Transistor confusion

Question

I am trying to control a 6v DC motor using Arduino, by feeding a PWM signal to the base of an NPN transistor hooked up to the motor circuit as shown in the schematic below.

^{simulate this circuit – Schematic created using CircuitLab}

Individually, both circuits work fine; if I remove the transistor the motor spins and if I try to control the brightness of an LED using the output of D5 that also works, but when I introduce the transistor, it fails. I've tried both with the motor + transistor and an LED + transistor, but same problem: nothing happens.

My assumption is that it has to do with my choice of transistor - that a 2N4401 is just not the right kind for this - but if that's the case, I don't really understand why this one doesn't work, or what to look for in one that would work.

(I would also be willing to accept that I have completely misunderstood how transistors work and that there is something else wrong.)

Answer 1

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Make sure that you have a ground connection between the two power supplies.

Without the V1- connection to the 5V- there is no return path for Q1's base current.

^{simulate this circuit}

Figure 2. The circuit re-drawn in the conventional format.

With the redrawn layout it is more clear what is happening.

• We can see that R1 is pulling D2 down to ground and that SW1 will pull it high when pressed.
• Positive voltages are at the top of the schematic. See +5 V and V1+.
• Base resistor added for Q1 to limit the current drawn from D5. Without it the output will be overloaded.
• DC motor coil current flows from top to bottom.
• It's now very clear that D1 is normally reverse biased (since it's point upwards). It will allow a circulating current through the motor when Q1 switches off. This will protect Q1.
• Using the ground symbols makes it instantly clear which points are connected to 0 V and eliminates some wires.