91{{nbsp}}kΩ resistors in series" in series with "two 91{{nbsp}}kΩ resistors in parallel"),
* 30{{nbsp}}ms (3x) ≅ 100{{nbsp}}nF and 273{{nbsp}}kΩ (three 91{{nbsp}}kΩ resistors in series),
* 40{{nbsp}}ms (4x) ≅ 100{{nbsp}}nF and 364{{nbsp}}kΩ (four 91{{nbsp}}kΩ resistors in series).
{{clear}}

=== Bistable SR latch ===
{{stack|[[File:555 Bistabiel digitaal.svg|thumb|Schematic of a 555 in bistable [[SR latch]] mode]]}}{{stack|[[File:Inverted SR Flip-flop.svg|thumb|100px|[[Active-low]] [[SR latch]] symbol, but lacks /Q output]]}}<!-- NOTE: Need better version with /Q removed. user:sbmeirow -->

{{see also|Set-Reset latch}}

A 555 timer can act as an [[Active low|active-low]] [[Set-Reset latch|SR latch]] (though without an inverted {{overline|Q}} output) with two outputs: output pin is a [[Push–pull output|push-pull]] output, discharge pin is an [[open-collector]] output (requires a [[pull-up resistor]]).

For the schematic on the right, a {{overline|Reset}} input signal connects to the {{overline|RESET}} pin and connecting a {{overline|Set}} input signal to the {{overline|TR}} pin. Thus, pulling {{overline|Set}} momentarily low acts as a "set" and transitions the output to the high state (''V''<sub>CC</sub>). Conversely, pulling {{overline|Reset}} momentarily low acts as a "reset" and transitions the Out pin to the low state (GND).

No timing capacitors are required in a bistable configuration. The threshold input is grounded because it is unused.<ref>{{Cite book |last=Buiting |first=Jan |url=https://books.google.com/books?id=rZDbnQ0siCYC&q=555+pin+7+%22open+collector%22&pg=PA84 |title=308 Circuits |date=2003 |publisher=Elektor International Media |isbn=978-0-905705-66-8 |language=en}}</ref> The trigger and reset inputs may be held high via [[pull-up resistor]]s if they are normally [[Hi-Z]] and only enabled by connecting to ground.
{{clear}}

=== Bistable Schmitt trigger inverter gate ===
{{stack|[[File:555 Bistabiel analoog.svg|thumb|Schematic of a 555 timer in bistable Schmitt trigger inverter mode. Example values C = 100 nF, R1 & R2 = 100 kΩ.]]}}
{{stack|[[File:Inverting Schmitt trigger symbol.svg|thumb|125px|Schmitt trigger inverter symbol]]}}
{{see also|Inverter gate}}

A 555 timer can be used to create a [[Schmitt trigger]] [[Inverter (logic gate)|inverter gate]] with two outputs: output pin is a [[Push–pull output|push-pull]] output, discharge pin is an [[open-collector]] output (requires a [[pull-up resistor]]).

For the schematic on the right, an input signal is [[Capacitive coupling|AC-coupled]] through a low value series capacitor, then biased by identical high-resistance resistors <math>R_1</math> and <math>R_2</math>, which causes the signal to be centered at {{Frac|1|2}} ''V''<sub >cc</sub>.  This centered signal is connected to both the trigger and threshold input pins of the timer.  The input signal must be strong enough to excite the trigger levels of the comparators to exceed the lower {{Frac|1|3}} ''V''<sub>CC</sub> and upper {{Frac|2|3}} ''V''<sub>CC</sub> thresholds in order to cause them to change state, thus providing the Schmitt trigger feature.<ref>{{cite web |title=555 Timer as Schmitt Trigger |url=https://www.electronicshub.org/555-timer-as-schmitt-trigger/ |website=Electronics Hub |archive-url=https://web.archive.org/web/20230315063926/https://www.electronicshub.org/555-timer-as-schmitt-trigger/ |archive-date=March 15, 2023 |date=June 19, 2015 |url-status=live}}</ref>

No timing capacitors are required in a bistable configuration.
{{clear}}

==Packages==
[[File:NE555 DIP & SOIC.jpg|thumb|right|Texas Instruments NE555 in [[Dual in-line package|DIP]]-8 and SO-8 packages<ref name="TI_NE555_Datasheet"/>]]

In 1972, [[Signetics]] originally released the 555 timer in [[Dual in-line package|DIP]]-8 and [[TO-5|TO5]]-8 metal can packages, and the 556 timer was released in a DIP-14 package.<ref name="Signetics_1972_Linear1_Databook"/>

In 2006, the dual 556 timer was available in through-hole packages as DIP-14 (2.54&nbsp;mm pitch),<ref name="TI_NE556_Datasheet"/> and surface-mount packages as SO-14 (1.27&nbsp;mm pitch) and SSOP-14 (0.65&nbsp;mm pitch).

In 2012, the 555 was available in through-hole packages as DIP-8 (2.54&nbsp;mm pitch),<ref name="JRC_NJM555_Datasheet"/> and surface-mount packages as SO-8 (1.27&nbsp;mm pitch), SSOP-8 / [[Small Outline Integrated Circuit|TSSOP]]-8 / VSSOP-8 (0.65&nbsp;mm pitch), [[Ball grid array|BGA]] (0.5&nbsp;mm pitch).<ref name="TI_NE555_Datasheet"/>

The MIC1555 is a [[CMOS]] 555-type timer with three fewer pins available in [[Small-outline transistor|SOT23]]-5 (0.95&nbsp;mm pitch) surface-mount package.<ref name="Microchip_MIC1555_Datasheet"/>

== Specifications ==
[[File:BjornR breadboard 01.jpg|thumb|555 timer circuit in a [[solderless breadboard]]]]
These specifications apply to the original bipolar NE555. Other 555 timers can have different specifications depending on the grade (industrial, military, medical, etc.).

{| class="wikitable"
| Part number
| NE555
|-
| IC Process
| Bipolar
|-
| Supply voltage (''V''<sub>CC</sub>)
| 4.5 to 16 [[Volt|V]]
|-
| Supply current (''V''<sub>CC</sub> = +5 V)
| 3 to 6 [[Ampere|mA]]
|-
| Supply current (''V''<sub>CC</sub> = +15 V)
| 10 to 15 mA
|-
| Output current (maximum)
| 200 mA
|-
| Maximum Power dissipation
| 600&nbsp;[[Watt|mW]]
|-
| Power consumption (minimum operating)
| 30&nbsp;mW @ 5&nbsp;V,<br/>225&nbsp;mW @ 15&nbsp;V
|-
| [[Operating temperature]]
| 0 to 70 °[[Celsius|C]]
|}

==Derivatives==
Numerous companies have manufactured one or more variants of the 555, 556, 558 timers over the past decades, under many different part numbers.  The following is a partial list:

{| class="wikitable sortable" style="text-align: center;"
|-
! Manufacturer !! Part<br/>number !! Production<br/>status !! IC<br/>process !! Timers<br/>total !! Supply<br/>min. ([[volt]]) !! Supply<br/>max. (volt) !! [[Quiescent current|''I''<sub>q</sub>]] ([[Electric current|μA]])<br/>at 5&nbsp;V<br/>supply<br/> !! Frequency<br/>max. ([[MHz]]) || class="unsortable" | Remarks || class="unsortable" | Datasheet
|-
| [https://customsiliconsolutions.com Custom Silicon Solutions (CSS)]|| CSS555 || {{Yes|Active}} || [[CMOS]] || 1 || 1.2 || 5.5 || 4.3 || 1.0 || Internal [[EEPROM]], requires programmer ||<ref name="CSS_CSS555_Datasheet">{{cite web |title=CSS555 Datasheet |url=http://www.customsiliconsolutions.com/downloads/Revised%20Standard%20products/CSS555_Spec_2.pdf |date=July 2012 |website=Custom Silicon Solutions |archive-url=https://web.archive.org/web/20170629194001/http://www.customsiliconsolutions.com/downloads/Revised%20Standard%20products/CSS555_Spec_2.pdf |archive-date=June 29, 2017 |url-status=live}}</ref><ref>{{cite web |title=CSS555 Part Search |url=https://www.jameco.com/shop/StoreCatalogDrillDownView?langId=-1&storeId=10001&catalogId=10001&freeText=css555&search_type=jamecoall |website=Jameco Electronics }}</ref><ref>{{cite journal |first=James |last=Senft |title=The Remarkable CSS555 |url=https://www.nutsvolts.com/magazine/article/february2016_CSS555TimerICs |journal=Nuts & Volts Magazine |archive-url=https://web.archive.org/web/20200527061138/https://www.nutsvolts.com/magazine/article/february2016_CSS555TimerICs |archive-date=May 27, 2020 |date=February 2016 |url-status=live}}</ref>
|-
| [[Diodes Incorporated|Diodes Inc]] || ZSCT1555 || {{No|Discontinued}} || Bipolar || 1 || 0.9 || 6 || 150 || 0.33 || Designed by [[Hans Camenzind]] || <ref name="DiodesInc_ZSCT1555_Datasheet">{{cite web |title=ZSCT1555 Datasheet |url=https://www.diodes.com/assets/Datasheets/ZSCT1555.pdf |date=July 2006 |website=[[Diodes Incorporated]] |archive-url=https://web.archive.org/web/20170629181234/https://www.diodes.com/assets/Datasheets/ZSCT1555.pdf |archive-date=June 29, 2017 |url-status=live}}</ref>
|-
| [[Japan Radio Company]] (JRC) || NJM555 || {{No|Discontinued}} || Bipolar || 1 || 4.5 || 16 || 3000 || 0.1* || Also available in [[Single in-line package|SIP]]-8 package. || <ref name="JRC_NJM555_Datasheet">{{cite web |title=NJM555 Datasheet |url=http://www.njr.com/semicon/PDF/NJM555_E.pdf |date=November 2012 |website=[[Japan Radio Company]] |archive-url=https://web.archive.org/web/20170629170934/http://www.njr.com/semicon/PDF/NJM555_E.pdf |archive-date=June 29, 2017 |url-status=dead}}</ref>
|-
| [[Microchip Technology|Microchip]] || MIC1555/7 || {{Yes|Active}} || CMOS || 1* || 2.7 || 18 || 240 || 5.0* || Reduced pins & features (only astable & monostable & no reset for MIC1555, astable only for MIC1557), only available in [[Small-outline transistor#SOT23-5|SOT23-5]], TSOT23-5, [[Flat no-leads package|UTDFN]]-10 packages. || <ref name="Microchip_MIC1555_Datasheet">{{cite web |title=MIC1555 Datasheet |url=http://ww1.microchip.com/downloads/en/DeviceDoc/20005730A.pdf |date=March 2017 |website=[[Microchip Technology]] |archive-url=https://web.archive.org/web/20210421210319/http://ww1.microchip.com/downloads/en/DeviceDoc/20005730A.pdf |archive-date=April 21, 2021 |url-status=live}}</ref>
|-
| [[ON Semiconductor|ON]] || MC1455 || {{Yes|Active}} || Bipolar || 1 || 4.5 || 16 || 3000 || 0.1* || {{sdash}} || <ref name="ON_MC1455_Datasheet">{{cite web |title=MC1455 Datasheet |url=https://www.onsemi.com/pub/Collateral/MC1455-D.PDF |date=December 2009 |website=ON Semiconductor |archive-url=https://web.archive.org/web/20200522114538/https://www.onsemi.com/pub/Collateral/MC1455-D.PDF |archive-date=May 22, 2020 |url-status=live}}</ref>
|-
| [[Renesas Electronics|Renesas]] || ICM7555 || {{Yes|Active}} || CMOS || 1 || 2 || 18 || 40 || 1.0 ||  ||<ref name="Intersil_ICM7555-556_Datasheet">{{cite web |title=ICM7555-556 Datasheet |url=http://www.intersil.com/content/dam/Intersil/documents/icm7/icm7555-56.pdf |date=June 2016 |website=[[Intersil]] |archive-url=https://web.archive.org/web/20170629211905/http://www.intersil.com/content/dam/Intersil/documents/icm7/icm7555-56.pdf |archive-date=June 29, 2017 |url-status=dead}}</ref>
|-
| Renesas || ICM7556 || {{Yes|Active}} || CMOS || 2 || 2 || 18 || 80 || 1.0 ||  ||<ref name="Intersil_ICM7555-556_Datasheet"/>
|-
| '''[[Signetics]]''' || '''NE555''' || {{Yes|Active (TI)}} || Bipolar || 1 || 4.5 || 16 || 3000 || 0.1* || '''First 555 timer''', [[Dual in-line package|DIP]]-8 or [[TO-5|TO5]]-8 packages. || <ref name="Signetics_1972_Linear1_Databook">{{cite web |title=Linear Vol1 Databook |url=https://archive.org/details/bitsavers_signeticsdcsLinearVol1_11470058 |date=1972 |website=[[Signetics]]}}</ref><ref name="Signetics_1973_555-556_Databook">{{cite web |title=555/556 Timers Databook |url=https://www.rfcafe.com/miscellany/cool-products/Signetics-555-556-Timer-1973-Databook.pdf |date=1973 |website=[[Signetics]] |archive-url=https://web.archive.org/web/20210511041359/https://www.rfcafe.com/miscellany/cool-products/Signetics-555-556-Timer-1973-Databook.pdf |archive-date=May 11, 2021 |url-status=live}}</ref><ref name="Signetics_1979_App_Manual">{{cite web |title=Analog Applications Manual |url=https://archive.org/details/bitsavers_signeticsdcsAnalogApplications_33415016 |date=1979 |website=[[Signetics]]}}</ref><ref name="Signetics_1985_Linear_Databook">{{cite web |title=Linear LSI Data and Applications Manual |url=https://archive.org/details/bitsavers_signeticsdcsLinearLSI_66682786 |date=1985 |website=[[Signetics]]}}</ref>
|-
| '''Signetics''' || '''NE556''' || {{Yes|Active (TI)}}  || Bipolar || 2 || 4.5 || 16 || 6000 || 0.1* || '''First 556 timer''', DIP-14 package. || <ref name="Signetics_1973_555-556_Databook"/><ref name="Signetics_1985_Linear_Databook"/>
|-
| '''Signetics''' || '''NE558''' || {{No|Discontinued}} || Bipolar || 4* || 4.5 || 16 || 4800* || 0.1* || '''First 558 timer''', DIP-16 package. || <ref name="Signetics_1985_Linear_Databook"/>
|-
| [[STMicroelectronics]] (ST) || TS555 || {{Yes|Active}} || CMOS || 1 || 2 || 16 || 110 || 2.7 || {{sdash}} || <ref name="ST_TS555_Datasheet">{{cite web |title=TS555 Datasheet |url=https://www.st.com/resource/en/datasheet/ts555.pdf |date=June 2015 |website=[[STMicroelectronics]]  |archive-url=https://web.archive.org/web/20200526095316/https://www.st.com/content/ccc/resource/technical/document/datasheet/45/d5/c6/11/46/d3/4b/a3/CD00000893.pdf/files/CD00000893.pdf/jcr:content/translations/en.CD00000893.pdf |archive-date=May 26, 2020 |url-status=live}}</ref>
|-
| [[Texas Instruments]] (TI) || LM555 || {{Yes|Active}} || Bipolar || 1 || 4.5 || 16 || 3000 || 0.1 ||  || <ref name="TI_LM555_Datasheet">{{cite web |title=LM555 Datasheet |url=http://www.ti.com/lit/ds/symlink/lm555.pdf |date=January 2015 |website=[[Texas Instruments]] |archive-url=https://web.archive.org/web/20170629043122/http://www.ti.com/lit/ds/symlink/lm555.pdf |archive-date=June 29, 2017 |url-status=live}}</ref>
|-
| Texas Instruments || LM556 || {{No|Discontinued}} || Bipolar || 2 || 4.5 || 16 || 6000 || 0.1 ||  || <ref name="TI_LM556_Datasheet">{{cite web |title=LM556 Datasheet |url=http://www.ti.com/lit/ds/symlink/lm556.pdf |date=October 2015 |website=[[Texas Instruments]] |archive-url=https://web.archive.org/web/20170629111822/http://www.ti.com/lit/ds/symlink/lm556.pdf |archive-date=June 29, 2017 |url-status=dead }}</ref>
|-
| Texas Instruments || LMC555 || {{Yes|Active}} || CMOS || 1 || 1.5 || 15 || 100 || 3.0 || Also available in [[Ball grid array|DSBGA]]-8 package. || <ref name="TI_LMC555_Datasheet">{{cite web |title=LMC555 Datasheet |url=http://www.ti.com/lit/ds/symlink/lmc555.pdf |date=July 2016 |website=[[Texas Instruments]] |archive-url=https://web.archive.org/web/20170628232001/http://www.ti.com/lit/ds/symlink/lmc555.pdf |archive-date=June 28, 2017 |url-status=live}}</ref>
|-
| Texas Instruments || NE555 || {{Yes|Active}} || Bipolar || 1 || 4.5 || 16 || 3000 || 0.1* || {{sdash}} || <ref name="TI_NE555_Datasheet">{{cite web |title=NE555 Datasheet |url=http://www.ti.com/lit/ds/symlink/ne555.pdf |date=September 2014 |website=[[Texas Instruments]] |archive-url=https://web.archive.org/web/20170628232239/http://www.ti.com/lit/ds/symlink/ne555.pdf |archive-date=June 28, 2017 |url-status=live}}</ref>
|-
| Texas Instruments || NE556 || {{Yes|Active}} || Bipolar || 2 || 4.5 || 16 || 6000 || 0.1* || {{sdash}} || <ref name="TI_NE556_Datasheet">{{cite web |title=NE556 Datasheet |url=http://www.ti.com/lit/ds/symlink/ne556.pdf |date=June 2006 |website=[[Texas Instruments]]  |archive-url=https://web.archive.org/web/20170629045640/http://www.ti.com/lit/ds/symlink/ne556.pdf |archive-date=June 29, 2017 |url-status=live}}</ref>
|-
| Texas Instruments || TLC551 || {{Yes|Active}} || CMOS || 1 || 1 || 15 || 170 || 1.8 ||  ||<ref name="TI_TLC551_Datasheet">{{cite web |title=TLC551 Datasheet |url=http://www.ti.com/lit/ds/symlink/tlc551.pdf |date=September 1997 |website=[[Texas Instruments]] |archive-url=https://web.archive.org/web/20170629111015/http://www.ti.com/lit/ds/symlink/tlc551.pdf |archive-date=June 29, 2017 |url-status=live}}</ref>
|-
| Texas Instruments || TLC552 || {{Yes|Active}} || CMOS || 2 || 1 || 15 || 340 || 1.8 ||  ||<ref name="TI_TLC552_Datasheet">{{cite web |title=TLC552 Datasheet |url=http://www.ti.com/lit/ds/symlink/tlc552.pdf |date=May 1988 |website=[[Texas Instruments]] |archive-url=https://web.archive.org/web/20170629111022/http://www.ti.com/lit/ds/symlink/tlc552.pdf |archive-date=June 29, 2017 |url-status=live}}</ref>
|-
| Texas Instruments || TLC555 || {{Yes|Active}} || CMOS || 1 || 2 || 15 || 170 || 2.1 || {{sdash}} || <ref name="TI_TLC555_Datasheet">{{cite web |title=TLC555 Datasheet |url=http://www.ti.com/lit/ds/symlink/tlc555.pdf |date=August 2016 |website=[[Texas Instruments]] |archive-url=https://web.archive.org/web/20170628232326/http://www.ti.com/lit/ds/symlink/tlc555.pdf |archive-date=June 28, 2017 |url-status=live}}</ref>
|-
| Texas Instruments || TLC556 || {{Yes|Active}} || CMOS || 2 || 2 || 15 || 340 || 2.1 || {{sdash}} || <ref name="TI_TLC556_Datasheet">{{cite web |title=TLC556 Datasheet |url=http://www.ti.com/lit/ds/symlink/tlc556.pdf |date=September 1997 |website=[[Texas Instruments]] |archive-url=https://web.archive.org/web/20170629111759/http://www.ti.com/lit/ds/symlink/tlc556.pdf |archive-date=June 29, 2017 |url-status=live}}</ref>
|-
| X-REL || XTR655 || {{Yes|Active}} || [[Silicon on insulator|SOI]] || 1 || 2.8 || 5.5 || 170 || 4.0 || Extreme (−60&nbsp;°[[Celsius|C]] to +230&nbsp;°C), ceramic DIP-8 package or bare die. || <ref name="X-REL_XTR655_Datasheet">{{cite web |title=XTR655 Datasheet |url=https://www.easii-ic.com/wp-content/uploads/2021/08/EASii-IC-DS-XTR650-Versatile-Timer-Rev-3-August-21-DS-00100-11.pdf |date=August 2021 |website=X-REL Semiconductor |archive-url=https://web.archive.org/web/20230710152544/https://www.easii-ic.com/wp-content/uploads/2021/08/EASii-IC-DS-XTR650-Versatile-Timer-Rev-3-August-21-DS-00100-11.pdf |archive-date=July 10, 2023 |url-status=live}}</ref>
|}

; Table notes
* All information in the above table was pulled from references in the datasheet column, except where denoted below.
* For the "Total timers" column, a "*" denotes parts that are missing 555 timer features.
* For the "''I''<sub>q</sub>" 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 ''I''<sub>q</sub> at 5&nbsp;V.<ref name="Signetics_1985_Linear_Databook"/> The value listed in this table was estimated by comparing the 5&nbsp;V to 15&nbsp;V ratio of other bipolar datasheets, then derating the 15&nbsp;V parameter for the NE558 part, which is denoted by the "*".
* For the "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&nbsp;MHz.<ref name="Microchip_MIC1555_Datasheet"/> 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<ref name="TI_NE555_Datasheet"/> states a value of 100&nbsp;kHz, and their website shows a value of 100&nbsp;kHz in timer comparison tables. Signetics App Note 170 states that most devices will oscillate up to 1&nbsp;MHz; however, when considering temperature stability, it should be limited to about 500&nbsp;kHz.<ref name="Signetics_1985_Linear_Databook"/> The application note from [[Halbleiterwerk Frankfurt (Oder)|HFO]] mentions that at higher supply voltages the maximum power dissipation of the circuit might limit the operating frequency, as the supply current increases with frequency.<ref name=b555>{{cite book |title=Zeitgeber-IS B 555 / B 556 |first=Ullrich |last=Reick |language=de |publisher=Halbleiterwerk Frankfurt (Oder) |date=1986-03-01 |url=http://datasheet.datasheetarchive.com/originals/scans/Scans-048/DSAGER000125.pdf}}</ref>
* For the "Manufacturer" column, the following associates historical 555 timer manufacturers to current company names.
** [[Fairchild Semiconductor]] was sold to [[ON Semiconductor]] in 2016.<ref>{{cite web |title=ON Semiconductor Successfully Completes Acquisition of Fairchild Semiconductor |url=https://www.businesswire.com/news/home/20160919005796/en/Semiconductor-Successfully-Completes-Acquisition-Fairchild-Semiconductor-2.4 |website=[[Business Wire]] |archive-url=https://web.archive.org/web/20200128124808/https://www.businesswire.com/news/home/20160919005796/en/Semiconductor-Successfully-Completes-Acquisition-Fairchild-Semiconductor-2.4 |archive-date=January 28, 2020 |date=September 19, 2016 |url-status=live}}</ref>  ON Semiconductor was founded in 1999 as a spinoff of [[Motorola]] Semiconductor Components Group.<ref>{{cite web |title=Former Motorola group emerges as ON Semiconductor |url=https://www.eetimes.com/former-motorola-group-emerges-as-on-semiconductor/# |publisher=[[EE Times]] |archive-url=https://web.archive.org/web/20200608011400/https://www.eetimes.com/former-motorola-group-emerges-as-on-semiconductor/ |archive-date=June 8, 2020 |date=August 5, 1999 |url-status=live}}</ref>  The MC1455 started as a Motorola product.
** [[Intersil]] was sold to [[Renesas Electronics]] in 2017.<ref>{{cite web |title=Renesas and Intersil Announce Final Regulatory Approval for Renesas' Acquisition of Intersil |url=https://www.renesas.com/us/en/about/press-center/news/2017/news20170222.html |website=[[Renesas Electronics]] |archive-url=https://web.archive.org/web/20200613181551/https://www.renesas.com/us/en/about/press-center/news/2017/news20170222.html |archive-date=June 13, 2020 |date=February 22, 2017 |url-status=live}}</ref>  The ICM7555 and ICM7556 started as Intersil products.
** Micrel was sold to [[Microchip Technology]] in 2015.<ref>{{cite web |title=Microchip Technology Completes Micrel Acquisition |url=https://www.powerelectronics.com/news/industry/article/21862805/microchip-technology-completes-micrel-acquisition |website=Power Electronics |archive-url=https://web.archive.org/web/20200522100320/https://www.powerelectronics.com/news/industry/article/21862805/microchip-technology-completes-micrel-acquisition |archive-date=May 22, 2020 |date=August 12, 2015 |url-status=live}}</ref>  The MIC1555 started as a Micrel product.
** [[National Semiconductor]] was sold to [[Texas Instruments]] in 2011.<ref>{{cite web |title=Texas Instruments completes acquisition of National Semiconductor |url=https://investor.ti.com/news-releases/news-release-details/texas-instruments-completes-acquisition-national-semiconductor |website=[[Texas Instruments]] |archive-url=https://web.archive.org/web/20200522101031/https://investor.ti.com/news-releases/news-release-details/texas-instruments-completes-acquisition-national-semiconductor |archive-date=May 22, 2020 |date=September 23, 2011 |url-status=live}}</ref>  The LM555 and LM556 started as a National Semiconductor products.
** [[Signetics]] was sold to [[Philips|Philips Semiconductor]] in 1975, later to [[NXP Semiconductors]] in 2006.<ref>{{cite web |title=NXP Semiconductors history |url=https://www.siliconvalleyhistorical.org/nxp-semiconductors-history |website=Silicon Valley Historical Association |archive-url=https://web.archive.org/web/20200321080200/https://www.siliconvalleyhistorical.org/nxp-semiconductors-history |archive-date=March 21, 2020 |date=2008 |url-status=live}}</ref>
** [[Zetex Semiconductors]] was sold to [[Diodes Incorporated]] in 2008.<ref>{{cite web |title=Diodes Incorporated closes acquisition of Zetex |url=https://www.ledsmagazine.com/leds-ssl-design/driver-ics/article/16697918/diodes-incorporated-closes-acquisition-of-zetex-plc |website=LEDs Magazine |archive-url=https://web.archive.org/web/20200522102057/https://www.ledsmagazine.com/leds-ssl-design/driver-ics/article/16697918/diodes-incorporated-closes-acquisition-of-zetex-plc |archive-date=May 22, 2020 |date=June 13, 2008 |url-status=live}}</ref>  The ZSCT1555 started as a Zetex product.<ref name="Redesigning">{{cite journal |first=H.R. |last=Camenzind |title=Redesigning the old 555 [timer circuit] |journal=IEEE Spectrum |volume=34 |issue=9 |pages=80–85 |date=September 1997 |doi=10.1109/6.619384 }}</ref>

=== 556 dual timer ===
{{stack|[[File:STM-NE556-HD.jpg|thumb|[[Die (integrated circuit)|Die]] of a NE556 dual timer manufactured by [[STMicroelectronics]]]]}}
{{stack|[[File:NE556 pennen.svg|thumb|Pinout of 556 dual timer<ref name="TI_NE556_Datasheet"/><ref name="Signetics_1973_555-556_Databook"/>]]}}

The dual version is called 556. It features two complete 555 timers in a 14-pin package; only the two power-supply pins are shared between the two timers.<ref name="TI_NE556_Datasheet"/><ref name="Signetics_1973_555-556_Databook"/> In 2020, the bipolar version was available as the NE556,<ref name="TI_NE556_Datasheet"/> and the CMOS versions were available as the Intersil ICM7556 and Texas Instruments TLC556 and TLC552. See [[#Derivatives|derivatives table]] in this article.<ref name="Intersil_ICM7555-556_Datasheet"/><ref name="TI_TLC556_Datasheet"/><ref name="TI_TLC552_Datasheet"/>

{{clear}}

=== 558 quad timer ===
{{stack|[[File:Signetics Corporation NE558D 0136O07 9331KK KOREA.jpg|thumb|Die of a NE558 quad timer manufactured by [[Signetics]]]]}}
{{stack|[[File:NE558 pennen.svg|thumb|Pinout of 558 quad timer<ref name="Signetics_1985_Linear_Databook"/>]]}}
{{stack|[[File:NE558 intern.svg|thumb|558 internal block diagram. It is different from 555 and 556 timers.<ref name="Signetics_1985_Linear_Databook"/>]]}}

The quad version is called 558 and has four reduced-functionality timers in a 16-pin package designed primarily for [[monostable multivibrator]] applications.<ref>{{Cite book |last=Horn |first=Delton |title=Amplifiers, waveform generators, and other low-cost IC projects |publisher=TAB Books |year=1994 |isbn=0-07-030415-7 |location=New York |pages=27 |oclc=28676554 |quote=Not all functions are brought out to the 558's pins. This chip is designed primarily for monostable multivibrator applications}}</ref><ref name="Signetics_1985_Linear_Databook"/> By 2014, many versions of 16-pin NE558 have become obsolete.<ref name=":0">{{Cite book |last1=Platt |first1=Charles |url=https://books.google.com/books?id=FgRYBQAAQBAJ&pg=PT213 |series=Encyclopedia of Electronic Components |volume=2 |title=LEDs, LCDs, Audio, Thyristors, Digital Logic, and Amplification |last2=Jansson |first2=Fredrik |date=2014-11-13 |publisher=Maker Media |isbn=978-1-4493-3414-7 |language=en}}</ref>

Partial list of differences between 558 and 555 chips:<ref name="Signetics_1985_Linear_Databook"/><ref name=":0" />
* One ''V''<sub>CC</sub> 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 output|push–pull]] (P.P.) type (555).

== See also ==
{{Portal|Electronics}}
* [[RC circuit]]
* [[Counter (digital)]]
* [[Operational amplifier]]
* [[List of LM-series integrated circuits]]
* [[List of linear integrated circuits]]
* [[4000-series integrated circuits]], [[List of 4000-series integrated circuits]]
* [[7400-series integrated circuits]], [[List of 7400-series integrated circuits]]
* [[Push–pull output]], [[Open collector|Open-collector/drain output]], [[Three-state logic|Three-state output]]

== Notes ==
{{notelist}}

== References ==
{{Reflist}}

== Further reading ==
;Books
* ''555 Timer Applications Sourcebook With Experiments''; 2nd Ed; Howard Berlin; BPB Publications; 218 pages; 2008; {{ISBN|978-8176567909}}. <small>[https://archive.org/details/555timerapplicat0000berl/ ''(1st Ed in 1978)'']</small>
* ''Timer/Generator Circuits Manual''; 1st Ed; R.M. Marston; Newnes; 276 pages; 1990; {{ISBN|978-0434912919}}.
* ''Engineer's Mini-Notebook&nbsp;– 555 Timer IC Circuits''; 3rd Ed; [[Forrest Mims]] III; Radio Shack; 33 pages; 1989; ASIN B000MN54A6. <small>[https://archive.org/download/Forrest_Mims-engineers_mini-notebook_555_timer_circuits_radio_shack_electronics/Forrest%20Mims-engineer's%20mini-notebook%20555%20timer%20circuits%20(radio%20shack%20electronics).pdf ''(1st Ed in 1984)'']</small>
* ''IC Timer Cookbook''; 2nd Ed; [[Walt Jung]]; Sams Publishing; 384 pages; 1983; {{ISBN|978-0672219320}}. <small>''[https://web.archive.org/web/20170713174329/http://chiclassiccomp.org/docs/content/books/ICTimerCookbook1stEd1977_WalterGJung.pdf (1st Ed in 1977)]''</small>
* ''110 IC Timer Projects''; Jules Gilder; Hayden; 115 pages; 1979; {{ISBN|978-0810456884}}.
* ''IC 555 Projects''; E.A. Parr; Bernard Babani Publishing; 144 pages; 1978; {{ISBN|978-0859340472}}.

; Books with timer chapters
* ''[http://www.ibiblio.org/kuphaldt/electricCircuits/index.htm Lessons in Electric Circuits] - Volume VI - Experiments''; Tony Kuphaldt; Open Book Project; 423 pages; 2010. <small>[https://web.archive.org/web/20170714100348/http://www.ibiblio.org/kuphaldt/electricCircuits/Exper/EXP.pdf ''(Chapter 6 and 8)'']</small>
* ''Designing Analog Chips''; [[Hans Camenzind]] (inventor of 555 timer); Virtual Bookworm; 244 pages; 2005; {{ISBN|978-1589397187}}. <small>[https://web.archive.org/web/20170612055924/http://www.designinganalogchips.com/_count/designinganalogchips.pdf ''(Chapter 11)'']</small>
* ''Timer, Op Amp, and Optoelectronic Circuits and Projects''; [[Forrest Mims]] III; Master Publishing; 128 pages; 2004; {{ISBN|978-0945053293}}. <small>(Chapter 1)</small>
* ''Linear LSI Data and Applications Manual''; [[Signetics]]; 1250 pages; 1985. <small>[https://archive.org/download/bitsavers_signeticsdcsLinearLSI_66682786/1985_Signetics_Linear_LSI.pdf ''(Appnotes AN170/171 and Datasheets NE555/6/8)'']</small>
* ''Analog Applications Manual''; [[Signetics]]; 418 pages; 1979. <small>[https://archive.org/download/bitsavers_signeticsdcsAnalogApplications_33415016/1979_Signetics_Analog_Applications.pdf ''(Chapter 6)'']</small>
* ''TTL Cookbook''; [[Don Lancaster]]; Sams Publishing; 412 pages; 1974; {{ISBN|978-0672210358}}. <small>[https://web.archive.org/web/20190311005416/https://www.tinaja.com/ebooks/TTLCB1.pdf ''(Chapter 4, pages 171-188)'']</small>

; Datasheets
* See links in [[#Derivatives|"Derivatives"]] table and "References" section in this article.

== External links ==
{{Commons category}}
* [https://www.nutsvolts.com/magazine/article/using-the-555-timer-ic-in-special-or-unusual-circuits Using the 555 Timer IC in Special or Unusual Circuits] - Nuts & Volts magazine
* [https://www.learningelectronics.net/VA3AVR/gadgets/555/555.html 555 Timer Tutorial] - Tony van Roon
* [https://www.555-timer-circuits.com/common-mistakes.html Common Mistakes When Using a 555 Timer]
* 555 timing calculators: [https://web.archive.org/web/20240207162830/https://ohmslawcalculator.com/555-astable-calculator astable], [https://web.archive.org/web/20240207162955/https://ohmslawcalculator.com/555-monostable-calculator monostable]
* [http://www.righto.com/2016/02/555-timer-teardown-inside-worlds-most.html Teardown of a bipolar RCA LM555CH chip]
* [http://www.righto.com/2016/04/teardown-of-cmos-555-timer-chip-how.html Teardown of a CMOS TI LMC555 chip]

{{NXP Semiconductors}}
{{Authority control}}

[[Category:1972 introductions]]
[[Category:Electronic oscillators]]
[[Category:Linear integrated circuits]]