The NE555 or LM555 timer is a chip containing the semi-conductor components needed for timer and oscillator circuits. Several types of timers or oscillators can be built with this simple IC depending on the configuration of the external circuitry.

  • Monostable Multivibrator (one shot): when the trigger is activated, the output remains high for a set time. Uses: timers, bounce-free switches, touch switches, frequency divider, capacitance measurement, pulse-width modulation (PWM) signals.
  • Astable Multivibrator (free running oscillator): trigger is tied to threshold – output keeps oscillating at a set frequency. This can be used for e.g. lamp flashers, pulse generation, logic clocks, tone generation, security alarm and so on.
  • Bi-stable (flip-flop): the circuit can be in two distinct states and changes state depending on trigger and reset input. Used for cleaning noisy signal, bounce-free latched switches.

A CMOS (lower power) version is available under TLC555 or ICM7555 .
Double and quadruple versions are also available under number 556 and 558 respectively.

Most important specifications

  • Supply voltage: 4.5 – 16 V.
  • Supply current: around 10 mA, depending on supply voltage.
  • Maximum output current: 200 mA.
  • Always add a decoupling capacitor accross the power supply terminals, close to the 555 IC.