The 555 timer IC is an integrated circuit used in a variety of timer, delay, pulse generation, and oscillator 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. The design was first marketed in 1972 by Signetics and used bipolar junction transistors. Since then, numerous companies have made the original timers and later similar low-power CMOS 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".

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. 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. 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. This revised version passed a second design review, and the prototypes were completed in October 1971 as the NE555V (plastic DIP) and SE555T (metal TO-5). 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. 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."

Name

Several books report the name "555" timer IC derived from the three 5 kΩ 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' timer IC.."

Design

Depending on the manufacturer, the standard 555 package incorporated the equivalent of 25 transistors, 2 diodes, and 15 resistors on a silicon chip packaged into an 8-pin dual in-line package (DIP-8). Variants available included the 556 (a DIP-14 combining two complete 555s on one chip), and 558 / 559 (both variants were 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 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.

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 are split across two columns.

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 period. The astable configuration is implemented using two resistors, R_1 and R_2, and one capacitor C. 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 to 2/3 VCC, however, in later pulses, it only charges from 1/3 VCC to 2/3 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 R_2, 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: The output low time interval of each pulse is given by: Hence, the frequency f of the pulse is given by: and the duty cycle D is given by: where t is the time in seconds, R is the resistance in ohms, C is the capacitance in farads, and \ln(2) is the natural log of 2 constant. Resistor R_1 requirements:

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 on the voltage drop across the diode. The low time is unaffected by the diode and so remains But the diode's forward voltage drop Vdiode slows charging on the capacitor, so the high time is longer than the often-cited to become: where Vdiode is when the diode's "on" current is 1/2 of VCC/R1 (which depends on the type of diode and can be found in datasheets or measured). When Vdiode is small relative to Vcc, this charging is faster and approaches but is slower the closer Vdiode is to Vcc: "As an extreme example, when VCC = 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, when Vcc = 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 as Vdiode approaches 0 V."

Voltage-controlled pulse-width modulation

In the previous example schematics, the control pin was not used, thus it should connected to ground through a 10 nF decoupling 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.

Monostable

Monostable mode produces an output pulse when the trigger signals drops below 1/3 VCC. An RC circuit sets the output pulse's duration as the time t in seconds it takes to charge C to 2/3 VCC: where R is the resistance in ohms, C is the capacitance in farads, \ln(3) is the natural log of 3 constant. 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.

Example Values

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.

Bistable SR latch

A 555 timer can act as an active-low SR latch (though without an inverted Q output) with two outputs: output pin is a push-pull output, discharge pin is an open-collector output (requires a pull-up resistor). For the schematic on the right, a Reset input signal connects to the RESET pin and connecting a Set input signal to the TR pin. Thus, pulling Set momentarily low acts as a "set" and transitions the output to the high state (VCC). Conversely, pulling Reset 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. The trigger and reset inputs may be held high via pull-up resistors if they are normally Hi-Z and only enabled by connecting to ground.

Bistable schmitt trigger inverter gate

A 555 timer can be used to create a Schmitt trigger inverter gate with two outputs: output pin is a push-pull output, discharge pin is an open-collector output (requires a pull-up resistor). For the schematic on the right, an input signal is AC-coupled through a low value series capacitor, then biased by identical high-resistance resistors R_1 and R_2, which causes the signal to be centered at 1/2 V cc. 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 lower 1/3 VCC and upper 2/3 VCC thresholds in order to cause them to change state, thus providing the schmitt trigger feature. No timing capacitors are required in a bistable configuration.

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 a DIP-14 package. 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). 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). 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 the original 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, under 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 and 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: