At 15:14 02/23/99 -0800, Lynx {Glenn Jones} wrote:
>I need to calculate a bias resitor? i was just going to use 10K, is this
>not a good value?

that's a "base resistor." you have a current through the led, which is
(Vcc-V_led)/R_led, which is the current the transistor has to switch (Ic).
something between 1 and 20mA usually, depending on the led and how bright
you want it. to switch that current reliably, the base current must be at
least Ic/hfe (hfe or beta is the current amplification Ic/Ib). the voltage
at the base is around 0.6V (when the emitter is on ground), so your base
current is Ib=(Vcc-0.6V)/Rb. 10k gives you 0.4mA, with a plain vanilla
transistor (hfe=50) you get a max. Ic of 20mA, which should be ok.

if you use your first circuit, the base current would be considerably
lower, because the voltage across Rb would be reduced by the led voltage:
Ib=(Vcc-V_led-0.6V)/Rb, which could be as low as 0.2mA. is probably still
enough to drive your led, but you see the difference.

ge