Square wave generator - low cost

Hi,

I am constructing my own device for micro current treatment. My intention is to construct one similar to Sota Pulse Generator.

I already have the power source and a board to control voltage and current with high precision.

Now I am looking for a low cost bi-phasic SQUARE WAVE GENERATOR able to work with:

• voltage peak between around 3 and 30 volts
• current starting with 1 mA
• frequency around of 3.92 Hz (half Schumann frequency)

Do you have any suggestion of square frequency generator that may be suitable?

Sukhoi