555 Timer-Ramp Generator


Ramp Generator Circuit-using 555 Timer IC

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.

555 ramp generator

Ramp Generator Circuit

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


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