Good Transistor Site

http://www.zen22142.zen.co.uk/Design/bjtsw.htm

http://www.kpsec.freeuk.com/components/tran.htm#choosing

Great electrical online book
http://www.ibiblio.org/kuphaldt/electricCircuits/Semi/index.html



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.

Last edited by a

IC max. Maximum collector current.
VCE max. Maximum voltage across the collector-emitter junction.
You can ignore this rating in low voltage circuits.

hFE This is the current gain (strictly the DC current gain). The guaranteed minimum value is given because the actual value varies from transistor to transistor - even for those of the same type! Note that current gain is just a number so it has no units.
The gain is often quoted at a particular collector current IC which is usually in the middle of the transistor's range, for example '100@20mA' means the gain is at least 100 at 20mA. Sometimes minimum and maximum values are given. Since the gain is roughly constant for various currents but it varies from transistor to transistor this detail is only really of interest to experts.
Why hFE? It is one of a whole series of parameters for transistors, each with their own symbol. There are too many to explain here.