I know the ATMega328P is a pretty good driver compared to most microprocessors, but it is not a driver. The maximum package source current is 200mA. That is all I was getting at. In this case, really I want to drive LEDs (correctly)from a TTL package. Since I want to turn on an LED for logic 1, the fact that TTL has good sinking current is not meaningful as far as I can see, but maybe I am having tunnel vision here in some way.
Grumpy_Mike:
How do I know the transistor is open and not operating in the ohmic region?
No such region, do you mean saturated? It is down to the gain of the transistor.
Help me on that. How do I know if I saturated the transistor to keep the emitter to collector resistance minimal?
Can someone recommend a good NPN transistor for my scenario? 1mA to the base must allow 20mA from emitter to collector while saturating the transistor. Correctly stated?
Thank you all.