555 Timer

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555 Timer.jpg

This is a common 555 timer/oscillator from TI. A classic for all of those first year circuits projects where you need to blink an LED, generate tone, and thousands of other great beginning projects.

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.


  • 4.5V to 16V supply
  • 8-pin DIP package
  • Timing from microseconds to hours
  • Astable or monostable operation
  • Adjustable duty cycle
  • TTL compatible output
  • Sink or source up to 200mA


Hooking it up

The connection of the pins is as follows:

  1. GND - Ground, low level (0 V)
  2. TRIG - OUT rises, and interval starts, when this input falls below 1/3 VCC.
  3. OUT - This output is driven to +VCC or GND.
  4. RESET - A timing interval may be interrupted by driving this input to GND.
  5. CTRL - "Control" access to the internal voltage divider (by default, 2/3 VCC).
  6. THR - The interval ends when the voltage at THR is greater than at CTRL.
  7. DIS - Open collector output; may discharge a capacitor between intervals.
  8. V+, VCC - Positive supply voltage is usually between 3 and 15 V.


Modules using this component

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NOTE: All information contained within this article is pure opinion. Although this article is intended to help people, it may contain faulty or misleading information. This article is not to be considered professional opinion or advice, and is in no way a replacement for reading all safety/instructional documentation. Always remember to protect yourself when handling/using hazardous materials, as well as test new techniques before using them on projects/work intended to be handed in or used.

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