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There are many schmitt trigger inverter ICs in the market, some of which include: F40106B, SN7414, CD4010,...Frequency of this astable multivibrator depends not only on capacitor, C and resistor, R but also on the positive-going threshold voltage(VT+) and negative-going threshold voltage(VT-) of whichever ICs one uses. The capacitor, C charges and discharges in between VT+ and VT-. Deriving a relation or formula for calculating the frequency is done in terms of R, C, Vcc and VT+ & VT- characteristics of the IC. The frequency can be worked out with the formula defining voltage across the capacitor at any given time in a series R-C circuit. Below is the formula.
 Where Vc(t) is the voltage of the capacitor at a given time, t and it is taken as VT+; Vi is the initial voltage of the capacitor before charging and it is taken as VT-; Vf is the voltage of the capacitor at infinite time which is always Vcc and 0 when charging and discharging respectively, here it taken as Vcc. t is the time taken for the capacitor to charge up to VT+ from VT- or discharge to VT- from VT+; R is the resistance in ohm and C is the capacitor in farad. Therefore above equation now becomes
 Performing arithmetic manipulation
 Remember that the period of the oscillation, T is 2t and the frequency is the inverse of T. Therefore
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