Astable multivibrator using 555 timer

The key external component of the astable timer is the capacitor. An astable multivibrator can be designed as shown in the circuit diagram (with typical component values) using IC 555, for a duty cycle of more than 50%. The corresponding voltage across the capacitor and voltage at output is also shown. The astable function is achieved by charging/discharging a capacitor through resistors connected, respectively, either to VCC or GND. Switching between the charging and discharging modes is handled by resistor divider R1-R3, two Comparators, and an RS Flip-Flop in IC 555. The upper or lower comparator simply generates a positive pulse if VC goes above 2/3 VCC or below 1/3 VCC. And these positive pulses either SET or RESET the Q output. The time for charging C from 1/3 to 2/3 Vcc, i.e, ON Time = 0.693 (RA + RB). C The time for discharging C from 2/3 to 1/3 Vcc, i.e. OFF Time = 0.693 RB. C To get the total oscillation period, just add the two: Thus, Tosc = 0.693∙(RA+RB)∙C + 0.693∙(RB)∙C = 0.693 ∙ (RA + 2∙RB) ∙ C fosc = 1/ Tosc = 1.44/(RA + 2∙RB).C Duty cycle = RA+RB/ RA + 2∙RB