Astable 555 Multivibrator - doc00019
This is a demo of the popular 555 timer. you can try
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
Astable 555 Multivibrator xml use iframe
part for your Webpage or Blog.
These are the formulae used by 555 and same is used
T1 = 0.693 (Ra + Rb) * Ct charge time
T2 = 0.693 (Rb * Ct) discharge time of
T = T1 + T 2 total period
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