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[[File:Signetics NE555N.JPG|thumb|NE555 from [[Signetics]] in [[Dual in-line package|dual-in-line package]]]]
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[[File:NE555 Bloc Diagram.svg|thumb|Internal block diagram]]
The '''555 timer IC''' is an [[integrated circuit]] (chip) used in a variety of [[timer]], pulse generation, and [[Electronic oscillator|oscillator]] applications. The 555 can be used to provide time delays, as an [[oscillator]], and as a [[flip-flop element]]. Derivatives provide up to four timing circuits in one package.


Introduced in 1971 by [[Signetics]], the 555 is still in widespread use due to its ease of use, low price, and stability.  It is now made by many companies in the original [[Bipolar junction transistor|bipolar]] and also in low-power [[CMOS]] types. {{As of|2003}}, it was estimated that 1 billion units are manufactured every year.<ref name="semiconductormuseum.com">[http://www.semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Page2.htm Ward, Jack (2004). The 555 Timer IC&nbsp;– An Interview with Hans Camenzind. The Semiconductor Museum. Retrieved 2010-04-05]</ref>
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== Design ==
The IC was designed in 1971 by [[Hans Camenzind]] under contract to [[Signetics]], which was later acquired by [[Philips]] (now [[NXP]]).
 
Depending on the manufacturer, the standard 555 package includes 25 [[transistor]]s, 2 [[diode]]s and 15 [[resistor]]s on a [[silicon]] chip installed in an 8-pin mini dual-in-line package ([[DIP-8]]).<ref>van Roon, Fig 3 & related text.</ref> Variants available include the 556 (a 14-pin DIP combining two 555s on one chip), and the two 558 & 559s (both a 16-pin DIP combining four slightly modified 555s with DIS & THR connected internally, and TR is falling edge sensitive instead of level sensitive).
 
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. It has been hypothesized that the 555 got its name from the three 5&nbsp;[[ohm|kΩ]] resistors used within,<ref>Scherz, Paul (2000) "Practical Electronics for Inventors", p. 589. McGraw-Hill/TAB Electronics. ISBN 978-0-07-058078-7. Retrieved 2010-04-05.</ref> but Hans Camenzind has stated that the number was arbitrary.<ref name="semiconductormuseum.com"/>
 
Low-power versions of the 555 are also available, such as the 7555 and CMOS TLC555.<ref>Jung, Walter G. (1983) "IC Timer Cookbook, Second Edition", pp. 40–41. Sams Technical Publishing; 2nd ed. ISBN 978-0-672-21932-0. Retrieved 2010-04-05.</ref> The 7555 is designed to cause less supply noise than the classic 555 and the manufacturer claims that it usually does not require a "control" capacitor and in many cases does not require a [[decoupling capacitor]] on the power supply. Those parts should generally be included, however, 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.
 
=== Pins ===
[[File:555 Pinout.svg|thumb|Pinout diagram]]
The connection of the pins for a DIP package is as follows:
{| class="wikitable"
! Pin !! Name !! Purpose
|-
| 1
| GND
| Ground reference voltage, low level (0 V)
|-
| 2
| TRIG
| The OUT pin goes high and a timing interval starts when this input falls below 1/2 of CTRL voltage (which is typically 1/3 of ''V''<sub>CC</sub>, when CTRL is open).
|-
| 3
| OUT
| This output is driven to approximately 1.7 V below [[Vcc|+''V''<sub>CC</sub>]] or GND.
|-
| 4
| {{overline|RESET}}
| A timing interval may be reset by driving this input to GND, but the timing does not begin again until RESET rises above approximately 0.7 volts. Overrides TRIG which overrides THR.
|-
| 5
| CTRL
| Provides "control" access to the internal voltage divider (by default, 2/3 ''V''<sub>CC</sub>).
|-
| 6
| THR
| The timing (OUT high) interval ends when the voltage at THR is greater than that at CTRL (2/3 ''V''<sub>CC</sub> if CTRL is open).
|-
| 7
| DIS
| [[Open collector]] output which may discharge a capacitor between intervals.  In phase with output.
|-
| 8
| ''V''<sub>CC</sub>
| Positive supply voltage, which is usually between 3 and 15 V depending on the variation.
|}
 
Pin 5 is also sometimes called the CONTROL VOLTAGE pin.
By applying a voltage to the CONTROL VOLTAGE input one can alter the timing characteristics of the device.
In most applications, the CONTROL VOLTAGE input is not used. It is usual to connect a 10 nF capacitor between pin 5 and 0 V to prevent interference.
The CONTROL VOLTAGE input can be used to build an astable with a frequency modulated output.
 
=== Modes ===
The 555 has three operating modes:
* '''[[Monostable]]''' mode: In this mode, the 555 functions as a "one-shot" pulse generator. Applications include timers, missing pulse detection, bouncefree switches, touch switches, frequency divider, capacitance measurement, [[pulse-width modulation]] (PWM) and so on.
* '''[[Astable]]''' (free-running) mode: The 555 can operate as an [[oscillator]]. Uses include [[light-emitting diode|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 [[Analog-to-digital_converter|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: 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.
* '''[[Flip-flop (electronics)|Bistable]]''' mode or [[Schmitt trigger]]: The 555 can operate as a [[flip-flop (electronics)|flip-flop]], if the DIS pin is not connected and no capacitor is used. Uses include bounce-free latched switches.
 
=== Monostable ===
{{see also|RC circuit}}
[[File:555 Monostable.svg|thumb|Schematic of a 555 in monostable mode]]
[[File:NE555 Monotable Waveforms (English).png|thumb|left|The relationships of the trigger signal, the voltage on C and the pulse width in monostable mode]]
In the monostable mode, the 555 timer acts as a "one-shot" pulse generator. The pulse begins when the 555 timer receives a signal at the trigger input that falls below a third of the voltage supply. The width of the output pulse is determined by the time constant of an RC network, which consists of a [[capacitor]] (C) and a [[resistor]] (R). The output pulse ends when the voltage on the capacitor equals 2/3 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.<ref>van Roon, Chapter "Monostable Mode". (Using the 555 timer as a logic clock)</ref>
 
The output pulse width of time ''t'', which is the time it takes to charge C to 2/3 of the supply voltage, is given by
 
:<math>t = RC\ln(3) \approx 1.1 RC</math>
 
where t is in seconds, R is in [[ohm]]s and C is in [[farads]].
 
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.<ref>http://www.national.com/ds/LM/LM555.pdf</ref>
{{clear}}
 
=== Bistable ===
[[File:555 Bistable.svg|thumb|Schematic of a 555 in bistable mode]]
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 resistor]]s while the threshold input (pin 6) is simply floating. 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 capacitors are required in a bistable configuration. Pin 5 (control) is connected to ground via a small-value capacitor (usually 0.01 to 0.1 uF); pin 7 (discharge) is left floating.
{{clear}}
 
=== Astable ===
[[File:555 Astable Diagram.svg|thumb|Standard 555 astable circuit]]
In astable mode, the 555 timer puts out a continuous stream of rectangular pulses having a specified frequency. Resistor R<sub>1</sub> is connected between V<sub>CC</sub> and the discharge pin (pin 7) and another resistor (R<sub>2</sub>) is connected between the discharge pin (pin 7), and the trigger (pin 2) and threshold (pin 6) pins that share a common node. Hence the capacitor is charged through R<sub>1</sub> and R<sub>2</sub>, and discharged only through R<sub>2</sub>, since pin 7 has low impedance to ground during output low intervals of the cycle, therefore discharging the capacitor.
 
In the astable mode, the frequency of the pulse stream depends on the values of R<sub>1</sub>, R<sub>2</sub> and C:
 
:<math>f = \frac{1}{\ln(2) \cdot C \cdot (R_1 + 2R_2)}</math><ref>van Roon Chapter: "Astable operation".</ref>
 
The high time from each pulse is given by:
 
:<math>\mathrm{high} = \ln(2) \cdot (R_1 + R_2) \cdot C</math>
 
and the low time from each pulse is given by:
 
:<math>\mathrm{low} = \ln(2) \cdot R_2 \cdot C</math>
 
where R<sub>1</sub> and R<sub>2</sub> are the values of the resistors in [[ohm]]s and C is the value of the capacitor in [[farad]]s.
 
The power capability of R<sub>1</sub> must be greater than <math>\frac{V_{cc}^{2}}{R_1}</math>.
 
Particularly with bipolar 555s, low values of <math>R_1</math> 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.  It should be noted that the first cycle will take appreciably longer than the calculated time, as the capacitor must charge from 0V to 2/3 of V<sub>CC</sub> from power-up, but only from 1/3 of V<sub>CC</sub> to 2/3 of V<sub>CC</sub> on subsequent cycles.
 
To achieve a [[duty cycle]] of less than 50% is to use a small diode (that is fast enough for the application) in parallel with R<sub>2</sub> (instead of placing it on pin 7), with the cathode on the capacitor side. This bypasses R<sub>2</sub> during the high part of the cycle so that the high interval depends approximately only on R<sub>1</sub> and C. The presence of the diode is a voltage drop that slows charging on the capacitor so that the high time is longer than the expected and often-cited ln(2)*R<sub>1</sub>C = 0.693 R<sub>1</sub>C.  The low time will be the same as without the diode as shown above.  With a diode, the high time is
 
:<math>\mathrm{high} = R_1 C \cdot \ln\left(\frac{2V_{\textrm{cc}}-3V_{\textrm{diode}}}{V_{\textrm{cc}}-3V_{\textrm{diode}}}\right) </math>
 
where V<sub>diode</sub> is when the diode has a current of 1/2 of V<sub>cc</sub>/R<sub>1</sub> which can be determined from its datasheet or by testing.  As an extreme example, when V<sub>cc</sub>= 5 and V<sub>diode</sub>= 0.7, high time = 1.00 R<sub>1</sub>C which is 45% longer than the "expected" 0.693 R<sub>1</sub>C.  At the other extreme, when V<sub>cc</sub>= 15 and V<sub>diode</sub>= 0.3, the high time = 0.725 R<sub>1</sub>C which is closer to the expected 0.693 R<sub>1</sub>C.  The equation reduces to the expected 0.693 R<sub>1</sub>C if V<sub>diode</sub>= 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.
 
== Specifications ==
These specifications apply to the NE555. Other 555 timers can have different specifications depending on the grade (military, medical, etc.).
 
{| class="wikitable"
| Supply voltage (''V''<sub>CC</sub>)
| 4.5 to 15 V
|-
| Supply current (''V''<sub>CC</sub> = +5 V)
| 3 to 6 mA
|-
| Supply current (''V''<sub>CC</sub> = +15 V)
| 10 to 15 mA
|-
| Output current (maximum)
| 200 mA
|-
| Maximum Power dissipation
| 600&nbsp;mW
|-
| Power consumption (minimum operating)
| 30&nbsp;mW@5V, 225&nbsp;mW@15V
|-
| [[Operating temperature]]
| 0 to 70 °C
|}
 
== Derivatives ==
Many pin-compatible variants, including [[CMOS]] versions, have been built by various companies. Bigger packages also exist with two or four timers on the same chip. The 555 is also known under the following type numbers:
 
{| class="wikitable"
! Manufacturer
! Model
! Remark
|-
| Custom Silicon Solutions<ref>http://www.customsiliconsolutions.com/products-for-ASIC-solutions/standard-IC-products.aspx</ref>
| CSS555/CSS555C
| CMOS from 1.2&nbsp;V, IDD&nbsp;<&nbsp;5&nbsp;µA
|-
| [[CEMI]]
| ULY7855
|
|-
| [[ECG Philips]]
| ECG955M
|
|-
| [[Exar]]
| XR-555
|
|-
| [[Fairchild Semiconductor]]
| NE555/KA555
|
|-
| [[GoldStar]]
| GSC555
| CMOS
|-
| [[Harris Corporation|Harris]]
| HA555
|
|-
| [[IK Semicon]]
| ILC555
| CMOS from 2&nbsp;V
|-
| [[Intersil]]
| SE555/NE555
|
|-
| [[Intersil]]
| ICM7555
| CMOS
|-
| [[Lithic Systems]]
| LC555
|
|-
| [[Maxim Integrated Products|Maxim]]
| ICM7555
| CMOS from 2&nbsp;V
|-
| [[Motorola]]
| MC1455/MC1555
|
|-
| [[National Semiconductor]]
| LM1455/LM555/LM555C
|
|-
| [[National Semiconductor]]
| LMC555
| CMOS from 1.5&nbsp;V
|-
| [[Sylvania Electric Products|NTE Sylvania]]
| NTE955M
|
|-
| [[Raytheon]]
| RM555/RC555
|
|-
| [[RCA]]
| CA555/CA555C
|
|-
| [[STMicroelectronics]]
| NE555N/ K3T647
|
|-
| [[Texas Instruments]]
| SN52555/SN72555
|
|-
| [[Texas Instruments]]
| TLC555
| CMOS from 2&nbsp;V
|-
| [[USSR]]
| K1006ВИ1
|
|-
| [[X-REL Semiconductor]]
| XTR655
|Operation from -60°C to 250+°C
|-
| [[Zetex Semiconductors|Zetex]]
| ZSCT1555
| down to 0.9&nbsp;V
|-
| [[NXP Semiconductors]]
| ICM7555
| CMOS
|-
| HFO / [[East Germany]]
| B555
|
|}
 
=== 556 Dual timer ===
[[Image:STM-NE556-HD.jpg|thumb|[[Die (integrated circuit)|Die]] of a 556 dual timer manufactured by [[STMicroelectronics]].]]
The dual version is called 556. It features two complete 555s in a 14 pin DIL package.
 
=== 558 Quad timer ===
The quad version is called 558 and has 16 pins. To fit four 555s into a 16 pin package the control, voltage, and reset lines are shared by all four modules. Each module's discharge and threshold circuits are wired together internally.
 
=== XTR651 Extended Functionality HiRel HiTemp (-60°C to 250+°C)===
This version includes non-overlapped complementary outputs, coarse temperature sensor and on-chip 200pF timing capacitance.<ref>[http://www.x-relsemi.com/EN/Documentation/Datasheet/DS-00100-11-XTR650-HIGH-TEMPERATURE%20VERSATILE%20TIMER.pdf 15 X-REL Semiconductor Data Sheet], 38100 Grenoble France</ref>
 
== Example applications ==
 
=== Joystick interface circuit using the 558 quad timer ===
The [[Apple II]] [[microcomputer]] used a quad timer 558 in monostable (or "one-shot") mode to interface up to four "game paddles" or two [[joystick]]s to the host computer. It also used a single 555 for flashing the display cursor.
 
A similar circuit was used in the [[IBM PC]].<ref>Engdahl, pg 1.</ref> In the joystick interface circuit of the IBM PC, the [[capacitor]] (C) of the RC network (see Monostable Mode above) was generally a 10 nF capacitor. The [[resistor]] (R) of the RC network consisted of the [[potentiometer]] inside the joystick along with an external resistor of 2.2 kilohms.<ref>Engdahl, "Circuit diagram of PC joystick interface"</ref> The joystick potentiometer acted as a variable resistor. By moving the joystick, the resistance of the joystick increased from a small value up to about 100 kilohms. The joystick operated at 5 V.<ref name="EngdahlPC">http://www.epanorama.net/documents/joystick/pc_joystick.html</ref>
 
Software running in the host computer started the process of determining the joystick position by writing to a special address (ISA bus I/O address 201h).<ref name="EngdahlPC" /><ref>Eggebrecht, p. 197.</ref> This would result in a trigger signal to the quad timer, which would cause the capacitor (C) 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 C to charge up to 2/3 of 5 V (or about 3.33 V), which was in turn determined by the joystick position.<ref name="EngdahlPC"/><ref>Eggebrecht, pp. 197-99</ref> 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.<ref name="EngdahlPC"/>
 
== See also ==
{{Portal|Electronics}}
* [[Counter]]
* [[Operational amplifier|OpAmp]]
* [[Electronic oscillator|Oscillator]]
* [[RC circuit]]
 
== References ==
{{Reflist|30em}}
 
== Further reading ==
* ''555 Timer Applications Sourcebook Experiments''; H. Berlin; BPB Publications; 218 pages; 2008; ISBN 978-8176567909.
* ''Timer, Op Amp, and Optoelectronic Circuits and Projects''; [[Forrest Mims]] III; Master Publishing; 128 pages; 2004; ISBN 978-0-945053-29-3.
* ''Engineer's Mini-Notebook&nbsp;– 555 Timer IC Circuits''; [[Forrest Mims]] III; Radio Shack; 33 pages; 1989; ASIN B000MN54A6.
* ''IC Timer Cookbook''; 2nd Ed; Walter G Jung; Sams Publishing; 384 pages; 1983; ISBN 978-0-672-21932-0.
* ''555 Timer Applications Sourcebook with Experiments''; Howard M Berlin; Sams Publishing; 158 pages; 1979; ISBN 978-0-672-21538-4.
* ''IC 555 Projects''; E.A. Parr; Bernard Babani Publishing; 144 pages; 1978; ISBN 978-0-85934-047-2.
 
== External links ==
{{Commons category|555 timer IC}}
* [http://www.eleinmec.com/article.asp?1 555 Timer Circuits&nbsp;– the Astable, Monostable and Bistable]
* [http://www.electroniq.net/555-timer-circuits Simple 555 timer circuits]
* [http://www.falstad.com/circuit/e-555square.html Java simulation of 555 oscillator circuit]
* [http://www.daycounter.com/Calculators/NE555-Calculator.phtml NE555 Frequency and duty cycle calculator] for astable multivibrators
* [http://www.globu.net/pp/english/pp/ne555.htm Using NE555 as a Temperature DSP]
* [http://www.sentex.ca/~mec1995/gadgets/555/555.html 555 Timer Tutorial]
* [http://www.555-timer-circuits.com/common-mistakes.html Common Mistakes When Using a 555 Timer]
* [http://www.kpsec.freeuk.com/555timer.htm 555 and 556 Timer Circuits]
* [http://elektronikhobi.net/555-entegresinin-ozellikleri-ve-kullanim-alanlari/ 555 using areas and examples circuits]
* [http://www.engineersgarage.com/555-timer-circuits Working with 555 Timer Circuits] Engineers Garage
* [http://ekalk.info/555a_en.html Analysis and synthesis of a 555 astable multivibrator circuit - online calculator]
* [http://www.cirvirlab.com/simulation/ne555_astable_multivibrator_online.php Online simulations of a 555 astable multivibrator circuit - online simulator]
;IC Datasheets
* [http://focus.ti.com/lit/ds/symlink/ne555.pdf NE555, Single Bipolar Timer, Texas Instruments]
* [http://focus.ti.com/lit/ds/symlink/na556.pdf NE556, Dual Bipolar Timer, Texas Instruments]
* [http://www.nxp.com/documents/data_sheet/NE558_3.pdf NE558, Quad Bipolar Timer, NXP]
* [http://www.ti.com/lit/gpn/lmc555 LMC555, Single CMOS Timer, Texas Instruments] (operates down to 1.5 Volt at 50 uAmp)
* [http://www.intersil.com/data/fn/fn2867.pdf ICM755x, Single / Dual CMOS Timer, Intersil] (operates down to 2.0 Volt at 60 uAmp)
* [http://www.diodes.com/datasheets/ZSCT1555.pdf ZSCT1555, Single CMOS Timer, Diodes Inc] (operates down to 0.9 Volt at 74 uAmp)
* [http://touchstonesemi.com/products/timers TS300x, Single CMOS Timers, Touchstone] (operates down to 0.9 Volt at 1.0 uAmp)
* [http://www.x-relsemi.com/EN/Documentation/Datasheet/DS-00100-11-XTR650-HIGH-TEMPERATURE%20VERSATILE%20TIMER.pdf XTR65x, HiRel HiTemp Timer, X-REL] (operates from -60°C to 230°C)
 
[[Category:Oscillators]]
[[Category:Linear integrated circuits]]

Latest revision as of 20:59, 11 January 2015

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