This excellent 555 applet kindly provided by: Calculate Frequency by parts R1   Ohms KOhms MOhms R2   Ohms KOhms MOhms C   μF nF pF T1     μSec mSec Sec T2     μSec mSec Sec F     Hz KHz MHz Duty cycle       Calculate R2 by T1-T2 T1*   μSec mSec Sec T2*   μSec mSec Sec R1   Ohms KOhms MOhms C   μF nF pF F     Hz KHz MHz Duty cycle       R2     Ohms KOhms MOhms Calculate R1 by T1-T2 T1*   μSec mSec Sec T2*   μSec mSec Sec R2   Ohms KOhms MOhms C   μF nF pF F     Hz KHz MHz Duty cycle       R1     Ohms KOhms MOhms Calculate C by T1-T2 T1*   μSec mSec Sec T2*   μSec mSec Sec R1   Ohms KOhms MOhms R2   Ohms KOhms MOhms F     Hz KHz MHz Duty cycle       C     μF nF pF * If you want to use instead of the T1 and T2 variables the frequenct F and the Duty Cycle for your calculations, use the following form to calculate the T1 and T2 and then you can click the button 'Replace'. This button will replace the values for T1 and T2 to all above calculation forms. F   Hz KHz MHz Duty cycle     % T1     μSec mSec Sec T2     μSec mSec Sec

Comments:

• talking@tpg.com.au " The letter F should be f and Calculate Frequency by parts should be: Calculate Frequency by component-values
  KOhms should be: kOhms"