Question

1) A 555 timer is operating in an astable mode at a frequency f-40kHz and Duty cycle D(%)- 85, VCC = 5V, find the range of variation off and D(%) if the voltage at the control input is modulated by a.c. coupling to it an external sine wave with a peak amplitude of IV. Assume C 0.01uF Ir.

Answer 1

To 555 timer Also [oscillation frequency f = 40 K H z Duty cycle D = 85 %] (I . e when the control is zero) f_0 = 40 K H Z = 1.44/(R_A + 2 R_B) c Given c = 0.1 mu F Rightarrow (R_A + 2 R_B) = 1.44/0.1 times 40 k ohm R_A + 2R_B = 0.36 k ohm Duty cycle D = R_A + R_B/R_A + 2 R_B = 0.85 0.15 R_A = 0.7 R_B Rightarrow R_A = 4 . 67 R_B From 1 & 2 Rightarrow R_B = 0.054 k ohm = 54 ohm R_A = 0.252 k ohm = 252 ohm] NOTE: If we choose a capacitance c = 0.01 mu F = 10 mu F Rightarrow (R_A + 2 R_B) = 3.6 k ohm Rightarrow R_B = 540 ohm