Arduino: Control a DC motor with potentiometer and multiple power supplies

A quick circuit showing how to control the speed of a DC motor with a potentiometer with your Arduino board. Also shows how to use a TIP120 transistor to allow the Arduino control a larger power supply.

Transistors are 3 pin devices, which via the 3rd pin (Base) allow it to control the current passing through the other 2 pins (Collector and Emitter). So for this tutorial I am using the power from the Arduino Digital PWM pin 9 (+5V) to control the flow of current to a DC motor which uses an additional power supply with a much larger current than the Arduino board can supply or control. Of course like most electrical components each transistor is designed for a specfic operating range or current.

Below you can see TIP120 the pins and how they appear in a schematic:

So thats the transistor. Next up is the rectifier diode, I’m using this inbetween the power supply flowing from the motor. It acts like a one way valve to only allow the current to flow one way, so my circuit should be protected should the motor power supply cause a surge or if the motor draws too much current. The main thing to remember is that Diodes like LED’s have a correct orientation, shown to the left.

The other item is the potentiometer, which is basically a variable resistor. By turning it you control the flow of current by allowing more or less through. Potentiometers, like resistors have a resistance rating in Ohms and a power rating. For this I am using a pot with a 10K ohm rating.

Shopping list
1K Ohm resistor (Brown, Black, Red, Gold)
10k Potentiometer
TIP120 Transistor
1n4004 1A Diode
6V DC motor
Arduino Deumilanove w/ ATMEGA328
Breadboard / Prototyping board
Jumper/ Connector wires
4x AA battery holder
4x AA batteries
Optional 9V power supply (here) or use the USB power for the Arduino

The circuit
Pretty simple, but remember that the GND connection must be shared between the Arduino and the additional power supply and I’m using a 1k Ohm resistor between Arduino pin 9 and the Base pin of the transistor.

The sketch

int potPin = 0;           		// Analog pin 0 connected to the potentiometer
int transistorPin = 9;    		// connected from digital pin 9 to the base of the transistor
int potValue = 0;         		// value returned from the potentiometer
 
void setup() {  			// set  the transistor pin as an output
  pinMode(transistorPin, OUTPUT);
}
 
void loop() {  				// read the potentiometer, convert it to between 0 - 255 for the value accepted by the digital pin.
  potValue = analogRead(potPin) / 4;  	// potValue alters the supply from pin 9 which in turn controls the power running through the transistor
  analogWrite(9, potValue);
}

After thoughts
I’m going to use the above and the work I’ve done with the L293D chip so far to control the speed and direction to the motor with a potentiometer.

This content is published under the Attribution-Noncommercial-Share Alike 3.0 Unported license.

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