I just wanted to run this idea past you, and make sure I haven't overlooked something...
For the circuit I'm building, I have 14 LED's of which I need to illuminate two at a time, then cycle back to the first.
To do that, I'm looking at using this 4017 IC:
Now, the 4017 can handle 25mA max. And 80% of that is 20mA. So in theory it should source 20mA at 5v from each pin, in sequence, without a problem.
I'd like to power my LEDs with 20mA each, so the plan is to connect the 4017 to the Arduino's pin via a current limiting resistor, then connect two LEDs in serial, directly to each of seven pins, and then to ground... the idea being to make full use of that 5v coming off the pin to power both leds with 20mA without exceeing the 4017's output capability, and to reduce the number of resistors and arduino pins needed to just one.
What do you think? Everything I know so far tells me this ought to work without any problems, but I'm still learning so I want confirmation.
I plan to wire the eigth pin in the 4017 sequence to the 4017's reset pin, which I assume will trigger the first led in the sequence to illuminate the moment I hit the 8th clock in my sequence.
The current limiting resistor I've calculated I'll need for the 4017 is 47 ohms, based on a 5v source, a 2.1v drop, and 20mA of current, powering two leds per pin in serial. Also, according to the LED wizard, the leds in this configuration will dissipate 84mW, which is well below the 500mW which another CMOS 4017 I found said it could handle. (The TI one doesn't list the power dissipation from what I could tell, but I assume its similar.)