555 timer IC

The integrated circuit NE555 is an active electronic component that is frequently used in timers and clocks, sometimes even in switching power supplies. It was presented NE555 - often abbreviated also described as 555 - in 1972 by the U.S. semiconductor manufacturer Signetics. Due to its robustness and reliability, it is still used today, with its areas of application range from toys and spacecraft. The NE555 is the world's best-selling integrated circuit.

History

The NE555 (later today Philips Semiconductors NXP) developed in the years 1970 to 1971 by Swiss engineer Hans R. Camenzind for the U.S. semiconductor manufacturer Signetics. Initially, the project was very controversial: timer circuits were previously mostly built with operational amplifiers or comparators, which accounted for a large proportion of analog circuits produced by Signetics. On the one hand there was doubt that at all there would be a market for specialized Timer ICs, on the other hand, feared that one of the timer IC could reduce the sales of the operational amplifier - this is known in the business as Kannibalismuseffekt. The term NE555 is from Art Fury, his time marketing manager at Signetics. To him, the fact goes back to the development of the NE555 was given despite the company's internal resistors to Hans Camenzind in order.

Hans R. Camenzind had previously NE565/NE567 ( a phase locked loop, PLL), and the NE566 ( a voltage controlled oscillator, VCO) designed circuits, each of which contains a stable, temperature and supply voltage largely unaffected oscillator. The first draft of the NE555 therefore based on a similar oscillator, in which also an external capacitor from a voltage-current converter, which serves as a constant current source, and a plurality of current mirrors is linearly charged or discharged and produced a triangular voltage.

After completion of the design and its review and approval by Signetics Camenzind rejected his design again: He replaced the constant current source by a single external resistor. This was instead of the 14-pin housing an 8-pin can be used - with constant current source would be 9 connectors have been necessary; Signetics introduced but only 8 - or 14 -pin DIP ago. Although the charge or discharge of the capacitor through a resistor takes place a non-linear, a change in the supply voltage has no effect, since the timer of the charging state of the capacitor ratiometric, i.e. in relation to the supply voltage comparison. As a side effect, the new circuit was stable to temperature variations.

From 1972, the NE555 was mass produced. The demand exceeded all expectations in the first quarter Signetics sold more than half a million pieces. The other semiconductor manufacturer built the NE555 very quickly: Just six months after its release were 555 copies of eight different manufacturers on the market. Partially different type designations are used for the replicas, such as MC1455 by ON Semiconductor (formerly Motorola ) LM555 from National Semiconductor, KA555 Fairchild Semiconductor's (formerly Samsung) or the SN72555 from Texas Instruments. Even in the Soviet bloc replicas were prepared as the K1006ВИ1 from the former USSR. Such replicas were common in the 1970s until the Semiconductor Chip Protection Act of 1984 ( SCPA ). Even over 30 years after the launch about a billion pieces were in 2003 still produced annually.

The available chip housing changed over the years: The NE555 was from the beginning in a plastic casing, the dual in - line package (DIP), and the ( no longer available ) round metal case (TO -78 "metal can" ) made. For military and space applications, the NE555 in ceramic packages ( CERDIP, LCC) is packaged. In the 1980s, housing variants followed as Surface Mounted Device ( SMD) as SOIC, SSOP TSSOP or.

There were also versions with two and four timer circuits in a housing on the market, which allow the compact construction of more complex circuits. Dual variant NE556 includes a chip package with 14 pins of two identical and mutually independent on a 555 timer chip. The variant contains four NE558 timer in a 16- pin package, also known as " quad timer " means. Due to the combined terminals timers are not completely independent of each other in the latter variant.

A few years after the launch of the bipolar version Intersil brought a largely compatible CMOS version on the market. It has the same pinout and identical function at much lower power consumption. In chip design, no Darlington stages are required. Due to lower input current it allows for greater resistance values ​​and the outputs can be controlled up to the supply voltage. However, the CMOS variant delivers a smaller maximum output current compared to Bipolarversion. Today, CMOS versions from different manufacturers are available, such as the LMC555 (National Semiconductor ) TLC555 (Texas Instruments), ICM7555 (various manufacturers). Also, it there is a dual variant called ICM7556.

Also modified bipolar versions were brought out, for example, by the company of the ZSCT1555 Zetex working at 0.9 V supply voltage down and therefore is suitable for operation from a single battery cell. This design is also by Hans Camenzind.

The disadvantages of the original bipolar designs such as the relatively large quiescent current of up to 15 mA, and the today's standards, relatively high minimum supply voltage of 4.5 V, and the significant current peaks during switching of the output stage be partly avoided in recent replicas. Nevertheless, the IC from various manufacturers is still produced in unmodified circuit, only the manufacturing processes were changed to smaller structure sizes, resulting in the reduced size of the chip (english the shrink ), see Scaling ( microelectronics ). After a fire at a factory in Caen, where the last production line was destroyed in the processes used, Philips stopped production of the NE555 in 2003 and has since then ( as now NXP), only the CMOS version at ICM7555.

Construction

The NE555 contains the equivalent of 24 bipolar transistors, two diodes and 15 resistors (Small Scale Integration SSI), which together form six function blocks ( standing in below block diagram and circuit diagram are those highlighted ):

  • Between the supply voltage VCC ( ) and the ground GND (-) is a voltage divider consisting of three identical resistors which, when not connected from the outside, the two reference voltages of 1/3 Vcc and 2/3 VCC supplies. The latter is available at the port pin control voltage. (green)
  • Two comparators are connected respectively to one of the reference voltages while the other two inputs of which are fed directly to the terminals of trigger or threshold. (yellow and orange)
  • A flip-flop stores the status of the timer and is controlled by the two comparators. About the Reset input that overrides the other two inputs can be reset any time the flip-flop ( and thus the entire timer block). (Purple )
  • Parallel to the output stage of a transistor is connected, its collector is connected to terminal Discharge. This transistor is always turned on when the output is low. (light blue)

Circuit of the bipolar NE555

Circuit of the CMOS version

Basic circuits

The NE555 described below has three basic modes of operation. In addition, there are a lot more and derived circuit variants, such as electronic speedometer trigger switch in oscilloscopes, basic functions in cable testers or temperature controller, in which the NE555 is used as a circuit part.

Monostable multivibrator

As a monostable multivibrator NE555 works as the single pulse. Applications in this mode include timer frequency divider, capacitance measurement, and the pulse width modulation ( PWM).

The pulse starts when the NE555 timer receives a signal at the trigger input. This is the case when the voltage at the trigger input falls below 1/3 the supply voltage. The width of the output pulse is determined by the time constant of an external RC network, which consists of a capacitor C and a resistor R, as shown in the adjacent circuit diagram. The output pulse ends when the voltage across the capacitor 2/3 corresponds to the supply voltage. The pulse width τ of the output pulse may be varied by adjusting the values ​​of R and C according to the following equation:

In this simple basic circuit of the monostable multivibrator is not retriggerable. A new trigger signal is accepted only after the time τ. By expansion of the circuit can be realized with the NE555 and a retriggerable monostable multivibrator.

An application of the monostable multivibrator with four times the timer circuit NE558 existed in the analog joystick interface on the so-called game port of the IBM PC in the 1980s: The NE558 thus represents a part of a simple AD converter with four channels, which the resistance value of the potentiometer reacted per axis for two joysticks into one dependent on the deflection pulse duration. The triggering of the NE558 and the determination of the duration of a pulse is realized through software.

Bistable multivibrator

In operation, as the only flip-flop in the flip-flop integrated NE555 is used in principle. Areas of application are, for example, bounce-free switch.

The trigger input (pin 2 ) is used for the flip -flop as a set, whereby the flip-flop is set by a short pulse. A short pulse at the reset input (pin 4 ) is used as RESET, the flip-flop is now reset. Both the set pulse and the reset pulse corresponding to the negative logic - this situation is characterized by a sweep of the signal name.

Since no time-dependent function is available at a flip-flop, no additional capacitors are necessary in principle. The smoothing capacitor shown in the circuit with 10 nF is used at the terminal CTRL, as well as in the other Schaltungsmodi, only the smoothing of the internal reference voltage and can be dispensed with low accuracy requirements.

Astable multivibrator

As astable multivibrator of the NE555 operates as an oscillator and at its output is a periodic signal. This can be produced with a variable pulse-width ratio, depending on the configuration, various waveforms such as in the simplest form, a square wave. Applications include, among others, all forms of turn signals, pulse generators, electronic watches, application of the tone generation or as a clock source in DC-DC converters.

The circuit is formed by two resistors, R1 and R2, and a charging capacitor C. The storage capacitor is initially uncharged, which is set to the internal flip-flop on the TRIG input of the DIS output is held high impedance and the capacitor of the two resistors is loaded. On reaching 2/3 of the supply voltage on the capacitor C, the internal flip- flop is reset, whereby the output DIS from the ground (GND ) is connected. As a result, the capacitor discharges through the resistor R2 and the DIS output to ground. The discharge takes place until the capacitor is discharged to 1/3 the supply voltage. Then, the flip- flop is set again and the process starts anew.

Thus, the oscillator frequency can f depending on the components R1, R2 and C to determine:

Different choice of R1 and R2 is different pulse width ratios can choose. The length of time for the thigh high pulse at the output is

Tlow, the amount of time for the low pulse at the output is given as:

The resistance values ​​should not be set too low this to avoid overloading of the output stage. Thus, for example, be achieved with this simple basic circuit no pulse width ratio of 50:50 because then the resistor R1 would be necessary with a value of 0 Ω short-circuit.

Operating data and variants

Subsequent detailed technical specifications apply to the NE555. Other variants of the timer can have different specifications, which can be found in the respective data sheets.

The following table presents a selection of current and former manufacturers of the 555 timer circuit with the type designation and special together.

Datasheets

  • LMC555 CMOS version of the 555 timer. (PDF, 1.2 MB ) Texas Instruments, SNAS558H - MAY 2004 - REVISED November 2010
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