Find Component Values for a 555 Timer
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.
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%)