The cathode of the LED (the pin with the flat side on the case) connects to negative.
The power source is selected automatically. The Arduino Uno can be powered via the USB connection or with an external power supply.
#Arduino external power supply series
If you cannot get a DMM but you have an LED (note: a bulb will not work) and a suitable resistor (say 2.2K) you can connect the resistor in series with the LED then connect the two to the power supply. From the official Arduino website: Power. If it reads a negative value then the red probe is connected to negative. If the voltage reads positive then the black probe is connected to negative. I would think connecting VIN to ground would cause the power supply to fail. So remember to share grounds with external voltage supplies (DC-DC), that set the circuit to the same reference. Set it to DC Volts in the right range then connect the two probes (correctly inserted into the right sockets on the DMM) to the power supply's output connections. now, in your breadboard you have connected GND to VIN, that's the problem, Vin is an input voltage that is regulated to supply 3.3V and 5V. The simplest way is with a Digital MultiMeter (one of the most basic items you need to own if you are working with electronics - if you don't have one then go and buy one immediately). If you don't know which that is and you just randomly connected a random wire to a random pin then you need to investigate your power supply. Since you already have it wired up the negative is the one which connects to the emitter of your transistor. So yes, you need to know which is the "negative" connection and connect that to ground on the Arduino. Note that the "negative" isn't actually "negative", it is zero volts when measured from the reference point of itself. In your circuit that is the "negative" connection of the power supply. Ground is usually (though not always, but in your case it is) the point of lowest potential in a circuit.
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