IC555 based LED Flasher

IC 555 is used in astable multivibrator mode.

Since the timer IC555 can source / sink 200mA current, its output can drive an LED directly as shown in figure. R3 is used as current limiting resistor.

Now the OUTPUT HIGH and OUTPUT LOW duration, is determined by the Resistors R1 & R2 and capacitor C1. These are calculated using below formulas:

Time High (Seconds) T1 = 0.693 * (R1+R2) * C1

Time Low (Seconds) T2 = 0.693 * R2 * C1

For Time High = 4 sec and Time Low = 2 sec, assume C1=320uF, and calculate the values of R1 and R2.

If R1 = R2 = 9.1K,

Time High = 4.036 sec

Time Low = 2.018 sec