Calulating BJT base resistance for PIC led switching?

[xnihilo]

Connect your NPN transistor with the emitter to ground then you can calculate the resistor in this way:

R = (Vin - 0.7)/Ib

Where:
Vin = pic output (5 v)
0.7 = base/emitter diode threshold
Ib = desired base current ( 2mA is what you need for saturating 20 mA load)

Nearest value = 2,2 K

LM555 can be connected directly to 9 V PS and the output (pin 3) can drain your 20 mA load without any problem. So you can connect your led + 330 ohms directly to the IC.

Al.

Thank you but I beleive there has been a misunderstanding of my post:
The LM555 is not driving any led, I'm using a BJT for switching a led (with its current limiter because I use 9V) and another BJT for switching an LM555.
One pin of the pic is activating the first BJT to switch on the led and another pin is activating the second BJT to trigger the LM555 that will then switch a relay.