Stepping Down Voltage to power DC motor

Question

First post here, it's probably a very easy fix for most of you here. Right now I'm trying to use a digital potentiometer in a voltage divider to step down a voltage from 5v to a range of 3 - 1.5 volts that will go into a set of 4 DC vibrational motors. The resistance of the digipot is in the range 10Kohm - 1Kohm while the motors are only about 15ohms.

In order to supply enough current I put a voltage buffer in front of the motor however that still doesn't fix my problem. I'm able to control the output voltage of the buffer well without the motor attached in the 3 - 1.5 volt range however as soon as I plug in the motor the voltage drops to 0.420 V (according to an oscilloscope).

I'm using a UA741 for my op amp and TIP120 as well in the circuit. Power is being supplied from a simple DC power supply, I plan to eventually use a battery to supply power. I've attached a drawing of the schematic and links to the data sheets below.

TIP120 NPN Transistor:http://www.bc-robotics.com/datasheets/Tip-120.pdf

UA741 OP amp: http://www.ti.com/lit/ds/symlink/ua741.pdf

Motor: http://www.nfpmotor.com/spec/encapsulated-and-enclosed-vibration-motors/Encapsulated-vibration-motor-NFP-E0724.pdf

Answer 1

You have a few problems.

1. The 741 is a 40 year-old design. It's output can't swing close to the negative rail. You need something better.
2. The motor's coil resistance is only 15 Ω and requires 60 to 100 mA and 150 mA when stalled - and possibly on start.
3. Table 6.5 of the 741 datasheet states that the short-circuit current of the op-amp is 25 mA. It can't drive your motor.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. A more practical circuit.

The motor is 15 Ω and needs about 1.5 V which is 10 Ω/V so R1 would need to be about 30 Ω to drop the 5 V supply down by 3 V.

Answer 2

As Transistor said, Op-AMP 741 cannot deliver that much current according to the datasheet you have given. Instead you can use PWM concept to drive the Motor using Arduino. Circuit is here.

Calculation:

  R1 = (5V - 1.4V)/1mA 
     = 3.6k ohm. 

Using 2.2k ohm is also recommended. Here 1 mA current is taken as minimum value base current. But really for the circuit of yours, you only need 100uA (DC current gain is 1000 for TIP12x). Choosing little more current at base won't affect the motor at collector since, Motor's internal resistance will limit the current and draw only as much as it needs.
R2 is chosen for limiting current go to the load externally for safer side. It is chosen as follows.

R2 = (5v - 1.5V)/100 mA = 35 ohm which is equal to ~33 ohm. 

1N5819 is used as a flyback diode since the load is inductive.