How to Modulate Current without an Arduino
If you simply want to switch the current on and off at a certain rate, then you need an oscillator. A very common and cheap chip to do just that is the 555. Its full name is the LM555, but you will probably find it called just the 555 (or the 7555, a newer CMOS version). Many oscillator circuits (astable multivibrators) can be found on-line. You can vary the frequency of the 555 oscillator with a variable resistor.

If you want to do it without a chip, you can buld an astable multivibrator with two transistors. Again, frequency can be set by resistor and capacitor values.