 Astable multivibration circuit using 555 timer
Operation of this circuit base on charging and discharging of a capacitor via a resistor. To understand how this circuit operates and derivation of formula or equation for its frequency, one needs to know internal circuitry of 555 timer. Equation for determining voltage across the capacitor at any given time, t in a series R-C circuit will be used
 Where Vc(t) is the voltage of the capacitor at a given time, t; Vi is the initial voltage of the capacitor before charging or discharging; Vf is the voltage of the capacitor at infinite time which is always Vcc or 0 when charging or discharging respectively; t is the time taken for the capacitor to charge up or discharge to Vc(t); R is the resistance in ohm and C is the capacitor in farad.
 During the charging period, t1, the capacitor, C1 charges through R1 and R2 from Vcc/3 to 2Vcc/3
 During the discharging period, t2, the capacitor discharges through only R2 from 2Vcc/3 to Vcc/3 to give
 Period, T of the oscillation is t1 + t2 which gives
  Therefore, the frequency of the oscillation is
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