Find Component Values for a 555 Timer

555 Timer (or LM555), the ubiquitous Timer IC

Monostable or One-shot Timer

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.
      Pulse Length, in Seconds Seconds
      RA in Ohms Ohms
      C in µF µF

      Astable, or Free-Running Multivibrator

      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%)

      C in µF µF
      RA in K Ohms K Ohms
      RB in K Ohms K Ohms