At TJ-II, the working gas is puffed using a set of 8 piezoelectric valves, driven by a preprogrammed voltage signal. The aperture time and the desired density inside the vacuum vessel define the signal profile. This density depends in a complex way on the voltage applied to the valve and the accumulated effect of consecutive inlets during a shot. An VME waveform generator module generates the output signal. A graphical user interface has been developed based on JAVA; all system parameters can be fully configured using Internet applet viewers.