Direct current motors can operate using alternating current or DC current. AC current is the same as your home electricity supply, so a DC motor contains a rectifier to change the internal current to DC. Many DC motors use electricity from a battery or transformer to power them. If you want to control the speed of your DC motor you need to build a rheostat, but this should only be undertaken if the motor is battery- or transformer-powered. Home electric current is potentially dangerous.
Get a piece of strong cardboard and cut out a rectangular shape about three inches by four inches using a pair of scissors. The cardboard is the base for the motor speed controller, or rheostat.
Make a slight incision about a ¼ inch from the top and bottom of the cardboard. Ensure the incisions are on one of the two longest sides and both on the same side. Use a small knife to make the incision; it only needs to be about 1/8 inch.
Use bare AWG 12 wire and insert the end of the wire into one of the two incisions. Leave about three to four inches loose before the incision so you can connect the wire later. Wind the wire tightly around the width of the cardboard starting from the incision. You need to ensure the wire is wound neatly all the way down to the next incision. Ensure there are no gaps as you wind it around the card; the tighter the wire is wound the better performance you get from your speed controller.
Wind the wire back up from the bottom incision to the top again, so you form a second layer and then wind a third layer down to the bottom incision. Insert the wire into the incision and then cut the wire using a knife close to the incision; you don’t need any wire protruding.
Use a pen and mark the bottom of the card, where no wire is protruding with the word “Low Speed.” Mark the top of the card, where you have three to four inches of wire protruding from the incision and write the word “High Speed.”
Connect the wire that’s protruding from the top incision to a wire that connects to your battery or transformer. Use a strip of insulating tape to connect the two wires.
Strip about ½ inch off the end of the other wire that’s connected to the motor using wire strippers. Attach the wire to a paper clip or small nail using a strip of tape.
Place the paper clip or nail on the top of the card where it’s marked “High Speed” and the motor operates at its usual speed. Slowly move the paper clip or nail down the wire that’s wound around the card toward where you marked “Low Speed.” As you move the paper clip or nail down the wire winding the motor gradually gets slower and slower. This is because of the resistance in the wire; the farther the electric current has to travel the greater the resistance, so less electrical power gets to the motor, making it slow down.
This article is intending as educational and fun task. It must not be used to build a speed controller for a motor that operates using your home electricity supply. Ensure the voltage from the transformer or battery doesn't exceed about 12 volts.