Getting Started with Edison part 7 - Javascript Web Server

[cisotips]

Love it can't wait to see episode 8. Would be cool to make a simple file server

[artmezins]

The audio instructions are JUMBLED relative to what is happening in the video for connecting everything. For example, when the audio says "connect the emitter of the transistor to the 100 Ohm resistor", the video shows the 1 KOhm resistor being connected to GPIO12. If a newbie simply imitated the video rather than follow the (audio) "instructions", they could fry their board at worst or get really confused at best when it doesn't work.

Also, this uses an NPN transistor wired in an unconventional common collector configuration (with the 100 Ohm resistor connected to the emitter at one end, the LED anode at the other end, and the LED cathode going to ground). This provides NEGATIVE Voltage feedback relative to the base current drive, reducing the base drive and possibly preventing normal saturated operation. As the transistor turns on (if it does), its base Voltage (tries to) rise to: Vbe (0.7V) + the LED on Voltage (1.8-2.2V) + the Voltage drop across the 100 Ohm resistor (10mA * 100 Ohm = 1V) = 3.5 to 3.9V, so that can't happen, right? If the LED current is 1mA, then that lets the base Voltage go to about 0.7 + 1.8 + 0.1 = 2.6V (using the lower LED on Voltage) with Vce using the rest at 3.3-2.6=0.7V -- a more reasonable value. If this were a normal common emitter circuit, then Vce would be about 0.2V, so assuming an LED Voltage of 2.2V, that would give an LED current of: (3.3 - (0.2 + 2.2))V/100 Ohm = 9mA for a brighter LED (Sparkfun recommends 16-18 mA).

So, why wasn't a normal common emitter configuration used here? Was this just a simple goof up?