Astable multivibrator (oscillator) realized with ne555 timer circuit is given in figure bellow.

Simulate ne555 astable multivibrator online.


NE555 is perhaps most widely spread integrated circuit in the world. Reason for widespread is need to easily obtain timer circuit as default part of many practical electronic circuit and simple utilization of ne555 for that purpose. Let’s have a look at the inner structure of ne555:


NE555 has two-point voltage divider that consists of three resistors with same value each. It means that power supply voltage VCC is divided into (2/3)·VCC and (1/3)·VCC for inner comparators threshold voltages, respectively. Comparator CP2 controls reset input of S-R latch that is also integrated inside ne555. Comparator CP1 controls set input of S-R latch and has lower threshold voltage (1/3)·VCC then comparator CP2. Comparator CP2 is “high” when V6 voltage is higher then (2/3)·VCC and then reset is activated. Comparator CP1 is “high” when V2 voltage is lower then (1/3)·VCC and the set is activated. Transistor T2 is with “open collector”. Purpose of T2 is to discharge some outer capacitor. With such wise inner structure, it is easy to use ne555 as timer circuit for as astable or monostabile multivibrator with only few outer components added. Presented astable multivibrator operates as follows:

Initially, S-R latch is set and transistor T2 is blocked. Capacitor C1 is charging from zero to (2/3)·VCC, and charging path is through resistors R1 and R2.


Charging time constant is Ï„=(R1+R2)C1. Once when C1 reaches (2/3)·VCC, comparator CP2 is “high” and S-R letch is reset. Then, discharging transistor T2 is active and rapidly goes to saturation, discharging capacitor C1 through resistor R1.


Discharging time constant is Ï„=R1C1. So, duty cycle with this kind of astable multivibrator is always higher then 50% and it is exactly:


In stationary regime, capacitor is charging from (1/3)·VCC to (2/3)·VCC with time constant (R1+R2)C1 and discharging from (2/3)·VCC to (1/3)·VCC with time constant R1C1. Overall time for charging and discharging is


So, frequency of oscillation is:


Go to ne555 astable multivibrator online.

External Links:

NE555 Astable multivibrator – oscillator on Circuitstoday
NE555 Astable multivibrator – oscillator on 555-timer-circuits.com
NE555 Astable multivibrator – oscillator on Daycounter
NE555 Astable multivibrator – oscillator on Cogeco.ca
NE555 Astable multivibrator – oscillator on Circuit-innovations


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