Mumbai University > Information Technology > sem 3> Analog and Digital Circuits

Marks: 10M

Year: May16

Operation of Astable Multivibrator mode of 555 timer IC:

⦁ When initially power is turned ON, Trigger Pin voltage is below Vcc/3, that makes the lower comparator output HIGH and SETS the flip flop and output of the 555 chip is HIGH.

⦁ This makes the transistor Q1 OFF, because Qbar, Q’=0 is directly applied to base of transistor. As the transistor is OFF, capacitor C1 starts charging and when it gets charged to a voltage above than Vcc/3, then Lower comparator output becomes LOW (Upper comparator is also at LOW) and Flip flop output remains the same as previous (555 output remains HIGH).

⦁ Now when capacitor charging gets to voltage above than 2/3Vcc, then the voltage of non-inverting end (Threshold PIN 6) becomes higher than the inverting end of the comparator. This makes Upper comparator output HIGH and RESETs the Flip flop, output of 555 chip becomes LOW.

⦁ As soon as the output of 555 get LOW means Q’=1, then transistor Q1 becomes ON and short the capacitor C1 to the Ground. So the capacitor C1 starts discharging to the ground through the Discharge PIN 7 and resistor R2.

⦁ As capacitor voltage get down below the 2/3 Vcc, upper comparator output becomes LOW, now SR Flip flop remains in the previous state as both the comparators are LOW.

⦁ While discharging, when capacitor voltage gets down below Vcc/3, this makes the Lower comparator output HIGH (upper comparator remain LOW) and Sets the flip flop again and 555 output becomes HIGH.

⦁ Transistor Q1 becomes OFF and again capacitor C1 starts charging.

This charging and discharging of capacitor continues and a rectangular oscillating output wave for is generated. While capacitor is getting charge the output of 555 is HIGH, and while capacitor is getting discharge output will be LOW. So this is called Astable mode because none of the state is stable and 555 automatically interchange its state from HIGH to LOW and LOW to HIGH, so it is called Free running Multivibrator.   Now the OUTPUT HIGH and OUTPUT LOW duration, is determined by the Resistors R1 & R2 and capacitor C1. This can be calculated using below formulas:

Time High (Seconds) T1 = 0.693 * (R1+R2) * C1

Time Low (Seconds) T2 = 0.693 * R2 * C1

Time Period T = Time High + Time Low = 0.693 * (R1+2*R2) * C1

Freqeuncy f = 1/Time Period = 1/ 0.693 * (R1+2R2) * C1 = 1.44 / (R1+2R2) * C1

Duty Cycle: Duty cycle is the ratio of time for which the output is HIGH to the total time.

Duty cycle %: (Time HIGH/ Total time) * 100 = (T1/T) * 100 = (R1+R2)/ (R1+2*R2) *100

Here is the practical demonstration of the Astable mode of 555 timer IC, where we have connected a LED to the output of the 555 IC. In this 555 astable multivibrator circuit, LED will switch ON and OFF automatically with a particular duration. ON time, OFF time, Frequency etc can be calculated using above formulas.