This is the schematic of a simple DIY 15$ voltage variator with inversion switch.
The Ammeter can be found on Amazon for 12$ at
I'm using a 100Ohms potentiometer resistor, but the Ohm value can be whatever you like, as long as the resistor max currents holds the power supplied to the motor or circuit. One of the two ends of the potentiometer is not wired as shown in the schematic.
The relays should be at the nominal voltage of the power supply.In this example i'm using 12V G5V Omron relay.