I am attempting a binary clock,and are having some doubts on whether the schematic will end up blowing all my LEDs. I hope I can explain it sufficiently from what I think is happening.
Did up a rough schematic in EAGLE and attached the png file.
Ok, here goes:
The LEDs each draw 20mA, and the current is provided by one PNP connected to the +5V source and base is connected to a pin on the microcontroller
The PNP is actually a Darlington pair with hFE(gain) of 1000.
Supposedly I provide a current of 0.12mA to the base of the PNP and the output provides 120mA of current to the 6 LEDs.
Now the individual LEDs are "selected" to be lit up by each of the 6 NPN transistors connected to it. Similarly, the base is connected to a pin on the microcontroller.
A current of 0.08mA will be passed into the base, with a hFE of 250, an output current of 20mA should flow through it to ground (I think)
Now the question is, let's say I activate the PNP transistor and it starts providing 120mA of current to the array, and I activate only the first NPN transistor below.
As I understand, only one LED (connected to the activated NPN transistor) will light up.
However, will the whole 120mA start rushing through it, or will the NPN transistor limit the current flow to 20mA?