1. Astable Multivibrator

2. Introduction
An astable multivibrator, often called a free-running multivibrator, is a rectan­gular-wave generating cir­cuit. Unlike the monostable multivibrator, this circuit does not require any ex­ternal trigger to change the state of the output, hence the name free-running.
An astable multivibrator can be produced by adding resistors and a capacitor to the basic timer IC, as illustrated in figure. The timing during which the output is either high or low is determined by the externally connected two resistors and a capacitor. The details of the astable multivibrator circuit are given below.

4. Cont...,
Pin 1 is grounded;
pins 4 and 8 are shorted and then tied to supply +Vcc, output (VOUT is taken form pin 3;
pin 2 and 6 are shorted and the connected to ground through capacitor C,
pin 7 is connected to supply + VCC through a resistor RA;
and between pin 6 and 7 a resistor RB is connected.
At pin 5 either a bypass capacitor of 0.01
F is connected or modulation input is applied.

5. Astable Multivibrator & Waveforms

6. 2) Astable Multivibrator(Oscillator)
UCC + C1 C2 Q RD SD . 5K VA VB T 1 3 2 4 5 6 7 8
(grand) uO
charge . uC R1 R2 + – 1 1 1
>2/3 UCC
<1/3 UCC
first
uC=0 1
RD=1
SD=0
discharge

7. . uC uO uO 4 8 5 6 2 7 1 3 uC tp1 tp2 t tp1 tp2 With one stable state
+VCC uO . uC C R1 R2
tp1
tp2
2/3UCC
1/3UCC uC t uO
tp1
=(R1+R2)C ln2=0.7(R1+R2)C
tp2
=R2C ln2=0.7R2C
T=tp1+tp2
=0.7(R1+2R2)C
Duty Cycle = tp1/T
With one stable state

8. Astable Multivibrator Operation

9. In figure, when Q is low or output VOUT is high, the discharging transistor is cut-­off and the capacitor C begins charging toward VCC through resistances RA and RB. Because of this, the charging time constant is (RA + RB) C.
Eventually, the threshold voltage exceeds +2/3 VCC, the comparator 1 has a high output and triggers the flip-flop so that its Q is high and the timer output is low. With Q high, the discharge transistor saturates and pin 7 grounds so that the capacitor C discharges through resistance RB with a discharging time constant RB C.
With the discharging of capacitor, trigger voltage at inverting input of comparator 2 decreases.
When it drops below 1/3VCC, the output of comparator 2 goes high and this reset the flip-flop so that Q is low and the timer output is high. This proves the auto-transition in output from low to high and then to low as. Thus the cycle repeats.

10. 555 Timer Astable Multivibrator Circuit

11. Cont..,
This circuit diagram shows how a 555 timer IC is configured to function as an astable multivibrator.
An astable multivibrator is a timing circuit whose 'low' and 'high' states are both unstable. As such, the output of an astable multivibrator toggles between 'low' and 'high' continuously, in effect generating a train of pulses.
This circuit is therefore also known as a 'pulse generator' circuit.

12. Cont..,
In this circuit, capacitor C1 charges through R1 and R2, eventually building up enough voltage to trigger an internal comparator to toggle the output flip-flop. Once toggled, the flip-flop discharges C1 through R2 into pin 7, which is the discharge pin.
When C1's voltage becomes low enough, another internal comparator is triggered to toggle the output flip-flop. This once again allows C1 to charge up through R1 and R2 and the cycle starts all over again.
C1's charge-up time t1 is given by: t1 = 0.693(R1+R2)C1. C1's discharge time t2 is given by: t2 = 0.693(R2)C1. Thus, the total period of one cycle is t1+t2 = C1(R1+2R2).
The frequency f of the output wave is the reciprocal of this period, and is therefore given by: f = 1.44/(C1(R1+2R2)), wherein f is in Hz if R1 and R2 are in mega ohms and C1 is in microfarads.

13. Behavior of the Astable Multivibrator
The astable multivibrator is simply an oscillator. The astable multivibrator generates a continuous stream of rectangular off-on pulses that switch between two voltage levels.
The frequency of the pulses and their duty cycle are dependent upon the RC network values.
The capacitor C charges through the series resistors R1 and R2 with a time constant
(R1 + R2)C.
The capacitor discharges
through R2 with a time
constant of R2C

14. Uses of the Astable Multivibrator
Flashing LED’s
Pulse Width Modulation
Pulse Position Modulation
Periodic Timers (see mushroom timer in the experiment).