5V power supply negative terminal connection I am currently developing some basic led board using Arduino Uno R3. Basically,
the LEDs are connected in parallel so that 5V DC supply can light up about 3 LED. Each LED has its resistor and its connected to 3 Arduino digital pin where I ground it by using digitalWrite(pins, LOW). Each ground on the schematic is the Arduino digitalWrite(LOW). My question is where should the negative of the 5V should be connected? 

Also, if I wanted to measure the voltage of the LED, where should I connect my probe so that analogRead() can read the voltage? 

Thanks and sorry if my explanation is not correct since it's my first time using this site.







simulate this circuit &ndash; Schematic created using CircuitLab
The Arduino common, normally referred to as ground, although it is usually only accidentally grounded through external connection. In general ALL connections should have a common "ground".

You can not measure voltage across the LED given the circuit you have listed. (You could read the voltage at both ends and subtract.)

If you swap the LED and resistor you can get an approximation of the LED voltage (actually LED + Arduino pin) from the junction of LED and resistor.