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 Signetics NE555 in 8-pinDIP package | |
| Component type | Active,integrated circuit |
|---|---|
| Inventor | Hans Camenzind (1971) |
| First produced | 1972 |
| Electronic symbol | |
 Internal block diagram[1] | |
The555 timer IC is anintegrated circuit used in a variety oftimer, delay, pulse generation, andoscillator applications. It is one of the most popular timing ICs due to its flexibility and price. Derivatives provide two (556) or four (558) timing circuits in one package.[2] The design was first marketed in 1972 bySignetics[3][4] and usedbipolar junction transistors. Since then, numerous companies have made the original timers and later similar low-powerCMOS 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".[5]
The timer IC was designed in 1971 byHans Camenzind under contract toSignetics.[3] In 1968, he was hired by Signetics to develop aphase-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 the1970 recession, and development on the PLL was thus frozen.[6] 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.[7]
The first design for the 555 was reviewed in the summer of 1971.[8] 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.[8] 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.[8] This revised version passed a second design review, and the prototypes were completed in October 1971 as the NE555V (plasticDIP) and SE555T (metalTO-5).[9] 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.[6]
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."[8]
Several books report the name "555" timer IC derived from the three 5 kΩ resistors inside the chip.[10][11][12] However, in a recorded interview with an online transistor museum curator,[13] 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."[14]
Depending on the manufacturer, the standard 555 package incorporated the equivalent of 25transistors, 2diodes, and 15resistors on asilicon chip packaged into an 8-pindual in-line package (DIP-8).[15] Variants available included the 556 (a DIP-14 combining two complete 555s on one chip),[16] and 558 / 559 (both variants were a DIP-16 combining four reduced-functionality timers on one chip).[2]
TheNE555 parts were commercial temperature range, 0 °C to +70 °C, and theSE555 part number designated the military temperature range, −55 °C to +125 °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.[17][18][19][20]
The internalblock diagram andschematic of the 555 timer are highlighted with the same color across all three drawings to clarify how the chip is implemented:[2]
![555 internal block diagram[1]](/mt/?noimg=&dark=on&url=https%3a%2f%2fwikipedia.org%2fwiki%2fFile%3aNE555_Bloc_Diagram.svg)
Thepinout of the 8-pin 555 timer[1] and 14-pin 556 dual timer[21] 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.[2]
| 555 pin# | 556(unit 1) | 556(unit 2) | Pin name | Pin direction | Pin description[1][21][2] |
|---|---|---|---|---|---|
| 1 | 7 | GND | Power | Ground supply: this pin is theground reference voltage (zero volts).[22] | |
| 2 | 6 | 8 | TRIGGER | Input | Trigger: whenVTRIGGER falls below1⁄2VCONTROL (1⁄3VCC, except when CONTROL is driven by an external signal), OUTPUT goes to the high state and a timing interval starts.[22] As long as TRIGGER continues to be kept at a low voltage, OUTPUT will remain in the high state. |
| 3 | 5 | 9 | OUTPUT | Output | Output: this pin is apush-pull (P.P.) output that is driven to either a low state (GND) or a high state (VCC minus approximately 1.7 volts for bipolar timers, orVCC for CMOS timers). |
| 4 | 4 | 10 | RESET | 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 overridesTRIGGER, which in turn overrides THRESHOLD. If this pin is not used, it should be connected toVCC to prevent electrical noise accidentally causing a reset.[23][22] |
| 5 | 3 | 11 | CONTROL | Input | Control: this pin provides access to the internalvoltage divider (2⁄3VCC 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.[16] If this pin is not used, it should be connected to a 10nFdecoupling capacitor (between this pin and GND) to ensure electrical noise doesn't affect the internal voltage divider.[2][23][22] |
| 6 | 2 | 12 | THRESHOLD | Input | Threshold: when the voltage at this pin is greater thanVCONTROL (2⁄3VCC 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.[22] |
| 7 | 1 | 13 | DISCHARGE | Output | Discharge: This pin is anopen-collector (O.C.) output for bipolar timers, or an open-drain (O.D.) output for CMOS timers. This pin can be used to discharge acapacitor 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.[22] |
| 8 | 14 | VCC | 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 thederivatives table in this article.Decoupling capacitor(s) are generally applied (between this pin and GND) as a good practice.[24][23] | |
![Pinout of 555 single timer[1][2]](/mt/?noimg=&dark=on&url=https%3a%2f%2fwikipedia.org%2fwiki%2fFile%3a555_Pinout.svg)
![Pinout of 556 dual timer[21][2]](/mt/?noimg=&dark=on&url=https%3a%2f%2fwikipedia.org%2fwiki%2fFile%3aNE556_pennen.svg)
The 555 IC has the following operating modes:
| Frequency | C | R1 | R2 | Duty cycle |
|---|---|---|---|---|
| 0.1 Hz (+0.048%) | 100 μF | 8.2 kΩ | 68 kΩ | 52.8% |
| 1 Hz (+0.048%) | 10 μF | 8.2 kΩ | 68 kΩ | 52.8% |
| 10 Hz (+0.048%) | 1 μF | 8.2 kΩ | 68 kΩ | 52.8% |
| 100 Hz (+0.048%) | 100 nF | 8.2 kΩ | 68 kΩ | 52.8% |
| 1 kHz (+0.048%) | 10 nF | 8.2 kΩ | 68 kΩ | 52.8% |
| 10 kHz (+0.048%) | 1 nF | 8.2 kΩ | 68 kΩ | 52.8% |
| 100 kHz (+0.048%) | 100 pF | 8.2 kΩ | 68 kΩ | 52.8% |
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, and and one capacitor. 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:
During the first pulse, the capacitor charges from 0 V to2⁄3 VCC, however, in later pulses, it only charges from1⁄3 VCC to2⁄3 VCC. 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, 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:[16]
The output low time interval of each pulse is given by:[16]
Hence, thefrequency of the pulse is given by:[16]
and theduty cycle is given by:[16]
where is the time inseconds, is the resistance inohms, is the capacitance infarads, and is thenatural log of 2 constant.[a]
Resistor requirements:
To create an output high time shorter than the low time (i.e., aduty cycle less than 50%) a fast diode (i.e.1N4148 signal diode) can be placed in parallel with R2, with the cathode on the capacitor side.[16] This bypasses R2 during the high part of the cycle, so that the high interval depends only on R1 and C, with an adjustment based on the voltage drop across the diode. The low time is unaffected by the diode and so remains But the diode's forwardvoltage dropVdiode slows charging on the capacitor, so the high time is longer than the often-cited to become:
whereVdiode is when the diode's "on" current is1⁄2 ofVCC/R1 (whichdepends on the type of diode and can be found in datasheets or measured). When Vdiode is small relative toVcc, this charging is faster and approaches but is slower the closer Vdiode is toVcc:
As an extreme example, whenVCC = 5 V, and Vdiode = 0.7 V, high time is 1.00 R1C, which is 45% longer than the "expected" 0.693 R1C. At the other extreme, whenVcc = 15 V, and Vdiode = 0.3 V, the high time is 0.725 R1C, which is closer to the expected 0.693 R1C. The equation approaches 0.693 R1C asVdiode approaches 0 V.
In the previous example schematics, the control pin was not used, thus it should connected to ground through a 10 nFdecoupling 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.
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Monostable mode produces an output pulse when the trigger signals drops below1⁄3VCC. AnRC circuit sets the output pulse's duration as the time inseconds it takes to charge C to2⁄3VCC:[16]
where is the resistance inohms, is the capacitance infarads, is thenatural log of 3 constant.[b] 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.[25]
| Time | C | 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 |
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:
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.
A 555 timer can act as anactive-lowSR latch (though without an invertedQ output) with two outputs: output pin is apush-pull output, discharge pin is anopen-collector output (requires apull-up resistor).
For the schematic on the right, aReset input signal connects to theRESET pin and connecting aSet input signal to theTR pin. Thus, pullingSet momentarily low acts as a "set" and transitions the output to the high state (VCC). Conversely, pullingReset 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.[26] The trigger and reset inputs may be held high viapull-up resistors if they are normallyHi-Z and only enabled by connecting to ground.
A 555 timer can be used to create aSchmitt triggerinverter gate with two outputs: output pin is apush-pull output, discharge pin is anopen-collector output (requires apull-up resistor).
For the schematic on the right, an input signal isAC-coupled through a low value series capacitor, then biased by identical high-resistance resistors and, which causes the signal to be centered at1⁄2Vcc. 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 lower1⁄3VCC and upper2⁄3VCC thresholds in order to cause them to change state, thus providing the Schmitt trigger feature.[27]
No timing capacitors are required in a bistable configuration.
In 1972,Signetics originally released the 555 timer inDIP-8 andTO5-8 metal can packages, and the 556 timer was released in a DIP-14 package.[4]
In 2006, the dual 556 timer was available in through-hole packages as DIP-14 (2.54 mm pitch),[21] and surface-mount packages as SO-14 (1.27 mm pitch) and SSOP-14 (0.65 mm pitch).
In 2012, the 555 was available in through-hole packages as DIP-8 (2.54 mm pitch),[28] and surface-mount packages as SO-8 (1.27 mm pitch), SSOP-8 /TSSOP-8 / VSSOP-8 (0.65 mm pitch),BGA (0.5 mm pitch).[1]
The MIC1555 is aCMOS 555-type timer with three fewer pins available inSOT23-5 (0.95 mm pitch) surface-mount package.[29]
These specifications apply to the original bipolar NE555. Other 555 timers can have different specifications depending on the grade (industrial, military, medical, etc.).
| Part number | NE555 |
| IC Process | Bipolar |
| Supply voltage (VCC) | 4.5 to 16V |
| Supply current (VCC = +5 V) | 3 to 6mA |
| 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 @ 5 V, 225 mW @ 15 V |
| Operating temperature | 0 to 70 °C |
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:
| Manufacturer | Part number | Production status | IC process | Timers total | Supply min. (volt) | Supply max. (volt) | Iq (μA) at 5 V supply | Frequency max. (MHz) | Remarks | Datasheet |
|---|---|---|---|---|---|---|---|---|---|---|
| Custom Silicon Solutions (CSS) | CSS555 | Active | CMOS | 1 | 1.2 | 5.5 | 4.3 | 1.0 | InternalEEPROM, requires programmer | [30][31][32] |
| Diodes Inc | ZSCT1555 | Discontinued | Bipolar | 1 | 0.9 | 6 | 150 | 0.33 | Designed byHans Camenzind | [33] |
| Japan Radio Company (JRC) | NJM555 | Discontinued | Bipolar | 1 | 4.5 | 16 | 3000 | 0.1* | Also available inSIP-8 package. | [28] |
| Microchip | MIC1555/7 | 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 inSOT23-5, TSOT23-5,UTDFN-10 packages. | [29] |
| ON | MC1455 | Active | Bipolar | 1 | 4.5 | 16 | 3000 | 0.1* | — | [34] |
| Renesas | ICM7555 | Active | CMOS | 1 | 2 | 18 | 40 | 1.0 | [17] | |
| Renesas | ICM7556 | Active | CMOS | 2 | 2 | 18 | 80 | 1.0 | [17] | |
| Signetics | NE555 | Active (TI) | Bipolar | 1 | 4.5 | 16 | 3000 | 0.1* | First 555 timer,DIP-8 orTO5-8 packages. | [4][16][35][2] |
| Signetics | NE556 | Active (TI) | Bipolar | 2 | 4.5 | 16 | 6000 | 0.1* | First 556 timer, DIP-14 package. | [16][2] |
| Signetics | NE558 | Discontinued | Bipolar | 4* | 4.5 | 16 | 4800* | 0.1* | First 558 timer, DIP-16 package. | [2] |
| STMicroelectronics (ST) | TS555 | Active | CMOS | 1 | 2 | 16 | 110 | 2.7 | — | [36] |
| Texas Instruments (TI) | LM555 | Active | Bipolar | 1 | 4.5 | 16 | 3000 | 0.1 | [25] | |
| Texas Instruments | LM556 | Discontinued | Bipolar | 2 | 4.5 | 16 | 6000 | 0.1 | [37] | |
| Texas Instruments | LMC555 | Active | CMOS | 1 | 1.5 | 15 | 100 | 3.0 | Also available inDSBGA-8 package. | [18] |
| Texas Instruments | NE555 | Active | Bipolar | 1 | 4.5 | 16 | 3000 | 0.1* | — | [1] |
| Texas Instruments | NE556 | Active | Bipolar | 2 | 4.5 | 16 | 6000 | 0.1* | — | [21] |
| Texas Instruments | TLC551 | Active | CMOS | 1 | 1 | 15 | 170 | 1.8 | [20] | |
| Texas Instruments | TLC552 | Active | CMOS | 2 | 1 | 15 | 340 | 1.8 | [38] | |
| Texas Instruments | TLC555 | Active | CMOS | 1 | 2 | 15 | 170 | 2.1 | — | [19] |
| Texas Instruments | TLC556 | Active | CMOS | 2 | 2 | 15 | 340 | 2.1 | — | [39] |
| X-REL | XTR655 | Active | SOI | 1 | 2.8 | 5.5 | 170 | 4.0 | Extreme (−60 °C to +230 °C), ceramic DIP-8 package or bare die. | [40] |
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.[21][16] In 2020, the bipolar version was available as the NE556,[21] and the CMOS versions were available as the Intersil ICM7556 and Texas Instruments TLC556 and TLC552. Seederivatives table in this article.[17][39][38]
The quad version is called 558 and has four reduced-functionality timers in a 16-pin package designed primarily formonostable multivibrator applications.[49][2] By 2014, many versions of 16-pin NE558 have become obsolete.[50]
Partial list of differences between 558 and 555 chips:[2][50]
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.
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.
The 555 got its name from the three 5-kOhm resistors
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
Not all functions are brought out to the 558's pins. This chip is designed primarily for monostable multivibrator applications
