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# Astable Multivibrator with 555 Timer

2015-01-31 14:58
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The 555 Astable oscillator gives a square wave output at pin 3, The output drives two LEDs, LED1 lights up when pin 3 is low and LED2 when pin 3 is high.

de_gads_top 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 and see the change of frequency, period and duty cycle.

These are the formulae used by 555 .

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.

Datasheet of LM555 here pdfMore about 555 at Educypedia Timers

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