An Astable multivibrator, often a free-running multivibrator, is a rectangular-wave-generating circuit. Unlike the monostable multivibrator, this circuit does not require an external trigger to change the state of the output is either high or low is determined by the two resistors and a capacitor, which are externally connected to the 555 timer.
Astable operation:
Figure (a) shows the 555 timer configuration as an astable multivibrator. Initially, when the output is high, capacitor C starts charging toward VCC through RA and RB. However as soon as voltage across the capacitor equals 2/3 VCC, comparator 1 triggers the flip-flop, and the output switches low. Now capacitor C starts discharging through RB and transistor Q1. When the voltage across C equals 1/3 Vcc, comparator 2 output triggers the flip-flop, and the output goes high. Then the cycle repeats. The output voltage and capacitor voltage waveforms are shown in Figure (b).
As shown in this figure, the capacitor is periodically charged and discharged between 2/3 VCC and 1/3 VCC, respectively. The time during which the capacitor charges from 1/3 VCC to 2/3 VCC is equal to the time the output is high and is given by:
tc = 0.698*(RA + RB)*C
where RA and RB are in ohms and C is in farads. Similarly, the time during which the capacitor discharges from 2/3 VCC to 1/3 VCC is equal to the time the output is low and is given by:
td = 0.69*(RB)*C
where RB is in ohms and C is in farads. Thus the total period of the output waveform is
T = tc + td = 0.698*(RA + 2RB)*C
This, in turn, gives the frequency of oscillation as:
fo = 1 / T = 1.45 / [(RA + RB)*C]
The above equation indicates that the frequency fo is independent of the supply voltage VCC.
Often the term duty cycle is used in conjunction with the astable multivibrator. The duty cycle is the ratio of the time tc during which the output is high to the total time period T. It is generally expressed as a percentage. In equation form,
% duty cycle = (tc /
T)*100
= [ (RA + RB)
/ (RA + 2RB) ] * 100
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