555 timer circuit performing as an Astable multivibrator

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