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Home arrow How To arrow Oscillators arrow 555 timer IC
555 timer IC PDF Print E-mail
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555 timer IC

NE555 from Signetics in dual-in-line package
NE555 from Signetics in dual-in-line package
Schematic symbol of the 555 timer
Schematic symbol of the 555 timer

The 555 is an integrated circuit (chip) implementing a variety of timer and multivibrator applications. The IC was designed and invented by Hans R. Camenzind. It was designed in 1970 and introduced in 1971 by Signetics (later acquired by Philips). The original name was the SE555/NE555 and was called "The IC Time Machine". It is still in wide use, thanks to its ease of use, low price and good stability. Even today, Samsung in Korea manufactures over 1 billion units per year (2003).

The 555 timer is one of the most popular and versatile integrated circuits ever produced. It includes 23 transistors, 2 diodes and 16 resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP). The 556 is a 14-pin DIP that combines two 555s on a single chip. The 558 is a 16-pin DIP that combines four, slightly modified, 555s on a single chip (DIS & THR are connected internally, TR is falling edge sensitive instead of level sensitive). Also available are ultra-low power versions of the 555 such as the 7555. The 7555 requires slightly different wiring using fewer external components and less power.

The 555 has three operating modes:

  • Monostable mode: in this mode, the 555 functions as a "one-shot". Applications include timers, missing pulse detection, bouncefree switches, touch switches, etc.
  • Astable mode: the 555 can operate as an oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, etc.
  • Bistable mode: the 555 can operate as a flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bouncefree latched switches, etc.


  • 1 Usage
  • 2 Specifications
  • 3 Derivatives
  • 4 External links



The connection of the pins is as follows:

Nr. Name Purpose
1 GND Ground, low level
2 TR A short pulse high->low on the trigger starts the timer
3 Q During a timing interval, the output stays at +VCC
4 R A timing interval can be interrupted by applying a reset pulse to low (0V)
5 CV Control voltage allows access to the internal voltage divider (2/3 VCC)
6 THR The threshold at which the interval ends (it ends if U.thr > 2/3 VCC)
7 DIS Connected to a capacitor whose discharge time will influence the timing interval
8 V+, VCC The positive supply voltage which must be between 5 and 15 V, high level

Using simply a capacitor and a resistor, the timing interval, i.e. the time during which the output stays low, can be adjusted to the need of the specific application. An example configuration is shown below:

Example 555 schematic
Example 555 schematic

The interval time t is given by

t = RC

which is the time it takes to charge C to 63% of the applied voltage (exact figure: (1-1/e)V). See RC circuit for an explanation of this effect.


These specifications apply to the NE555. Other 555 timers can have better specifications depending on the grade (military, medical, etc).

Supply voltage (VCC) 4.5 to 15 V
Supply current (VCC = +5 V) 3 to 6 mA
Supply current (VCC = +15 V) 10 to 15 mA
Output current (maximum) 200 mA
Power dissipation 600 mW
Operating temperature 0 to 70° C


Many pin-compatible variants, including CMOS versions, have been built by various companies. The 555 is also known under the following type numbers:

Manufacturer Model Remark
ECG Philips ECG955M
Exar XR-555
Fairchild NE555/KA555
Harris HA555
Intersil SE555/NE555/ICM7555
Lithic Systems LC555
Maxim ICM7555
Motorola MC1455/MC1555
National LM1455/LM555C
NTE Sylvania NTE955M
Raytheon RM555/RC555
RCA CA555/CA555C
Sanyo LC7555
Texas Instruments SN52555/SN72555; TLC555
Zetex ZSCT1555 down to 0.9V

External links

From Wikipedia, the free encyclopedia

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