I'm testing the Vf of a red LED with these two simple circuits:
I use analog input 1 as a probe between the LED and the resistor to measure the voltage drop.
Assuming the red LED has 2V of Vf, the 150 Ohm resistor should limit the current at 20mA.
According to this, analog input should read 3V in circuit (A) and 2V in circuit (B), but that's not the case. I get 3V in circuit (A), but 4.5V in circuit (B), indicating a Vf of only 0.5V for the LED (?!).
What am I missing? Is it normal. I'ts the way I'm measuring the Vf (using analog input) what it's wrong?