I started a new motor related project recently with a new Arduino Uno, beginning with several tutorial circuits included in the Vilros Arduino guide. The circuit in question is #10, a simple DC motor circuit with a transistor switch (http://imgur.com/a/vaqUA). The Arduino is being powered for the moment via USB.
The circuit, completed according to the diagram/common sense, does not work. The voltage across the transistor base is clearly not sufficient to activate the switch with the motor in the circuit. 5V (more like 4.6V) from Pin 13 is running across a 330 ohm resistor to the transistor base, but there is no current between the collector and the base. Bypassing the collector/base gap with a piece of metal will complete the circuit and activate the motor.
For some reason, the transistor will switch properly when the motor is replaced by a simple LED. AND, switching the cable from Pin 13 to the +5V out is enough to activate the transistor switch. I was under the impression that the transistor should only require .7V above the common grounded element.
But, even more bizarrely, with a cable running from +5V to the transistor base (through a resistor), a voltmeter across the DC motor is reading a potential difference of anywhere from 9V to 1702V (see http://imgur.com/a/08I3W, yes, with 4 digits). The multimeter is otherwise working normally, so I don't see a good explanation. Is the Arduino pin just incapable of providing sufficient current/voltage to activate the transistor, or am I missing something obvious. And does anyone have some idea why the voltmeter is reading so high and fluctuating so wildly? It is curious that the Arduino disconnects from the PC whenever the circuit is completed with the motor in it, so the Arduino likely is overdrawing from the USB.
The transistor included was not the one listed, but a common MPS2222A.