anorton:
I was about to start a thread on this very topic, but apparently it has been asked before!After reading through the thread (and the links posted therein), I have a couple of questions:
- When performing Ohm's Law calculations, what specifies the values for V and I?
V, to me, should be the supply voltage (3.3V or 5V would be what I'm using in this case). And should I be the maximum allowable current to pull from the pin? Just wanting to make sure...
40ma is absolute maximum, but 20-30 ma is a better maximum limit standard to stick to. Keep in mind due to internal resistance of the output pins the output voltage will sag as current draw from the output pin is increased. Just shows that there is a difference from fundamental principal and electrical reality of real world components. That is why the datasheet for devices are so important to study and understand a chip limits and capabilities.
- What is the maximum allowable current draw for the 5V pin on an Arduino Uno? From this page, I see 50mA for the 3.3V pin, but nothing is listed for the 5V pin.
Because there is no exact answer. If the board is being powered via USB then there is a 500ma thermofuse that sets a maximum limit for the board and anything you wire up to it. But if powered from an external DC voltage source then there might be somewhat more but it depends on the exact DC voltage input as there is a heat dissipation limit for the on-board 5vdc voltage regulator. So hard answers are desirable but often allusive in the real world.
- Can one model a switch/pushbutton similarly to an LED? I ask this because a switch also has "infinite" resistance when not activated, but near 0 resistance when pressed.
No, totally different components, and a switch doesn't share all the characteristic of a switch and visa versa.