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555 timer as an astable multi vibrator, Summaries of Electronics

Electronics

555 timer as an astable multi vibrator

Typology: Summaries

2018/2019

Uploaded on 04/23/2019

basil-reji 🇮🇳

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555 TIMER AS AN ASTABLE MULTIVIBRATOR

An astable multivibrator, often called a free-running multivibrator, is a rectangular-wave generating cir-

cuit. 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.

FIG.1

 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

 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.

NOTE:

SQUARE WAVE OSCILLATOR: A symmetrical square wave can be obtained if a diode is

connected across resistor RB, as illustrated in dotted lines in figure 1. The capacitor C

charges through RA and diode D to approximately + 2/3VCC and discharges through resistor

RB and terminal 7 (transistor) until the capacitor voltage drops to 1/3 VCC. Then the cycle

is repeated. To obtain a square wave output, RA must be a combination of a fixed resistor

R and a pot, so that the pot can be adjusted to give the exact square wave.

FREE RUNNING RAMP GENERATOR: A free running ramp generator can be obtained if we

replace the the resistors RA and RB by a current mirror.

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555 TIMER AS AN ASTABLE MULTIVIBRATOR

An astable multivibrator, often called a free-running multivibrator, is a rectangular-wave generating cir- cuit. 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.

FIG. 1

 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  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.

NOTE:

SQUARE WAVE OSCILLATOR: A symmetrical square wave can be obtained if a diode is

connected across resistor RB, as illustrated in dotted lines in figure 1. The capacitor C

charges through RA and diode D to approximately + 2/3VCC and discharges through resistor

RB and terminal 7 (transistor) until the capacitor voltage drops to 1/3 VCC. Then the cycle

is repeated. To obtain a square wave output, RA must be a combination of a fixed resistor

R and a pot, so that the pot can be adjusted to give the exact square wave.

FREE RUNNING RAMP GENERATOR: A free running ramp generator can be obtained if we

replace the the resistors RA and RB by a current mirror.

ASTABLE MULTIVIBRATOR OPERATION

For explaining the operation of the timer 555 as an astable multivibrator, necessary internal circuitry with external connections are shown in figure.

FIG. 2  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, illustrated in the waveforms in the above figure 2. Thus the cycle repeats.

Overall period of oscillations, T = THIGH + TLOW = 0.693 (RA+ 2RB) C

The frequency of oscillations being the reciprocal of the overall period of oscillations T is given as

f = 1/ T = 1.44 / (RA+ 2RB) C

Equation indicates that the frequency of oscillation / is independent of the collector supply voltage +VCC.

DUTY CYCLE

Often the term duty cycle is used in conjunction with the astable multivibrator.

The duty cycle, is defined as the ratio of the time tc during which the output is high to the

total time period T.

It is given as:

% duty cycle, D = 𝒕 𝒕𝒄 X 100 = (^) ((𝐑𝐑𝐀𝐀 +^ + 𝟐𝐑^ 𝐑𝐁𝐁)) X 100

From the above equation it is obvious that square wave (50 % duty cycle) output can not be obtained unless RA is made zero. However, there is a danger in shorting resistance RA to zero. With RA = 0 ohm, terminal 7 is directly connected to + VCC. During the discharging of capacitor through RB and transistor, an extra current will be supplied to the transistor from VCC through a short between pin 7 and +VCC. It may damage the transistor and hence the timer.