Batsocks - Controlling Two LEDs per uC pin - Current Limiting Resistors

Controlling Two LEDs per uC pin

5. Current limiting resistors

OK, as promised, I'm finally here. Here are some back-of-fag-packet-calculations for you:

To calculate the resistor that each LED needs, we need to know:

What the supply voltage to the LED is (V_sup) (not to be confused with V_CC)
How much current you want to go through the LED (I_LED)
What the voltage drop is across the LED (V_F)#

Note: The "supply voltage to the LED" used in these calculations is VCC less 2 diode drops, because we're calculating the resistor required when the pin is controlling the LEDs. See Circuit Diagrams 3.2 and 3.3.

V_drop is 0.6v
V_CC is 5v
V_sup is V_CC - (2 * V_drop) = 3.8v
I_LED is 20mA (most LEDs' datasheets say that this is a reasonable current).
V_F is 1.9v
V_res is V_supply - V_F = 1.9v

Simple application of V=IR gives:

R = V_res/I_LED = 95 ohms

"So where do I stick the resistor(s)?"

The resistor needs to sit between the centre of the two LEDs and the pin that controls them.

Circuit Diagram 5.1

Normally it's not good practice to share a current limiting resistor between two LEDs, but in this case it's OK because we know that only one LED of the pair will ever be lit at one time.