applying a periodic biphasic square waveform to a surface electrode

Dear friends,

I want to apply an AC voltage/Current to a surface electrode placed on the human skin. I need a periodic waveform with unequal on and off time. How can I apply this stimulation pulse to a surface electrode at specific frequency?(I built the desired waveform from Definition node-> Function-> Piecewise, and set the electrode voltage from Electric Potential at 5V but when I change the duration of the pulse nothing changes. Is it correct way to apply desired waveform to an electrode?)

Please help me to solve this problem.

Best regards,