How to wire ground when using an arduino with transistors and an external power supply?
The issue is probably with the way you use the transistors. In your schematic you drew Bipolar NPN transistors. Those switch based on the base-emitter-voltage, where the base has to be more positive then the emitter. In your current setup the base is whatever the output pin is set to, but the emitter is floating because it is not connected to any voltage. This is because you trying to use an NPN transistor on the positive side of the LED, but you´re supposed to use them on the negative side like in this schematic:







simulate this circuit &ndash; Schematic created using CircuitLab

Here you can see that the base is whatever the output pin is (as long as there is no current going through R2) and the emitter is always grounded. Therefore a high output on the pin causes a positive base-emitter-voltage. Also note the resistors: R3 is the current limiting resistor for the LED to make sure it wont be destroyed because without it you´re basically shorting 3.5V to ground when the transistor is conducting. R1 is a pull-down resistor for the base of the transistor to make sure the base is never floating. R2 is a current limiting resistor to make sure you dont draw too much current from the output pin into the base of the resistor as this might damage the microcontroller.