555 timer IC

The 555 timer IC is an integrated circuit (chip) used in a variety of timer, delay, pulse generation, and oscillator applications. Derivatives provide two (556) or four (558) timing circuits in one package. It was commercialized in 1972 by Signetics. Numerous companies have made the original bipolar timers and similar low-power CMOS timers too. In 2017, it was said over a billion 555 timers are produced annually by some estimates, and "probably the most popular integrated circuit ever made."
History.
The timer IC was designed in 1971 by Hans Camenzind under contract to Signetics. 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 1970 recession, and development on the PLL was thus frozen. 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.
The first design for the 555 was reviewed in the summer of 1971. Assessed to be without error, it proceeded to layout design. A few days later, Camenzind got the idea of using a direct resistance instead of a constant current source finding later 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 revised design passed a second design review with the prototypes completed in October 1971 as the NE555V (plastic DIP) and SE555T (metal TO-5). The 9-pin copy had been already released by another company founded by an engineer who attended the first review and 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. 
Name.
Several books report the name 555 comes from the three 5 kilohm resistors inside the chip. However, in a recorded interview with an online transistor museum curator, 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'."
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; CMOS timers also cause less supply noise than bipolar version when the output switches states.
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:
Pinout.
The pinout of the 8-pin 555 timer and 14-pin 556 dual timer are shown in the following table. Since the 556 is conceptually two 555 timers that share power pins, the pin numbers for each half is split across two columns.
In the following table, longer pin names are used, because manufacturers never standardized the abbreviated pin names across all datasheets.
Modes.
The 555 IC has the following operating modes:
Astable.
In the astable configuration, the 555 timer puts out a continuous stream of rectangular pulses having a specific frequency. The astable configuration is implemented using two resistors, formula_1 and formula_2, and one capacitor formula_3. In this configuration, the control pin is not used, thus it is connected to ground through a 10 nF decoupling capacitor to shunt electrical noise. The threshold and trigger pins are connected to the capacitor formula_3, thus they have the same voltage. Initially, the capacitor formula_3 is not charged, thus the trigger pin receives zero voltage which is less than a third of the supply voltage. Consequently, the trigger pin causes the output to go high and the internal discharge transistor to go to cut-off mode. Since the discharge pin is no longer short-circuited to ground, the current flows through the two resistors, formula_1 and formula_2, to the capacitor charging it. The capacitor formula_3 starts charging until the voltage becomes two-thirds of the supply voltage. At this instance, the threshold pin causes the output to go low and the internal discharge transistor to go into saturation mode. Consequently, the capacitor starts discharging through formula_2 till it becomes less than a third of the supply voltage, in which case, the trigger pin causes the output to go high and the internal discharge transistor to go to cut-off mode once again. And the cycle repeats.
In the first pulse, the capacitor charges from zero to two-thirds of the supply voltage, however, in later pulses, it only charges from one-third to two-thirds of the supply voltage. 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 formula_2, thus the high interval is longer than the low interval. This is shown in the following equations.
The high time interval of each pulse is given by:
The low time interval of each pulse is given by:
Hence, the frequency formula_13 of the pulse is given by:
and the duty cycle (%) is given by:
where formula_16 is in seconds (time), formula_17 is in ohms (resistance), formula_3 is in farads (capacitance), formula_19 is the natural log of 2 constant, which is 0.693147 (rounded to 6 trailing digits), but it is commonly rounded to fewer digits in 555 timer books and datasheets as 0.7 or 0.69 or 0.693.
Resistor formula_1 requirements:
The first cycle will take appreciably longer than the calculated time, as the capacitor must charge from 0 V to of VCC from power-up, but only from of VCC to of VCC on subsequent cycles.
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 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 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
where Vdiode is when the diode's "on" current is of Vcc/R1 which can be determined from its datasheet or by testing. As an extreme example, when Vcc = 5 V and Vdiode= 0.7 V, high time = 1.00 R1C which is 45% longer than the "expected" 0.693 R1C. At the other extreme, when Vcc= 15 V and Vdiode= 0.3 V, 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 Vdiode = 0 V.
Monostable.
In monostable mode, the output pulse ends when the voltage on the capacitor equals 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.
The output pulse is of width "t", which is the time it takes to charge C to of the supply voltage. It is given by
where formula_16 is in seconds (time), formula_17 is in ohms (resistance), formula_3 is in farads (capacitance), formula_30 is the natural log of 3 constant, which is 1.098612 (rounded to 6 trailing digits), but it is commonly rounded to fewer digits in 555 timer books and datasheets as 1.1 or 1.099.
While using the timer IC in monostable mode, the time span between any two triggering pulses must be greater than the RC time constant. 
Bistable.
In bistable mode, the 555 timer acts as an SR flip-flop. The trigger and reset inputs are held high via pull-up resistors while the threshold input is grounded. Thus configured, pulling the trigger momentarily to ground acts as a 'set' and transitions the output pin to VCC (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. The discharge pin is left unconnected, or may be used as an open-collector output.
Schmitt trigger.
A 555 timer can be used to create a Schmitt trigger inverter gate 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 VCC to GND, is connected to the previous tied pins. The reset pin is tied to VCC.
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.
In 2012, the 555 was available in through-hole packages as DIP-8 (2.54 mm pitch), 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). 
In 2006, the dual 556 timer was available in through-hole packages as DIP-14 (2.54 mm pitch), and surface-mount packages as SO-14 (1.27 mm pitch) and SSOP-14 (0.65 mm pitch).
The MIC1555 is a CMOS 555-type timer with three fewer pins available in SOT23-5 (0.95 mm pitch) surface-mount package.
Specifications.
These specifications apply to bipolar NE555. Other 555 timers can have different specifications depending on the grade (industrial, military, medical, etc.).
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: 
556 dual timer.
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. In 2020, the bipolar version was available as the NE556, and the CMOS versions were available as the Intersil ICM7556 and Texas Instruments TLC556 and TLC552. See derivatives table in this article.
558 quad timer.
The quad version is called 558 that has four reduced-functionality timers in a 16 pin package designed primarily for "monostable multivibrator" applications. By 2014, many versions of 16-pin NE558 have become obsolete.
Partial list of differences between 558 and 555 chips: