555 timer IC

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The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation, and oscillator applications. The 555 can be used to provide time delays, as an oscillator, and as a flip-flop element. Derivatives provide two (556) or four (558) timing circuits in one package.

Introduced in 1972 by Signetics, the 555 is still in widespread use due to its low price, ease of use, and stability. It is now made by many companies in the original bipolar and in low-power CMOS technologies. As of 2003, it was estimated that 1 billion units were manufactured every year. The 555 is the most popular integrated circuit ever manufactured.

Video 555 timer IC

History

The IC was designed in 1971 by Hans R. Camenzind under contract to Signetics (later acquired by Philips Semiconductors, and now NXP).

In 1962, Camenzind joined PR Mallory's Laboratory for Physical Science in Burlington, Massachusetts. He designed a pulse-width modulation (PWM) amplifier for audio applications, but it was not successful in the market because there was no power transistor included. He became interested in tuners such as a gyrator and a phase-locked loop (PLL). He was hired by Signetics to develop a PLL IC in 1968. He designed an oscillator for PLLs such that the frequency did not depend on the power supply voltage or temperature. However, Signetics laid off half of its employees, and the development was frozen due to a recession.

Camenzind proposed the development of a universal circuit based on the oscillator for PLLs, and asked that he would develop it alone, borrowing their equipment instead of having his pay cut in half. Other engineers argued the product could be built from existing parts, but the marketing manager bought the idea. Among 5xx numbers that were assigned for analogue ICs, the special number "555" was chosen.

Camenzind also taught circuit design at Northeastern University in the morning, and went to the same university at night to get a master's degree in Business Administration. The first design was reviewed in the summer of 1971. There was no problem, so it proceeded to layout design. A few days later, he got the idea of using a direct resistance instead of a constant current source, and found that it worked. The change decreased the required 9 pins to 8, so the IC could be fit in an 8-pin package instead of a 14-pin package. This design passed the second design review, and the prototype was completed in October 1971. Its 9-pin copy had been already released by another company founded by an engineer who attended the first review and retired from Signetics, but they withdrew it soon after the 555 was released. The 555 timer was manufactured by 12 companies in 1972 and it became the best selling product.

Part name

It has been falsely hypothesized that the 555 got its name from the three 5 k? resistors used within, but Hans Camenzind has stated that the part number was arbitrary, thus it's just a coincidence they matched. The "NE" and "SE" letters of the original parts numbers (NE555 and SE555) were temperature designations for analog chips from Signetics, where "NE" was commercial temperature family and "SE" was military temperature family.

Maps 555 timer IC

Design

Depending on the manufacturer, the standard 555 package includes 25 transistors, 2 diodes and 15 resistors on a silicon chip installed in an 8-pin dual in-line package (DIP-8). Variants available include the 556 (a DIP-14 combining two complete 555s on one chip), and 558 / 559 (both a DIP-16 combining four reduced-functionality timers on one chip).

The NE555 parts were commercial temperature range, 0 °C to +70 °C, and the SE555 part number designated the military temperature range, -55 °C to +125 °C. These were available in both high-reliability metal can (T package) and inexpensive epoxy plastic (V package) packages. Thus the full part numbers were NE555V, NE555T, SE555V, and SE555T.

Low-power CMOS versions of the 555 are also available, such as the Intersil ICM7555 and Texas Instruments LMC555, TLC555, TLC551. CMOS timers use significantly less power than bipolar timers, also CMOS timers cause less supply noise than bipolar version when the output switches states. The ICM7555 datasheet claims that it usually doesn't require a "control" capacitor and in many cases does not require a decoupling capacitor across the power supply pins. For good design practices, a decoupling capacitor should 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.

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:

• Green: Between the positive supply voltage V_CC and the ground GND is a voltage divider consisting of three identical resistors, which create two reference voltages at ¹/₃ V_CC and ²/₃ V_CC. The latter is connected to the "Control Voltage" pin. All three resistors have the same resistance, 5 k? for bipolar timers, 40 k? (or other higher resistance values) for CMOS timers. It is a false myth that the 555 IC got its name from these three 5 k? resistors.
• Yellow: The comparator negative input is connected to the higher-reference voltage divider of ²/₃ V_CC (and "Control" pin), and comparator positive input is connected to the "Threshold" pin.
• Red: The comparator positive input is connected to the lower-reference voltage divider of ¹/₃ V_CC, and comparator negative input is connected to the "Trigger" pin.
• Purple: An SR flip-flop stores the state of the timer and is controlled by the two comparators. The "Reset" pin overrides the other two inputs, thus the flip-flop (and therefore the entire timer) can be reset at any time.
• Pink: The output of the flip-flop is followed by an output stage with push-pull (P.P.) output drivers that can load the "Output" pin with up to 200 mA (varies by device).
• Cyan: Also, the output of the flip-flop turns on a transistor that connects the "Discharge" pin to ground.

Pinout

The typical pinout of the 555 and 556 IC packages are as follows:

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Modes

The IC 555 has three operating modes:

1. Astable (free-running) mode - the 555 can operate as an electronic oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation and so on. The 555 can be used as a simple ADC, converting an analog value to a pulse length (e.g., selecting a thermistor as timing resistor allows the use of the 555 in a temperature sensor and the period of the output pulse is determined by the temperature). The use of a microprocessor-based circuit can then convert the pulse period to temperature, linearize it and even provide calibration means.
2. Monostable mode - in this mode, the 555 functions as a "one-shot" pulse generator. Applications include timers, missing pulse detection, bounce-free switches, touch switches, frequency divider, capacitance measurement, pulse-width modulation (PWM) and so on.
3. Bistable (schmitt trigger) mode - the 555 can operate as a flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bounce-free latched switches.

Astable

In astable mode, the 555 timer puts out a continuous stream of rectangular pulses having a specified frequency. Resistor R₁ is connected between V_CC and the discharge pin (pin 7) and another resistor (R₂) 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 R₁ and R₂, and discharged only through R₂, 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 R₁, R₂ and C:

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

The high time from each pulse is given by:

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

and the low time from each pulse is given by:

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

where R₁ and R₂ are the values of the resistors in ohms and C is the value of the capacitor in farads.

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

Particularly with bipolar 555s, low values of ${\displaystyle R_{1}}$ must be avoided so that the output stays saturated near zero volts during discharge, as assumed by the above equation. Otherwise the output low time will be greater than calculated above. The first cycle will take appreciably longer than the calculated time, as the capacitor must charge from 0V to ²/₃ of V_CC from power-up, but only from ¹/₃ of V_CC to ²/₃ of V_CC on subsequent cycles.

To have an output high time shorter than the low time (i.e., a duty cycle less than 50%) a fast diode (i.e. 1N4148 signal diode) can be placed in parallel with R₂, with the cathode on the capacitor side. This bypasses R₂ during the high part of the cycle so that the high interval depends only on R₁ and C, with an adjustment based the voltage drop across the diode. The voltage drop across the diode slows charging on the capacitor so that the high time is a longer than the expected and often-cited ln(2)*R₁C = 0.693 R₁C. The low time will be the same as above, 0.693 R₂C. With the bypass diode, the high time is

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

where V_diode is when the diode's "on" current is ¹/₂ of V_cc/R₁ which can be determined from its datasheet or by testing. As an extreme example, when V_cc= 5 and V_diode= 0.7, high time = 1.00 R₁C which is 45% longer than the "expected" 0.693 R₁C. At the other extreme, when V_cc= 15 and V_diode= 0.3, the high time = 0.725 R₁C which is closer to the expected 0.693 R₁C. The equation reduces to the expected 0.693 R₁C if V_diode= 0.

The operation of RESET in this mode is not well-defined. Some manufacturers' parts will hold the output state to what it was when RESET is taken low, others will send the output either high or low.

The astable configuration, with two resistors, cannot produce a 50% duty cycle. To produce a 50% duty cycle, eliminate R1, disconnect pin 7 and connect the supply end of R2 to pin 3, the output pin. This circuit is similar to using an inverter gate as an oscillator, but with fewer components than the astable configuration, and a much higher power output than a TTL or CMOS gate. The duty cycle for either the 555 or inverter-gate timer will not be precisely 50% and will change depending on the load that the output is also driving while high (longer duty cycles for greater loads) due to the fact the timing network is supplied from the device's output pin, which has different internal resistances depending on whether it is in the high or low state (high side drivers tend to be more resistive). An alternate method to set the duty cycle practically, is to connect a diode parallel to pin 6 & 7. The operation of the diode when connected is explained above. The resultant duty cycle is given as D=R2/(R1+R2). If a potentiometer is used to supply R1 and R2, R1 + R2 is constant. The duty cycle then varies with the potentiometer at a constant frequency. A series resistor of 100 ohms must be added to each R1 and R2 to limit peak current of the transistor(within) when R1 and R2 are at minimum level. This method of adding a diode has a restriction of choosing R1 and R2 values. An alternate way is to add a JK flip-flop to the output of non-symmetrical square wave generator. But, with this the output frequency is one half of the timer.

Monostable

In monostable mode, the output pulse ends when the voltage on the capacitor equals 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.

Assume initially the output of the monostable is zero, the output of flip-flop(Q bar) is 1 so that the discharging transistor is on and voltage across capacitor is zero. One of the input of upper comparator is at 2/3 of supply voltage and other is connected to capacitor. For lower comparator, one of the input is trigger pulse and other is connected at 1/3 of supply voltage. Now the capacitor charges towards supply voltage(Vcc). when the trigger input is applied at trigger pin the output of lower comparator is 0 and upper comparator is 0. The output of flip-flop remains unchanged therefore the output is 0. when the voltage across capacitor crosses the 1/3 of the vcc the output of lower comparator changes from 0 to 1. Therefore, the output of monostable is one and the discharging transistor is still off and voltage across capacitor charges towards vcc from 1/3 of vcc,

When the voltage across capacitor crosses 2/3 of VCC, the output of upper comparator changes from 0 to 1, therefore the output of monostable is 0 and the discharging transistor is on and capacitor discharges through this transistor as it offers low resistance path. The cycle repeats continuously. The charging and discharging of capacitor depends on the time constant RC.

The voltage across capacitor is given by vc = Vcc(1-e^(-t/RC)) at t=T, vc =(2/3)Vcc therefore, 2/3Vcc=Vcc(1-e^(-T/RC)), T=RC ln(1/3), T=1.1 RC (seconds)

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

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

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

While using the timer IC in monostable mode, the main disadvantage is that the time span between any two triggering pulses must be greater than the RC time constant. Conversely, ignoring closely spaced pulses is done by setting the RC time constant to be larger than the span between spurious triggers. (Example: ignoring switch contact bouncing.)

Bistable

In bistable mode, the 555 timer acts as a basic flip-flop. The trigger and reset inputs (pins 2 and 4 respectively on a 555) are held high via pull-up resistors while the threshold input (pin 6) is grounded. Thus configured, pulling the trigger momentarily to ground acts as a 'set' and transitions the output pin (pin 3) to V_CC (high state). Pulling the reset input to ground acts as a 'reset' and transitions the output pin to ground (low state). No timing capacitors are required in a bistable configuration. Pin 7 (discharge) is left unconnected, or may be used as an open-collector output.

A 555 timer can be used to create a Schmitt trigger which converts a noisy input into a clean digital output. The input signal should be connected through a series capacitor which then connects to the trigger and threshold pins. A resistor divider, from V_CC to GND, is connected to the previous tied pins. The reset pin is tied to V_CC.

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Specifications

These specifications apply to the NE555. Other 555 timers can have different specifications depending on the grade (military, medical, etc.). These values should be considered "ball park" values, instead the current official datasheet from the exact manufacturer of each chip should be consulted for parameter limitation recommendations.

src: howtomechatronics.com

Packages

In 1972, Signetics originally released the 555 timer in DIP-8 and TO5-8 metal can packages, and the 556 timer was released in DIP-14 package.

Currently, the 555 is available in through-hole packages as DIP-8 and SIP-8 (both 2.54mm pitch), and surface-mount packages as SO-8 (1.27mm pitch), SSOP-8 / TSSOP-8 / VSSOP-8 (0.65mm pitch), BGA (0.5mm pitch). The Microchip Technology MIC1555 is a 555 CMOS timer with 3 fewer pins available in SOT23-5 (0.95mm pitch) surface mount package.

The dual 556 timer is available in through hole packages as DIP-14 (2.54mm pitch), and surface-mount packages as SO-14 (1.27mm pitch) and SSOP-14 (0.65mm pitch).

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Derivatives

Numerous companies have manufactured one or more variants of the 555, 556, 558 timers over the past decades as many different part numbers. The following is a partial list: AMD, California Eastern Labs, CEMI, Custom Silicon Solutions, Diodes Inc, ECG Philips, Estek, Exar, Fairchild, Gemini, GoldStar, Harris, HFO, Hitachi, IK Semicon, Intersil, JRC, Lithic Systems, Maxim, Micrel, MOS, Motorola, ON, Microchip, National, NEC, NTE Sylvania, NXP, Philips, Raytheon, RCA, Renesas, Sanyo, Signetics, Silicon General, Solid State Scientific, STMicroelectronics, Teledyne, TI, Unisonic, Wing Shing, X-REL, Zetex.

Table notes

• All information in the above table was pulled from references in the datasheet column, except where denoted below.
• For "Timer Total" column, a "*" denotes parts that are missing 555 timer features.
• For "I_q" 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_q at 5V. The value listed in this table was estimated by comparing the 5V to 15V ratio of other bipolar datasheets, then derating the 15V parameter for the NE558 part, which is denoted by the "*".
• For "Frequency Max" column, a "*" denotes values that may not be the actual maximum frequency limit of the part. The MIC1555 datasheet discusses limitations from 1 to 5 MHz. 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 states a value of 100 kHz, and their website shows a value of 100 kHz in timer comparison tables, which is overly conservative. In Signetics App Note 170, states that most devices will oscillate up to 1 MHz, however when considering temperature stability it should be limited to about 500 kHz.

Table manufacturer notes

Over the years, numerous IC companies have merged. The new parent company inherits everything from the previous company then datasheets and chip logos are changed over a period of time to the new company. This information is useful when tracking down datasheets for older parts. Instead of including every related company in the above table, only one name is listed, and the following list can be used to determine the relationship.

• Fairchild Semiconductor was sold to ON Semiconductor in 2016.
• Micrel was sold to Microchip Technology in 2015.
• National Semiconductor was sold to Texas Instruments in 2011.
• Signetics was sold to Philips Semiconductor in 1975, later to NXP Semiconductors in 2006.
• Zetex Semiconductors was sold to Diodes Incorporated in 2008.

556 dual timer

The dual version is called 556. It features two complete 555s in a 14 pin package. Only the two power supply pins are shared between the two timers. Bipolar version are currently available, such as the NE556 and LM556. CMOS versions are currently available, such as the Intersil ICM7556 and Texas Instruments TLC556 and TLC552, see derivatives table.

558 quad timer

The quad version is called 558. It has four reduced-functionality timers in a 16 pin package (four complete 555 timer circuits would have required 26 pins). Since the 558 is uniquely different than the 555 and 556, the 558 was not as popular. Currently the 558 is not manufactured by any major chip companies (possibly not by any companies), thus the 558 should be treated as obsolete. Parts are still available from a limited number of sellers as "new old stock" (N.O.S.).

Partial list of differences between 558 and 555 chips:

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

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Example applications

Stepped tone generator

This example uses one 556 or two 555 chips.

Joystick and game paddles

The Apple II microcomputer used a quad timer 558 in monostable (or "one-shot") mode to interface up to four "game paddles" or two joysticks to the host computer. It also used a single 555 for flashing the display cursor.

The original IBM PC used a similar circuit for the game port on the "Game Control Adapter" 8-bit ISA card (IBM part number 1501300). In this joystick interface circuit, the capacitor of the RC network (see Monostable Mode above) was generally a 10 nF capacitor to ground with a series 2.2 K? resistor to the game port connector. The external joystick was plugged into the adapter card. Internally it had two potentiometers (100 to 150 K? each), one for X and other for Y direction. The center wiper pin of the potentiometer was connected to an Axis wire in the cord and one end of the potentiometer was connected to the 5 Volt wire in the cord. The joystick potentiometer acted as a variable resistor in the RC network. By moving the joystick, the resistance of the joystick increased from a small value up to about 100 k?.

Software running in the IBM PC computer started the process of determining the joystick position by writing to a special address (ISA bus I/O address 201h). This would result in a trigger signal to the quad timer, which would cause the capacitor of the RC network to begin charging and cause the quad timer to output a pulse. The width of the pulse was determined by how long it took the capacitor to charge up to ²/₃ of 5 V (or about 3.33 V), which was in turn determined by the joystick position. 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.

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See also

• 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/drain output, Three-state output

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References

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Further reading

Books

• Lessons In Electric Circuits - Volume VI - Experiments; Tony Kuphaldt; Open Book Project; 423 pages; 2010. (see chapter 6 and 8)
• 555 Timer Applications Sourcebook Experiments; 2nd Ed; Howard Berlin; BPB Publications; 218 pages; 2008; ISBN 978-8176567909. (1st Ed in 1979)
• Designing Analog Chips; Hans Camenzind (inventor of 555 timer); Virtual Bookworm; 244 pages; 2005; ISBN 978-1589397187. (see chapter 11)
• Timer, Op Amp, and Optoelectronic Circuits and Projects; Forrest Mims III; Master Publishing; 128 pages; 2004; ISBN 978-0945053293.
• Engineer's Mini-Notebook - 555 Timer IC Circuits; 3rd Ed; Forrest Mims III; Radio Shack; 33 pages; 1989; ASIN B000MN54A6. (1st Ed in 1984)
• IC Timer Cookbook; 2nd Ed; Walter Jung; Sams Publishing; 384 pages; 1983; ISBN 978-0672219320. (1st Ed in 1977)
• 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.
• TTL Cookbook; Don Lancaster; Sams Publishing; 412 pages; 1974; ISBN 978-0672210358. (see chapter 4)

Applications Manuals

• Linear LSI Data and Applications Manual; Signetics; 1250 pages; 1985.(see appnotes AN170/171 and NE555/6/8 datasheets)
• Analog Applications Manual; Signetics; 418 pages; 1979.(see chapter 6)

Datasheets

• see links in "Derivatives" section and "References" section above

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External links

• Common Mistakes When Using a 555 Timer
• 555 timer online calculators: astable multivibrator, monostable multivibrator

Source of article : Wikipedia