The document discusses the 555 timer integrated circuit and its use as an astable multivibrator. The 555 timer was designed in 1971 by Hans Camenzind and is used in oscillator circuits to provide timing delays. It works by charging and discharging a capacitor through two resistors, causing the output to switch between high and low states. When the capacitor voltage reaches 2/3 or 1/3 of the supply voltage, it triggers the internal flip-flop to change the output. This creates a continuous square wave at the output with a frequency determined by the resistor and capacitor values.

1. “555 TIMER AS ASTABLE MULTIVIBRATOR”
Presented by,
Chandu L Y
(18BBTEC011)
Department of Electronics and Communication Engineering
1
LINEAR INTEGRATED CIRCUITS
4BEC402

2. Introduction
• It was designed in 1971 by Hans R. Camenzind.
• 555 timer is used in variety timer, pulse generator,
oscillators.
• Used to provide delays in oscillator circuit.
• It consists of voltage divider network form by 3 resistors
of 5K (reason for 555), two comparators, flip flop and
transistor.
• NE555 used at 0º to 70º.
• SE555 used at -55º to 125º.

3. 555 Timer IC pin configuration

4. 555 timer as Astable Multivibrator

5. How does it work!..?
• Initially, when the output is high, capacitor C starts charging towards
Vcc through Ra and Rb.
• As soon as voltage across the capacitor equals to 2/3 Vcc, comparator 1
triggers the flip-flop, and the output switches low.
• Now capacitor C starts discharging through Rb and transistor Q1.
• When the voltage across C equals 1/3 Vcc, comparator 2’s output
triggers the flip-flop and the output goes high.
• The cycle repeats.

6. Output waveforms

7. Formulas
• Charging time
Tc = 0.69(Ra + Rb)C
• Discharging time
Td = 0.69(Rb)C
• Total period
T = Tc + Td
• Frequency
f = 1/T = 1.45/(Ra + 2Rb)C
• Duty cycle
D = Tc/T = Ra + Rb/Ra + 2Rb
• During charging capacitor voltage is:
Vc(t) = Vcc( 1 – exp(-t/(Ra + Rb)C )
• During discharging capacitor voltage is:
Vc(t) = Vcc( exp(-t/RbC) )

8. Applications
1.Square wave generation.
2.Pulse position modulation.
3.Pulse train.
4.Frequency modulation.

9. THANK YOU