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Astable 555 Multivibrator - doc00019


This is a demo of the popular 555 timer. you can try out the project on this page by a javascript simulation.

The Astable Configuration is like an Oscillator with a Square wave output. You can adjust the Frequency and Duty Cycle (means off time and on time pulse width as a %). The variation is done by playing with the caps and resistors while looking in One Eye at the Sacred Formula.

Do not overload the output pin or let high voltage/spike reach any pin.. Do not reduce the the resistor to Pin 7 or Discharge transistor, too much, it will burn out!

Press the Red button below to turn on the circuit press it again to turn off. The 555 can source (LED2) or Sink (LED1) upto 200mA.

It can even drive a small motor or lamp with diodes added to protect from inductive kickback.

Vary Ra, Rb and Ct with the controls given and see the change of frequency, period and duty cycle.

If LEDs are not lighting up, refresh the page with javascript and images enabled.



Astable 555 Multivibrator xml  use iframe part for your Webpage or Blog.

These are the formulae used by 555 and same is used in javascript without any change.

T1 = 0.693 (Ra + Rb) * Ct   charge time of Ct

T2 = 0.693 (Rb * Ct)  discharge time of Ct

T = T1 + T 2    total period in seconds

F = 1 / T = 1.44 / ((Ra + (2 * Rb)) * Ct)   Frequency in Hertz

D = T 2 / T  duty cycle, multiply by 100 to get  %.

Ct in farads and Ra-Rb in ohms.

The max power dissipation of 555 is 700mW so overload of more than 200mA will damage the device, connecting the battery in the reverse or wrong polarity will also damage device, ensure also Ra and Rb do not go less than 2.2K (use 4.7K minimum) as it may damage the discharge transistor at pin 7. The supply voltage can go upto 18V. For CMOS 555 like 7555 see the datasheet they are different.

the above circuit in pdf format   del00018.pdf
the cadsoft eagle source of the circuit   del00018.zip

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Updated 12-Aug-14