Yes, transistor a little silly for 1 LED, but the same design carries right over into 2-3-4 LEDs in series from 12V source.
OP, you notice how analog design is really still an "art" [meaning 1/2 subjective and
based upon personal experience, :-)].
While there can be some "art" associated with design of analog circuits, we're talking primarily about a digital switch here and the design procedure is not that mysterious.
CrossRoads:
Yes, transistor a little silly for 1 LED, but the same design carries right over into 2-3-4 LEDs in series from 12V source.
Well, I'm not going to argue with you about your "Hail Mary" design approach.
oric_dan:
PapaG:
OP, you notice how analog design is really still an "art" [meaning 1/2 subjective and
based upon personal experience, :-)].While there can be some "art" associated with design of analog circuits, we're talking primarily about a digital switch here and the design procedure is not that mysterious.
Unfortunately, although we're only building a digital switch, we're using an analog
component to do it, so we need to pay attention to the analog characteristics.
That's what all those resistors are for.
Well, my first job out of university was with Texas Instruments on their 7400 logic family team and I can assure you that all those ICs have transistors and resistors inside and the same design criteria that you use with discrete components apply. Technically speaking, a transistor in saturation is not an analog component.
I get your point, I just don't rely on magic when science will do the job. 
A newer version of the schematic then, incorporating some of the feedback. The simulator reports 13.5 mA at the LED and 430 ?A at the base for an hFE of 32. I didn't change the 10kOhm resistor of the base to 4.7kOhm as that would give an hFE of 16. It is still not entirely clear what the hFE should be and since it is a loose value anyway and the datasheet does not give the value of hFE for 5V i guess an approximate value of 30-70 should suffice.
Look at the On Characterics on sheet 2.
With Ib of 15mA, the base-emitter voltage will be ~0.6V and the collector-emitter voltage will be ~0.3V.
Use those numbers to work the resister values.
(5V - 0.6V)/15mA = Base resistor
(5V - Vf - 0.3V)/20mA = LED resistor.
The simulator reports 13.5 mA at the LED and 430 ?A at the base for an hFE of 32.
No, no. hFE is a characteristic of the transistor, not the overall circuit. You're looking at the current gain of the circuit.
A major point of designing a transistor circuit is that you minimize the effects of variations in the characteristics of the transistors, since they can vary quite a bit. So Rbase is chosen to limit the current in the base/emitter circuit, and Rled is chosen to limit the current in the LED through the Collector/Emitter circuit, and everything will work just swell as long as hFE is "much larger" than Ic/Ib
westfw:
The simulator reports 13.5 mA at the LED and 430 ?A at the base for an hFE of 32.
No, no. hFE is a characteristic of the transistor, not the overall circuit. You're looking at the current gain of the circuit.
A major point of designing a transistor circuit is that you minimize the effects of variations in the characteristics of the transistors, since they can vary quite a bit. So Rbase is chosen to limit the current in the base/emitter circuit, and Rled is chosen to limit the current in the LED through the Collector/Emitter circuit, and everything will work just swell as long as hFE is "much larger" than Ic/Ib
Excellent description of the situation.
CrossRoads:
I already did XD
This is from an upstairs window - can't really tell, but the undrifted snow is 20" deep, and the pile from the city plows at the end of the drive is 4 feet and 8-10' wide.
Gee - I remember those days... And then I moved to South Texas... Been a little chilly down here in the mornings of late. Temps around 70....
Temps in the 70s... but it reaches the 'teens' in the summer, right?
115, 117?
Little warm for me 8)
Can always dress a little warmer in the winter, but in the summer, its hard to dress cooler once you're naked!