@Transistor It's an LM393, and it has an open collector output.

Thanks. As it turns out, the square wave is very low frequency(~2Hz from a heart rate sensor). I tried using the 3.3V regulator but it never gets down to zero volts due to the internal reference. So I can either use your zener circuit, or use an npn transistor with the 3.3V regulator connected to the collector and the 5V square wave connected to the base. Is there any benefit to the latter circuit? Another thing is my comparator is outputs high when idle, and I don't want to waste power when idle.

@EE_socal I don't wan't to use a voltage divider because the voltage I want to shift will not be exactly 5V. It could be a little above or below.

@Sphero Pefhany Measuring at the 'out' pin and the 'in' pin. How could I check if the motor current is continuous?

Added some more info

Yep, I'll do that.

I added the capacitors in the schematic. I tried 10n and 4.7u.

The 3.3V and the 9V are separate sources.

What is the purpose of buffering the output of the filter. Also, will this produce a well defined cutoff frequency?

It's not a completely pure tone. Its going to be from a tone generator app on my phone.