555 timer

555 timer IC

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". The 555 gets its name from the three 5-kOhm resistors used in typical early implementations (Reference 1). It is still in wide use, thanks to its ease of use, low price and good stability. , 1 billion units are manufactured every year.

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-8). 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 and TLC555. 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, Frequency Divider,Capacitance Measurement, Pulse Width Modulation (PWM) etc
  • Astable - Free Running mode: the 555 can operate as an oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation, etc.
  • Bistable mode or Schmitt trigger: 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.


The connection of the pins is as follows:
Nr. Name Purpose
1 GND Ground, low level (OV ) ZERO VOLTS.
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 3 and 15 V

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. Thus, the 555 operates in monostable mode.
The interval time t is given by

t = 1.1 cdot 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.

When in astable mode, a resistor (call it R1) is connected between Vcc and the discharge pin (pin 7) and another (R2) is connected between the discharge pin (pin 7) and the trigger (pin 2) and threshold (pin 6) pins that share a common node. Hence the capacitor is charged through R1 and R2, and discharged only through R2, since pin 7 has low impedance to ground during output low intervals of the cycle, therefore discharging the capacitor. The use of R2 is mandatory, since without it the high current spikes from the capacitor may damage the internal discharge transistor.

In the astable mode, the high time from each pulse is given by

high = 0.693 cdot (R1 + R2) cdot C

and the low time from each pulse is given by

low = 0.693 cdot R2 cdot C

where R1 and R2 are the values of the resistors in ohms and C is the value of the capacitor in farads.

It has a voltage varying gate. The gate is open when the output voltage is low, and the gate is closed when output voltage is high.


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. There also exists bigger packages with two or four timers on the same chip. The 555 is also known under the following type numbers:

Manufacturer Model Remark
ECG Philips ECG955M
Exar XR-555
Fairchild Semiconductor NE555/KA555
Harris HA555
IK Semicon ILC555 CMOS from 2V
Intersil SE555/NE555/ICM7555
Lithic Systems LC555
Maxim ICM7555 CMOS from 2V
Motorola MC1455/MC1555
National Semiconductor LM1455/LM555/LM555C
National Semiconductor LMC555 CMOS from 1.5V
NTE Sylvania NTE955M
Raytheon RM555/RC555
RCA CA555/CA555C
Sanyo LC7555
STMicroelectronics NE555N/ K3T647
Texas Instruments SN52555/SN72555; TLC555 latter: CMOS from 2V
Zetex ZSCT1555 down to 0.9V

Dual Timer 556

The dual version is called 556. It features two complete 555:s in a 14 pin DIL package.

Quad Timer 558

The quad version is called 558 and has 16 pins. To fit four 555:s into a 16 pin package the control voltage and reset lines are shared by all four modules. Also for each module the discharge and threshold are internally wired together and called timing.


1) Scherz, Paul, "Practical Electronics for Inventors," p. 589.

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