Parts Required

1 TIP 120 Darlington transistor (Radioshack Catalog #: 276-2068) Note you can also use a TIP 122 or TIP that can handle larger loads.
1 DC motor either a gearhead motor or a small DC motor scrapped from surplus machine.
1 12 volt power supply for motor power.
1 1N4001 diode (Radioshack Catalog #: 276-1101)
1 Ik ohm resistor

1. Place the TIP 120 Darlington transistor in the board as shown in the schematicabove
2. Identify the base, collector, and emitter of the TIP 120 Darlington transistor from a data sheet you download. 
3. Connect a 1k-ohm resistor from the output pin # 11 of the BS2 to the base of the TIP 120 transistor.
4. Connect the anode end of the diode to the collector of the TIP 120 and the cathode end into the same row on the breadboard that you will connect the power source for the motor. Do not connect the motor power to the breadboard yet. The diode is necessary to stop the electrons in the motor from reversing and entering the microprocessor at the output pin when the motor stops.
5. Remember, diodes can be used as one-way gates for electrons. This reverse current will damage most microprocessors, so make sure your diode is correctly oriented with the cathode (white line on diode) toward the V+ supply of the motor power.

 

3D model of circuit

 

3D model of the circuit

1. Use your multimeter to take readings from your motor power supply and confirm the V+ and ground power of the power supply.
2. Unplug the motor power supply and use the schematic to determine placement of the V+ line in the same breadboard row as the cathode of the diode.
3. Place a connector wire from the emitter (E) of the TIP 120 to VSS on the board.
4. Make sure to wire in the ground of the power supply to the ground of the BS2 or the same row as the emitter.
5. NOTE: When you use two power supplies, it is important to have common grounds on the circuit meaning the grounds are connected to each other.
6. Now attach your motor to the breadboard making sure not to cross any wires. One lead connects to the emitter (E) of the TIP and the other lead connects to the cathode row of the breadboard. You can reverse the motor leads in any way and this will just reverse the direction the motor will turn with this circuit.