What is astable multivibrator mode of 555 timer IC?

 

Astable Multivibrator using 555 Timer | Circuit, Duty Cycle, Applications

The 555 Timer IC can be used in a variety of circuits like Time Delays, Oscillation, Pulse Generation, Pulse Width Modulation etc. Now, we will learn about the Astable Multivibrator Mode of 555 Timer IC. We will learn the circuit of Astable Multivibrator using 555 Timer IC, its operation, calculate the duty cycle and also take a look at few important applications of Astable Mode of 555 Timer IC.

Astable Multivibrator Mode of 555 Timer IC:

Astable multivibrator is also called as Free Running Multivibrator. It has no stable states and continuously switches between the two states without application of any external trigger. The IC 555 can be made to work as an astable multivibrator with the addition of three external components: two resistors (R1 and R2) and a capacitor (C). The schematic of the IC 555 as an astable multivibrator along with the three external components is shown below.

The pins 2 and 6 are connected and hence there is no need for an external trigger pulse. It will self trigger and act as a free running multivibrator (oscillator). The rest of the connections are as follows: pin 8 is connected to supply voltage (VCC). Pin 3 is the output terminal and hence the output is available at this pin. Pin 4 is the external reset pin. A momentary low on this pin will reset the timer. Hence, when not in use, pin 4 is usually tied to VCC.

The control voltage applied at pin 5 will change the threshold voltage level. But for normal use, pin 5 is connected to ground via a capacitor (usually 0.01µF), so the external noise from the terminal is filtered out. Pin 1 is ground terminal. The timing circuit that determines the width of the output pulse is made up of R1, R2 and C.

Operation

The following schematic depicts the internal circuit of the IC 555 operating in astable mode. The RC timing circuit incorporates R1, R2 and C.

Initially, on power-up, the flip-flop is RESET (and hence the output of the timer is low). As a result, the discharge transistor is driven to saturation (as it is connected to Q’). The capacitor C of the timing circuit is connected at Pin 7 of the IC 555 and will discharge through the transistor. The output of the timer at this point is low. The voltage across the capacitor is nothing but the trigger voltage. So, while discharging, if the capacitor voltage becomes less than 1/3 VCC, which is the reference voltage to trigger comparator (comparator 2), the output of the comparator 2 will become high. This will SET the flip-flop and hence the output of the timer at pin 3 goes to HIGH.

This high output will turn OFF the transistor. As a result, the capacitor C starts charging through the resistors R1 and R2. Now, the capacitor voltage is same as the threshold voltage (as pin 6 is connected to the capacitor resistor junction). While charging, the capacitor voltage increases exponentially towards VCC and the moment it crosses 2/3 VCC, which is the reference voltage to threshold comparator (comparator 1), its output becomes high.

As a result, the flip-flop is RESET. The output of the timer falls to LOW. This low output will once again turn on the transistor which provides a discharge path to the capacitor. Hence the capacitor C will discharge through the resistor R2. And hence the cycle continues.

Thus, when the capacitor is charging, the voltage across the capacitor rises exponentially and the output voltage at pin 3 is high. Similarly, when the capacitor is discharging, the voltage across the capacitor falls exponentially and the output voltage at pin 3 is low. The shape of the output waveform is a train of rectangular pulses. The waveforms of capacitor voltage and the output in the astable mode are shown below.

While charging, the capacitor charges through the resistors R1 and R2. Therefore the charging time constant is (R1 + R2) C as the total resistance in the charging path is R1 + R2. While discharging, the capacitor discharges through the resistor R2 only. Hence, the discharge time constant is R2C.

Duty Cycle

The charging and discharging time constants depends on the values of the resistors R1 and R2. Generally, the charging time constant is more than the discharging time constant. Hence the HIGH output remains longer than the LOW output and therefore the output waveform is not symmetric. Duty cycle is the mathematical parameter that forms a relation between the high output and the low output. Duty Cycle is defined as the ratio of time of HIGH output i.e., the ON time to the total time of a cycle.

If TON is the time for high output and T is the time period of one cycle, then the duty cycle D is given by:

 D = TON/ T 

Therefore, percentage Duty Cycle is given by:

 %D = (TON / T) * 100 

T is sum of TON (charge time) and TOFF (discharge time).

The value of TON or the charge time (for high output) TC is given by:

 TON = TC = 0.693 * (R1 + R2) C 

The value of TOFF or the discharge time (for low output) TD is given by

 TOFF = TD = 0.693 * R2C 

Therefore, the time period for one cycle T is given by

 T = TON + TOFF = TC + TD 

 T = 0.693 * (R1 + R2) C + 0.693 * R2C 

 T = 0.693 * (R1 + 2R2) C 

 Therefore, %D = (TON/ T) * 100 

 %D = (0.693 * (R1 + R2) C)/(0.693 * (R1 + 2R2) C) * 100 

 %D = ((R1 + R2)/(R1 + 2R2)) * 100 

 If T = 0.693 * (R1 + 2R2) C, then the frequency f is given by 

 f = 1 / T = 1 / 0.693 * (R1 + 2R2) C 

 f = 1.44/( (R1 + 2R2) C) Hz 

Selection of R1, R2 and C1

The Selection of values of R1, R2 and C1 for different frequency range are as follow:

R1 and R2 should be in the range 1KΩ to 1MΩ. It is best to Choose C1 first (because capacitors are available in just a few values and are usually not adjustable, unlike resistors) as per the frequency range  from the following table.

Choose R2 to give the frequency (f) you require.

R2 = 0.7 /(f × C1)‏

Choose R1 to be about a tenth of R2 (1KΩ min.)

Applications of Astable Multivibrator

• Square Wave Generation
• Pulse Position Modulation