Designing 555 Timer IC as an Astable Multivibrator

What is Timer IC 555?

The 555 timer is an extremely versatile integrated circuit which can be used to build lots of different circuits. You can use the 555 effectively without understanding the function of each pin in detail.

Pin configuration of Timer IC 555

Pin configuration of Timer IC 555

Frequently, the 555 is used in astable mode to generate a continuous series of pulses, but you can also use the 555 to make a one-shot or monostable circuit. The 555 can source or sink 200 mA of output current, and is capable of driving wide range of output devices.

Astable – 555 Timer IC

If the 555 timer circuit is connected as shown in the next figure (THRESH and \TRIG connected together at the upper terminal of the timing capacitor C), then it will run as a multivibrator. In this case three external components, RA, RB and C are required. If you build this circuit, it is recommended to use a 10 nF capacitor from pin 5 (CTRL) to GND and a 100 nF capacitor from +Vcc to GND.

Construction and Connection of 555 Timer IC

Circuit Diagram of 555 timer in Astable Mode

Circuit Diagram of 555 timer in Astable Mode

  • Pin 1 is Grounded
  • Pin 4 and Pin 8 are shorted and then tied to supply +Vcc
  • Output (Vout) is taken from Pin 3
  • Pin 2 and Pin 6 are shorted and then connected to ground through a capacitior C
  • Pin 7 is connected to supply +Vcc through resistor (RA)
  • Between Pin 6 and Pin 7 a resistor RB is connected
  • At Pin 5 either a bypass capacitor of 0.01µF is connected or modulation input is applied

Astable Multivibrator operation of 555 Timer IC

Necessary circuit connection is shown in figure. When Q is LOW or output Vout is HIGH, the discharging transistor is cut-off and the Capacitor C begins to charge towards Vcc through resistance RA and RB. Because of this, the charging time constant is (RA+RB)xC.

Timer IC 555 in Astable mode as Multivibrator operation

Timer IC 555 in Astable mode as Multivibrator operation

As the capacitor charges the threshold voltage exceeds +2/3 of 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 RCxC.

With the discharging of Capacitor, trigger voltage at inverting input comparator 2 decreases. When it drops below 1/3 of Vcc. the output of Comparator 2 goes high and this resets the flip-flop so that Q is low and the timer output is high. This proves that auto transition in output from low to high and then to low as illustrated in second diagram. Thus, the cycle repeats.

Capacitor and Output voltage waveforms of Timer IC 555 in Astable mode

Capacitor and Output voltage waveforms of Timer IC 555 in Astable mode

Astable Multivibrator using 555 IC -Design method

The time during which the capacitor C charges from 1/3 VCC to 2/3 VCC is equal to the time the output is high and is given as tc or THIGH = 0.693 (RA + RB) C, which is proved below.

Voltage across the capacitor at any instant during charging period is given as,vc=VCC(1-et/RC)

The time taken by the capacitor to charge from 0 to +1/3 VCC

1/3 VCC = VCC (1-et/RC)

The time taken by the capacitor to charge from 0 to +2/3 VCC

or t2 = RC loge 3 = 1.0986 RC

So the time taken by the capacitor to charge from +1/3 VCC to +2/3 VCC

tc = (t2 – t1) =  (10986 – 0.405) RC = 0.693 RC

Substituting R = (RA + RB) in above equation we have

THIGH = tc = 0.693 (RA + RB) C

where RA and RB are in ohms and C is in farads.

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 as

td or  TL0W = 0.693 RB C where RB is in ohms and C is in farads The above equation is worked out as follows: Voltage across the capacitor at any instant during discharging period is given as

vc = 2/3 VCC e- td/ RBC

Substituting vc = 1/3 VCC and t = td in above equation we have

+1/3 VCC = +2/3 VCC e- td/ RBC

Or  td = 0.693 RBC

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.

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

The duty cycle, the ratio of the time tc during which the output is high to the total time period T is given as

% duty cycle, D = tc / T * 100 = (RA + RB) / (RA + 2RB) * 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.

See also: Pin Configuration of Intel 8085 Microprocessor

However, a symmetrical square wave can be obtained if a diode is connected across resistor RB, as illustrated in dotted lines in figure. 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.

Although the timer 555 has been used in a wide variety of often unique applications it is very hard on its power supply lines, requiring quite a bit of current, and injecting many noise transients. This noise will often be coupled into adjacent ICs falsely triggering them. The 7555 is a CMOS version of the 555. Its quiescent current requirements are considerably lower than that of 555, and the 7555 does not contaminate the power supply lines. It is pin compatible with the 555. So this CMOS version of the 555 should be the first choice when a 555 timer IC is to be used.

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