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Component Values for a 555 Timer

Monostable or One-shot Timer

LM555 IC Setup for or One-Shot Operation.

Choose Two Values and Find the Third Value

Referring to 555 Datasheet:

  • Pulse Period is defined as: t = 1.1 * RA * C
  • Normal RA values are between 1 K and 10 Meg Ohm.
  • Normal C values are between 1 nanofarad and 100µF.

Astable or Free-Running Multivibrator

LM555 IC Setup for Astable, Free-Running Operation.

Referring to the 555 Datasheet:

  • The charge time (output high) is given by: t1 = 0.693 (RA + RB) C
  • The discharge time (output low) by: t2 = 0.693 (RB) C
  • Thus the total period is: T = t1 + t2 = 0.693 (RA + 2RB) C

Some Tips on chosing values for C, RA, and RB include:

  • Increasing C will increase the cycle time (or, reduce the frequency).
  • Increasing RA will increase Time High, but will leave Time Low unaffected.
  • Increasing RB will increase Time High, increase Time Low, and decrease the duty cycle (down to a minimum of 50%)