We apperceive that if a capacitor is answerable from a voltage antecedent through a resistor, an exponential waveform is produced while charging of a capacitor from a connected accepted antecedent produces a ramp. This is the abstraction abaft the circuit. The circuit of a access architect application timer 555 is apparent in figure. Here the resistor of previous circuits is replaced by a PNP transistor that produces a connected charging current.Charging current produced by PNP constant current source is
iC = Vcc-VE / RE
where VE = R2 / (R1 + R2) * VCC + VBE
When a trigger starts the monostable multivibrator timer 555 as shown in figure, the PNP current source forces a constant charging into the capacitor C. The voltage across the capacitor is, therefore, a ramp as illustrated in the figure. The slope of the ramp is given as
