### Astable Multivibrator using NE 555 timer IC -Circuit diagram and working

Astable Multivibrator can be designed by using 555 timer IC, Op Amps and also using transistors. The 555 IC provide accurate time delay from mille seconds to hours. The frequency of oscillation can be controlled manually by simple modification. It suitable for circuit designers with a relatively stable, cheap, and user-friendly integrated circuit for both monostable and astable applications.
The 555 timer IC was first introduced around 1971 by the Signetics Corporation as the SE555/NE555. This is a simple 555 timer circuit project.
Astable Multivibrator is simply an oscillator circuit that produces continuous pulses. The frequency can be controlled by changing the values of R1, R2 and C1. You can construct Astable multivibrator using transistors also, but the 555 circuit is comparatively simple.
Components:
1. NE 555 or SE 555
2. Resistor (1MΩx2, 1KΩ)
3. Capacitors (0.01Fµ, 1Fµ)
4. LED

## Design of Astable Multivibrator

Capacitor charges through R1 and R2 there for charging time is given by Tcharge=0.69(R1+R2)C1, during this time the output is High, so Tcharge=Ton=0.69(R1+R2)C1=1.38 sec

Capacitor discharges through R2 only there for discharging time is given by Tdischarge=0.69 R2 C1,   during this time the output is Low, so
Tdischarge=Toff =0.69 R2 C1=0.69 sec

Time Period=Ton+Toff=2.07sec
Frequency of oscillation=.483Hz

Working of Astable Multivibrator:
• Consider the flip flop is initially cleared, when the power is switched on, then the output of inverter will be HIGH.
• Now the capacitor C1 starts charging through R1 and R2. (Discharge transistor Q1 is OFF)
• When the capacitor voltage exceeds 2/3 Vcc, the upper comparator output will be High, it Reset the control flip flop.
• So the Q output of control flip flop will be LOW and Q’ will be High. So the final output from Inverter is LOW
• At the same time, the discharge transistor Q1 turns ON and the capacitor starts discharge through R2
• When the capacitor voltage less than 1/3 Vcc, the lower comparator output will be high, then the control flip flop get set to High. (Q=1, Q’=0, Final output=1)
• Now the discharge transistor Q1 if OFF and then capacitor starts charging. This process continues.
• The LED connected at the output will glows according to the output status.
• 4th pin is Reset pin, a Low voltage at this pin resets the IC. The Low signal is applied to the base terminal of reset transistor Q2. Then it turns ON followed by Discharge capacitor Q1 and capacitor discharges. See the images below.
Working Principle Images:
Charging                           Discharging
Resetting
Click on the image for enlarged view