Astable Multivibrator

This is a tutorial with calculator to help learn the application of 555 which is a very rare chip innovation. It is simple, versatile, flexible and programmable (by tweaking of pots).

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.

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.


Above is a canvas view of the Astable 555 gadget. In Google Sites and some other services, canvas view is available. Normal view alo can be set with only the calculator.

Astable 555 Oscillator xml code

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.

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

Astable Multivibrator with 555

If you are starting with electronics, this is the first step. So do it now ! Do-It-Yourself.