We know that if a capacitor is charged from a voltage source through a
resistor, an exponential waveform is produced while charging of a
capacitor from a constant current source produces a ramp. This is the
idea behind the circuit. The circuit of a ramp generator using timer 555
is shown in figure. Here the resistor of previous circuits is replaced
by a PNP transistor that produces a constant charging current.
Ramp Generator Circuit-using 555 Timer IC
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
Slope, s = I/C
