555 timer IC 1

555 timer IC

NE555 from Signetics in dual-in-line package

Internal block diagram

The 555 Timer IC is an integrated circuit (chip) implementing avariety of timer and multivibrator applications. The IC was designedby Hans R. Camenzind in 1970 and brought to market in 1971 bySignetics (later acquired by Philips). The original name was the SE555(metal can)/NE555 (plastic DIP) and the part was described as "The ICTime Machine".[1] It has been claimed that the 555 gets its name fromthe three 5 kΩ resistors used in typical early implementations,[2] butHans Camenzind has stated that the number was arbitrary.[3] The partis still in wide use, thanks to its ease of use, low price and goodstability. As of 2003, it is estimated that 1 billion units aremanufactured every year.[3]

Depending on the manufacturer, the standard 555 package includesover 20 transistors, 2 diodes and 15 resistors on a silicon chip installedin an 8-pin mini dual-in-line package (DIP-8).[4] Variants availableinclude the 556 (a 14-pin DIP combining two 555s on one chip), andthe 558 (a 16-pin DIP combining four slightly modified 555s with DIS& THR connected internally, and TR falling edge sensitive instead oflevel sensitive).

Ultra-low power versions of the 555 are also available, such as the7555 and TLC555.[5] The 7555 requires slightly different wiring usingfewer external components and less power.

The 555 has three operating modes:• Monostable mode: in this mode, the 555 functions as a "one-shot". Applications include timers, missing pulse

detection, bouncefree switches, touch switches, frequency divider, capacitance measurement, pulse-widthmodulation (PWM) etc

• Astable - free running mode: the 555 can operate as an oscillator. Uses include LED and lamp flashers, pulsegeneration, logic clocks, tone generation, security alarms, pulse position modulation, etc.

• Bistable mode or Schmitt trigger: the 555 can operate as a flip-flop, if the DIS pin is not connected and nocapacitor is used. Uses include bouncefree latched switches, etc.

Usage

Pinout diagram

The connection of the pins is as follows:

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Pin Name Purpose

1 GND Ground, low level (0 V)

2 TRIG OUT rises, and interval starts, when this input falls below 1/3 VCC.

3 OUT This output is driven to +VCC or GND.

4 RESET A timing interval may be interrupted by driving this input to GND.

5 CTRL "Control" access to the internal voltage divider (by default, 2/3VCC).

6 THR The interval ends when the voltage at THR is greater than at CTRL.

7 DIS Open collector output; may discharge a capacitor between intervals.

8 V+,VCC

Positive supply voltage is usually between 3 and 15 V.

Monostable mode

Schematic of a 555 in monostable mode

The relationships of the trigger signal, the voltageon C and the pulse width in monostable mode

In the monostable mode, the 555 timer acts as a “one-shot” pulsegenerator. The pulse begins when the 555 timer receives a signal at thetrigger input that falls below a third of the voltage supply. The width ofthe pulse is determined by the time constant of an RC network, whichconsists of a capacitor (C) and a resistor (R). The pulse ends when thecharge on the C equals 2/3 of the supply voltage. The pulse width canbe lengthened or shortened to the need of the specific application byadjusting the values of R and C.[6]

The pulse width of time t, which is the time it takes to charge C to 2/3of the supply voltage, is given by

where t is in seconds, R is in ohms and C is in farads. See RC circuit for an explanation of this effect.

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Bistable ModeIn bistable mode, the 555 timer acts as a basic flip-flop. The trigger and reset inputs (pins 2 and 4 respectively on a555) are held high via pull-up resistors while the threshold input (pin 6) is simply 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 capacitorsare required in a bistable configuration. Pins 5 and 7 (control and discharge) are left floating.

Astable mode

Standard 555 Astable Circuit

In astable mode, the '555 timer ' puts out a continuous stream ofrectangular pulses having a specified frequency. Resistor R1 isconnected between VCC and the discharge pin (pin 7) and anotherresistor (R2) is connected between the discharge pin (pin 7), and thetrigger (pin 2) and threshold (pin 6) pins that share a common node.Hence the capacitor is charged through R1 and R2, and discharged onlythrough R2, since pin 7 has low impedance to ground during outputlow intervals of the cycle, therefore discharging the capacitor.

In the astable mode, the frequency of the pulse stream depends on thevalues of R1, R2 and C:

[7]

The high time from each pulse is given by

and the low time from each pulse is given by

where R1 and R2 are the values of the resistors in ohms and C is the value of the capacitor in farads.To achieve a duty cycle of less than 50% a diode can be added in parallel with R2 towards the capacitor. Thisbypasses R2 during the high part of the cycle so that the high interval depends only on R1 and C1.

SpecificationsThese specifications apply to the NE555. Other 555 timers can have better specifications depending on the grade(military, medical, etc).

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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 70 °C

DerivativesMany pin-compatible variants, including CMOS versions, have been built by various companies. Bigger packagesalso exist with two or four timers on the same chip. The 555 is also known under the following type numbers:

Manufacturer Model Remark

Custom Silicon Solutions[8]

CSS555/CSS555C CMOS from 1.2 V, IDD < 5 µA

Avago Technologies Av-555M

ECG Philips ECG955M

Exar XR-555

Fairchild Semiconductor NE555/KA555

Harris HA555

IK Semicon ILC555 CMOS from 2 V

Intersil SE555/NE555

Intersil ICM7555 CMOS

Lithic Systems LC555

Maxim ICM7555 CMOS from 2 V

Motorola MC1455/MC1555

National Semiconductor LM1455/LM555/LM555C

National Semiconductor LMC555 CMOS from 1.5 V

NTE Sylvania NTE955M

Raytheon RM555/RC555

RCA CA555/CA555C

STMicroelectronics NE555N/ K3T647

Texas Instruments SN52555/SN72555

Texas Instruments TLC555 CMOS from 2 V

USSR K1006ВИ1

Zetex ZSCT1555 down to 0.9 V

NXP Semiconductors ICM7555 CMOS

555 timer IC 5

Dual timer 556The dual version is called 556. It features two complete 555s in a 14 pin DIL package.

Quad timer 558The quad version is called 558 and has 16 pins. To fit four 555s into a 16 pin package the control voltage and resetlines are shared by all four modules. Also for each module the discharge and threshold are internally wired togetherand called timing.

Example applications

Joystick interface circuit using quad timer 558The original IBM personal computer used a quad timer 558 in monostable (or "one-shot") mode to interface up totwo joysticks to the host computer.[9] In the joystick interface circuit of the IBM PC, the capacitor (C) of the RCnetwork (see Monostable Mode above) was generally a 10 nF capacitor. The resistor (R) of the RC networkconsisted of the potentiometer inside the joystick along with an external resistor of 2.2 kilohms.[10] The joystickpotentiometer acted as a variable resistor. By moving the joystick, the resistance of the joystick increased from asmall value up to about 100 kilohms. The joystick operated at 5 V.[11]

Software running in the host computer started the process of determining the joystick position by writing to a specialaddress (ISA bus I/O address 201h).[12] [13] This would result in a trigger signal to the quad timer, which wouldcause the capacitor (C) of the RC network to begin charging and cause the quad timer to output a pulse. The width ofthe 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 turndetermined by the joystick position.[12] [14]

Software running in the host computer measured the pulse width to determine the joystick position. A wide pulserepresented the full-right joystick position, for example, while a narrow pulse represented the full-left joystickposition.[12]

Atari Punk ConsoleOne of Forrest M. Mims III's many books was dedicated to the 555 timer. In it, he first published the "Stepped ToneGenerator" circuit which has been adopted as a popular circuit, known as the Atari Punk Console, by circuit bendersfor its distinctive low-fi sound similar to classic Atari games.

Pulse Width ModulationThe 555 can be used to generate a variable PWM signal using a few external components. The chip alone can drivesmall external loads or an amplifying transistor for larger loads.

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Bibliography• van Roon, Tony (1995). "555 Timer Tutorial" [15] Tony van Roon (VA3AVR) Website. Retrieved 2010-04-05.• Engdahl, Tomi (1994). "PC analogue joystick interface" [16]. EPanorama.net. Retrieved 2009-06-06.• Eggebrecht, Lewis C. (1983). "Interfacing to the IBM Personal Computer". Sams Publishing. ISBN:

978-0672220272. Retrieved 2010-04-05.

External links• Surtell, Tim (2001). 555 Timer Circuits - the Astable, Monostable and Bistable [17] Electronics in Meccano.• Hewes, John (2010) 555 and 556 Timer Circuits [18] The Electronics Club.• LF/LM555 Data Sheet(PDF) [19] Fairchild Semiconductor, 2002.• Falstad, John (2010)Java simulation [20] of 555 oscillator circuit. Falstad.com• NE555 datasheet (PDF) [21] Collection of 555 Datasheets. DataSheetArchive.com.• Roca, Juan Carlos Galarza (2007) Using NE 555 as a Temperature DSP [22] "The Parallel port as an Input/output

Interface" (unpublished book)• NE555 Frequency and duty cycle calculator [23] for astable multivibrators. Daycounter.com. 2004. Notes 20%

inaccuracy.

• "Eagleapex" (2007) Time-lapse [[intervalometer [24]] for SLRs using a 555]. Instructables.com.

References[1] van Roon, "pg. 1" (http:/ / www. sentex. ca/ ~mec1995/ gadgets/ 555/ 555. html)[2] Scherz, Paul (2000) "Practical Electronics for Inventors," p. 589. McGraw-Hill/TAB Electronics. ISBN: 978-0070580787. Retrieved

2010-04-05.[3] Ward, Jack (2004). The 555 Timer IC - An Interview with Hans Camenzind (http:/ / www. semiconductormuseum. com/ Transistors/

LectureHall/ Camenzind/ Camenzind_Page2. htm). The Semiconductor Museum (http:/ / www. semiconductormuseum. com/ ). Retrieved2010-04-05.

[4] van Roon, Fig 3 & related text.[5] Jung, Walter G. (1983) "IC Timer Cookbook, Second Edition," pp. 40–41. Sams Technical Publishing; 2nd ed. ISBN: 978-0672219320.

Retrieved 2010-04-05.[6] van Roon, Chapter "Monostable Mode."[7] van Roon Chapter: "Astable operation."[8] http:/ / www. customsiliconsolutions. com[9] Engdahl, pg 1.[10] Engdahl, "Circuit diagram of PC joystick interface"[11] Engdahl, "Joystick construction" (http:/ / www. epanorama. net/ documents/ joystick/ pc_joystick. html#introduction).[12] Engdahl, "PC analogue joystick interface" (http:/ / www. epanorama. net/ documents/ joystick/ pc_joystick. html#pc_interface).[13] Eggebrecht, p. 197.[14] Eggebrecht, pp. 197-99[15] http:/ / www. sentex. ca/ ~mec1995/ gadgets/ 555/ 555. html[16] http:/ / www. epanorama. net/ documents/ joystick/ pc_joystick. html[17] http:/ / www. eleinmec. com/ article. asp?1[18] http:/ / www. kpsec. freeuk. com/ 555timer. htm[19] http:/ / www. fairchildsemi. com/ ds/ LM%2FLM555. pdf[20] http:/ / www. falstad. com/ circuit/ e-555square. html[21] http:/ / www. datasheetarchive. com/ search. php?q=NE555& sType=part& ExactDS=Starts[22] http:/ / www. globu. net/ pp/ english/ pp/ ne555. htm[23] http:/ / www. daycounter. com/ Calculators/ NE555-Calculator. phtml[24] http:/ / www. instructables. com/ id/ SORIJ3MFAQDX2PW/

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Image Sources, Licenses and ContributorsImage:Signetics NE555N.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Signetics_NE555N.JPG  License: GNU Free Documentation License  Contributors: Gvf, Shooke,Stefan506, 1 anonymous editsImage:NE555 Bloc Diagram.svg  Source: http://en.wikipedia.org/w/index.php?title=File:NE555_Bloc_Diagram.svg  License: Public Domain  Contributors: User:BlanchardJImage:555 Pinout.svg  Source: http://en.wikipedia.org/w/index.php?title=File:555_Pinout.svg  License: Public Domain  Contributors: User:InductiveloadImage:555 Monostable.svg  Source: http://en.wikipedia.org/w/index.php?title=File:555_Monostable.svg  License: Public Domain  Contributors: User:InductiveloadImage:NE555 Monotable Waveforms (English).png  Source: http://en.wikipedia.org/w/index.php?title=File:NE555_Monotable_Waveforms_(English).png  License: Public Domain Contributors: User:BlanchardJ, User:JacksonmissImage:555 Astable Diagram.svg  Source: http://en.wikipedia.org/w/index.php?title=File:555_Astable_Diagram.svg  License: Public Domain  Contributors: jjbeard

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