5VDC power supply is from IC 78L05.
LED: Vf =3.0V (3.3V Max) @ If=15mA, Ir =10uA@ Vr =5V.
The point a, b, c voltage is measured with respect to ground.
Maximum output current sourced by I/O pin is 20mA.
Then, one question: Why Va is 2.7V while the I/O voltage output is high? In my opinion Va is supposed to 5V.