The Harmonic Quad is a pulse generator that produces a multiplexed electrical signal at a fixed rate. The output is a dual frequency composite square wave. If you test Harmonic Quad unit output with a voltmeter, it will typically read around 2.5 volts, because it is measuring average voltage. The peak-to-peak voltage is about 8 volts, and can be seen on an oscilloscope. The “low battery” light will come on when the strength falls below 5.7 volts. Most zappers are powered with a 9 volt battery. The Harmonic Quad uses (2) 9 volt batteries.