1.2.5

Astable Multivibrators are free running oscillators which oscillate between two states continually producing two square wave output waveforms. An astable multivibrator is often called as a free running multivibrator i.e. rectangular wave generating circuit. As that of monostable multivibrator, it does not requires an external trigger to change the state of output hence it is called as free running. However, the time during which output is either high or low is determined by two external resistors and a capacitor externally connected to the 555 timer. As shown in the circuit diagram, initially when output is high, capacitor C start charging towards Vcc through R1 & R2. However as soon as voltage across capacitor equals to 2/3Vcc, threshold comparator triggers the flip-flop and output switches to low. Now Capacitor starts discharging through R2 and internal transistor. When Voltage across C become less than or equal to 1/3Vcc, trigger comparator output changes the state of flip-flop and output goes high. This cycle repeats again and rectangular waveform is observed at the output of timer. The time during which the capacitor charges from 1/3Vcc to 2/3Vcc is equal to the time the output is high and is given by tc = 0.693 (R1+R2) C. Similarly the time during which the capacitor discharges from 2/3Vcc to 1/3Vcc is equal to the time the output is low and is given by td = 0.693 (R2) C. The total period of output waveform is given by T= tc +td= 0.693 (R1+2R2) C.