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This is a 50% Duty Cycle Astable Multivibrator using the 555 timer IC. An approximately 50% duty cycle is achieved by discharging the timing capacitor through a voltage divider from Vcc to the discharge p‌in. This allows the discharging period to be equal to the charging period by finding the proper combination of Ra and Rb. However, this is not as versatile as the other 50% duty cycle 555 oscillator circuits since there are constraints to the values of the resistances. The ratio of Ra to Rb should always be greater than 2 so that the capacitor can be discharged lower than 1/3 Vcc. In conjunction with this, the Thevenin equivalent circuit formed by the voltage divider should discharge the timing capacitor in such a way that the period is equal to the charging interval. Few standard resistor values can satisfy this requirement.