555 timer IC
The 555 timer IC is an integrated circuit used in a variety of timer, delay, pulse generation, and oscillator applications. Derivatives provide two or four 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. Numerous companies have made the original bipolar timers and similar low-power CMOS timers too. , it was estimated that 1 billion units were manufactured every year. The 555 is the most popular integrated circuit ever manufactured.
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 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. Other engineers argued the product could be built from existing parts; 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 and SE555T. 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.
Later, Hans Camenzind designed the ZSCT1555 timer for Zetex Semiconductors, which is no longer manufactured.
Part name
It has been falsely hypothesized that the 555 got its name from the three 5 kΩ resistors within the bipolar IC. Hans Camenzind has stated that the part number was arbitrary, thus it was simply a coincidence they matched. The "NE" and "SE" prefix letters of the original Signetic parts numbers, NE555 and SE555, were temperature designations for analog chips, where "NE" was commercial temperature family and "SE" was military temperature family.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. Variants available include the [|556], and [|558] / 559.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 and inexpensive epoxy plastic 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:- Green: Between the positive supply voltage VCC and the ground GND is a voltage divider consisting of three identical resistors, which create two reference voltages at VCC and VCC. The latter is connected to the "Control Voltage" pin. All three resistors have the same resistance, 5 kΩ for bipolar timers, 100 kΩ for CMOS timers. It is a false myth that the 555 IC got its name from these three 5 kΩ resistors.
- Yellow: The comparator negative input is connected to the higher-reference voltage divider of VCC, and comparator positive input is connected to the "Threshold" pin.
- Red: The comparator positive input is connected to the lower reference voltage divider of VCC, 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 can be reset at any time.
- Pink: The output of the flip-flop is followed by an output stage with push-pull output drivers that can load the "Output" pin with up to 200 mA for bipolar timers, lower for CMOS timers.
- Cyan: Also, the output of the flip-flop turns on a transistor that connects the "Discharge" pin to the ground.
Pinout
| 555 pin# | 556:1st pin# | 556:2nd pin# | Pin name | Pin direction | Pin description |
| 7 | 7 | GND | Power | Ground supply: this pin is the ground reference voltage. For bipolar timers, one or more decoupling capacitor are required because of current surges during output switching. | |
| 6 | 8 | Input | Trigger: when the voltage at this pin falls below of CONT pin voltage, the OUT pin goes high and a timing interval starts. As long as this pin continues to be kept at a low voltage, the OUT pin will remain high. | ||
| 5 | 9 | OUT | Output | Output: this pin is a push-pull output that is driven to either a low state or a high state . For bipolar timers, this pin can drive up to 200mA, but CMOS timers are able to drive less. For bipolar timers, if this pin drives an edge-sensitive input of a digital logic chip, a 100 to 1000 pF decoupling capacitor may need to be added to prevent double triggering, though a higher bypass capacitor on VCC power pin can help alleviate this problem. | |
| 4 | 10 | 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 overrides the trigger, which in turn overrides the threshold. If this pin is not used, it should be connected to VCC to prevent electrical noise causing a reset. | ||
| 3 | 11 | CONT | Input | Control : this pin provides access to the internal voltage divider. 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. If this pin is not used, it should be connected to a 10 nF decoupling capacitor to ensure electrical noise doesn't affect the internal voltage divider. | |
| 2 | 12 | THRES | Input | Threshold: when the voltage at this pin is greater than the voltage at CONT pin, then the "OUT high" timing interval ends, causing the output to go low. | |
| 1 | 13 | DISCH | Output | Discharge: For bipolar timers, this pin is an open-collector output, CMOS timers are open-drain. This pin can be used to discharge a capacitor between intervals, in phase with output. In bistable mode and schmitt trigger mode, this pin is unused, which allows it to be used as an alternate output. | |
| 14 | 14 | VCC | Power | Positive supply: For bipolar timers, the 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 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 the [|derivatives table] in this article. For bipolar timers, one or more decoupling capacitor are required because of current surges during output switching. |
Modes
The 555 IC has the following operating modes:- Astable 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. The use of a microprocessor-based circuit can then convert the pulse period to temperature, linearize it and even provide calibration means.
- 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, and so on.
- Bistable mode – the 555 operates as a SR flip-flop. Uses include bounce-free latched switches.
- Schmitt Trigger mode - the 555 operates as a Schmitt trigger inverter gate which converts a noisy input into a clean digital output.
Astable
| Frequency | C | R1 | R2 | Duty cycle |
| 0.1 Hz | 100uF | 8.2K | 68K | 52.8% |
| 1 Hz | 10uF | 8.2K | 68K | 52.8% |
| 10 Hz | 1uF | 8.2K | 68K | 52.8% |
| 100 Hz | 100nF | 8.2K | 68K | 52.8% |
| 1 kHz | 10nF | 8.2K | 68K | 52.8% |
| 10 kHz | 1nF | 8.2K | 68K | 52.8% |
| 100 kHz | 100pF | 8.2K | 68K | 52.8% |
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, and, and one capacitor. 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, thus they have the same voltage. Initially, the capacitor 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, and, to the capacitor charging it. The capacitor 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 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, 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 of the pulse is given by:
and the duty cycle is given by:
where is in seconds, is in ohms, is in farads, is the natural log of 2 constant, which is 0.693147181 but commonly is rounded to fewer digits in 555 timer books and datasheets as 0.7 or 0.69 or 0.693.
Resistor requirements:
- power capability of must be greater than, per Ohm's law.
- Particularly with bipolar 555s, low values of 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.
To have an output high time shorter than the low time a fast 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*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 and Vdiode= 0.7, high time = 1.00 R1C which is 45% longer than the "expected" 0.693 R1C. At the other extreme, when Vcc= 15 and Vdiode= 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 Vdiode= 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.
Monostable
| Time | C | R |
| 100 uS | 1nF | 91K |
| 1 mS | 10nF | 91K |
| 10 mS | 100nF | 91K |
| 100 mS | 1uF | 91K |
| 1 S | 10uF | 91K |
| 10 S | 100uF | 91K |
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.
Assume initially the output of the monostable is zero, the output of flip-flop is 1 so that the discharging transistor is on and the voltage across a capacitor is zero. One of the inputs of the upper comparator is at 2/3 of supply voltage and the other is connected to the capacitor. For a lower comparator, one of the inputs is trigger pulse and the other is connected at 1/3 of the supply voltage. Now the capacitor charges towards supply voltage. When the trigger input is applied at the trigger pin the output of the lower comparator is 0 and the upper comparator is 0. The output of flip-flop remains unchanged therefore the output is 0. when the voltage across the 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 the voltage across capacitor charges towards Vcc from 1/3 of Vcc,
When the voltage across the capacitor crosses 2/3 of VCC, the output of the upper comparator changes from 0 to 1, therefore the output of monostable is 0 and the discharging transistor is on and the capacitor discharges through this transistor as it offers low resistance path. The cycle repeats continuously.
The charging and discharging of the capacitor depends on the time constant RC.
The voltage across capacitor is given by vc = Vcc at t=T. Since vc =Vcc, therefore 2/3Vcc = Vcc, thus reduced to T = RC ln seconds.
The output pulse width of time t, which is the time it takes to charge C to of the supply voltage, is given by
where is in seconds, is in ohms, is in farads, is the natural log of 3 constant, which is 1.098612289 but commonly is 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 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.
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. Pulling the reset input to ground acts as a 'reset' and transitions the output pin to ground. No timing capacitors are required in a bistable configuration. Pin 7 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 2020, the 555 was available in through-hole packages as DIP-8, and surface-mount packages as SO-8, SSOP-8 / TSSOP-8 / VSSOP-8, BGA.
In 2020, the dual 556 timer was available in through-hole packages as DIP-14, and surface-mount packages as SO-14 and SSOP-14.
The MIC1555 is a CMOS 555-type timer with 3 fewer pins available in SOT23-5 surface-mount package.
Specifications
These specifications apply to bipolar NE555. Other 555 timers can have different specifications depending on the grade. These values should be considered "ballpark" values; the current official datasheet from the exact manufacturer of each chip should be consulted instead for parameter limitation recommendations.| Part number | NE555 |
| IC Process | Bipolar |
| Supply voltage | 4.5 to 16 V |
| Supply current | 3 to 6 mA |
| Supply current | 10 to 15 mA |
| Output current | 200 mA |
| Maximum Power dissipation | 600 mW |
| Power consumption | 30 mW @ 5V, 225 mW @ 15V |
| Operating temperature | 0 to 70 °C |
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:| Manufacturer | Part Number | Production Status | IC Process | Timer Total | Supply Min | Supply Max | 5V Supply Iq | Frequency Max | Remarks | Datasheet |
| Custom Silicon Solutions | CSS555 | CMOS | 1 | 1.2 | 5.5 | 4.3 | 1.0 | Lowest current, internal EEPROM, requires programmer | ||
| Diodes Inc | ZSCT1555 | Bipolar | 1 | 0.9 | 6 | 150 | 0.33 | Lowest voltage, designed by Camenzind | ||
| Japan Radio Company | NJM555 | Bipolar | 1 | 4.5 | 16 | 3000 | 0.1* | Also available in SIP-8 | ||
| Microchip | MIC1555 | CMOS | 1* | 2.7 | 18 | 240 | 5.0* | Reduced features, only available in SOT23-5 | ||
| ON | MC1455 | Bipolar | 1 | 4.5 | 16 | 3000 | 0.1* | |||
| Renesas | ICM7555 | CMOS | 1 | 2 | 18 | 40 | 1.0 | Lowest current of common parts | ||
| Renesas | ICM7556 | CMOS | 2 | 2 | 18 | 80 | 1.0 | Lowest current of common parts | ||
| Signetics | NE555 | Bipolar | 1 | 4.5 | 16 | 3000 | 0.1* | First 555 timer, DIP-8 or TO5-8 | ||
| Signetics | NE556 | Bipolar | 2 | 4.5 | 16 | 6000 | 0.1* | First 556 timer, DIP-14 | ||
| Signetics | NE558 | Bipolar | 4* | 4.5 | 16 | 4800* | 0.1* | First 558 timer, DIP-16 | ||
| STMicroelectronics | TS555 | CMOS | 1 | 2 | 16 | 110 | 2.7 | |||
| Texas Instruments | LM555 | Bipolar | 1 | 4.5 | 16 | 3000 | 0.1* | Slightly different than NE555 | ||
| Texas Instruments | LM556 | Bipolar | 2 | 4.5 | 16 | 6000 | 0.1* | Slightly different than NE556 | ||
| Texas Instruments | LMC555 | CMOS | 1 | 1.5 | 15 | 100 | 3.0 | Also available in DSBGA-8 | ||
| Texas Instruments | NE555 | Bipolar | 1 | 4.5 | 16 | 3000 | 0.1* | |||
| Texas Instruments | NE556 | Bipolar | 2 | 4.5 | 16 | 6000 | 0.1* | |||
| Texas Instruments | TLC551 | CMOS | 1 | 1 | 15 | 170 | 1.8 | Lowest voltage of common parts | ||
| Texas Instruments | TLC552 | CMOS | 2 | 1 | 15 | 340 | 1.8 | Lowest voltage of common parts | ||
| Texas Instruments | TLC555 | CMOS | 1 | 2 | 15 | 170 | 2.1 | |||
| Texas Instruments | TLC556 | CMOS | 2 | 2 | 15 | 340 | 2.1 | |||
| X-REL | XTR655 | SOI | 1 | 2.8 | 5.5 | 170 | 4.0 | Extreme, ceramic DIP-8 or bare die |
; Table notes
- All information in the above table was pulled from references in the datasheet column, except where denoted below.
- For the "Timer Total" column, a "*" denotes parts that are missing 555 timer features.
- For "Iq" 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 Iq 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 the "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. The application note from HFO mentions that at higher supply voltages the maximum power dissipation of the circuit might limit the operating frequency, as the supply current increases with frequency.
- For the "Manufacturer" column, the following associates historical 555 timer manufacturers to current company names.
- * Fairchild Semiconductor was sold to ON Semiconductor in 2016. ON Semiconductor was founded in 1999 as a spinoff of Motorola Semiconductor Components Group. The MC1455 started as a Motorola product.
- * Intersil was sold to Renesas Electronics in 2017. The ICM7555 and ICM7556 started as Intersil products.
- * Micrel was sold to Microchip Technology in 2015. The MIC1555 started as a Micrel product.
- * National Semiconductor was sold to Texas Instruments in 2011. The LM555 and LM556 started as a National Semiconductor products.
- * Signetics was sold to Philips Semiconductor in 1975, later to NXP Semiconductors in 2006.
- * Zetex Semiconductors was sold to Diodes Incorporated in 2008. The ZSCT1555 started as a Zetex product.
556 dual timer
558 quad timer
The quad version is called 558. It has four reduced-functionality timers in a 16 pin package. Since the timers in 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, thus the 558 should be treated as obsolete. Parts are still available from a limited number of sellers as "new old stock".Partial list of differences between 558 and 555 chips:
- One VCC and one GND, similar to 556 chip.
- Four "Reset" are tied together internally to one external pin.
- Four "Control Voltage" are tied together internally to one external pin.
- Four "Triggers" are falling-edge sensitive, instead of level sensitive.
- Two resistors in the voltage divider, instead of three resistors.
- One comparator, instead of two comparators.
- Four "Output" are open-collector type, instead of push-pull type. 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.
Example applications
Joystick and game paddles
The Apple II microcomputer used a quad timer 558 in monostable 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. In this joystick interface circuit, the capacitor of the RC network was generally a 10 nF capacitor to ground with a series 2.2 KΩ resistor to the game port connector. The external joystick was plugged into the adapter card. Internally it had two potentiometers, 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 kΩ.
Software running in the IBM PC computer started the process of determining the joystick position by writing to a special address. 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 of 5 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.