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Getting started, episode 2

Added by mvdswaluw about 4 years ago

I hope you use a low current led. I guess the microcontroller wouldn’t like sinking 20mAmps.


Replies (8)

RE: Getting started, episode 2 - Added by JohnO about 4 years ago

Innovative design covering many dimensions.

Getting started, episode 2 - Added by martynj about 4 years ago

mvdswaluw,
Actually it is the other way around - the processor will attempt to draw the current it wants, the LED will act like a near constant voltage drop (~1.7 V).
The side-effect is the healthy red glow from the LED. 

RE: Getting started, episode 2 - Added by jcw about 4 years ago

Yes, there is a lot to cover on the periphery of all this. I’m trying to separate the “get going” part from the notes about the how’s and why’s of it all.

The distinction (and confusion) between whether components affect current or voltage is extremely important, IMO. An LED, tied to a power supply will - usually - just self-destruct (which is why there always has to be a current limiting resistor in there).

In this “bare blinker” circuit, it’s the microcontroller that’s defining the current consumption (as long as you don’t exceed its voltage specs). And now that you mention it, I’m at a loss as to how to make that more intuitively clear.

Let me try anyway, but I’ll also keep this in mind for a future post (it really is at the heart of understanding voltage vs. current, I suspect):

  • an ideal LED enforces a fixed voltage drop
  • put a smaller voltage on it, and nothing happens - no current, no light, it might as well have been disconnected
  • put at least that minimum voltage on it, and it will draw as much current as the supply can deliver
  • the amount of light produced is proportional to the amount of current flowing through it
  • anything over say 20 mA for a normal small LED, and it’ll go “poof” (internally, and not very spectacularly, alas)
  • so for normal use, i.e. to produce light, we add a resistor in series
  • when no current flows, a resistor acts the same as a short (which sounds weird, but a short without current is actually meaningless)
  • the more current through the resistor, the more voltage it “insists” on having across itself
  • this is the essence of Ohm’s Law: voltage (across resistor) = current (through resistor) x resistance (AT ALL TIMES)
  • with the proper resistor value, you end up with say 20 mA through the LED, at a given total voltage

It’s actually quite useful to try this out with an adjustable power supply, and measure voltages and currents.

Trivial as it might seem, to me, something as simple as a LED + resistor + power supply is actually the essence of engineering: components with totally predictable behaviour, acting together in ways which are - again - totally predictable. The engineering part comes from choosing just the right values to accomplish what you want.

In the case of the “bare blinker”, I intentionally messed it all up by re-using components in unintended ways, yet the result is still what I was after, and completely predictable. That’s why it’s so useful to build up intuitions about all this - it lets you play games and fool around with creative new experiments. And of course, sometimes it all breaks down - but in a way, that’s even more useful because that’s where the learning happens… you get to sharpen and re-calibrate your intuition.

RE: Getting started, episode 2 - Added by mvdswaluw about 4 years ago

Quote: “The worst that can happen is a “bricked” FTDI board – take your time!"

Or insert him in to a windows PC with the new “ftdigate” drivers. And safe yourself some time if you have a fake FTDI chip. ;-)

RE: Getting started, episode 2 - Added by JohnO about 4 years ago

Do you have more detail?

RE: Getting started, episode 2 - Added by mvdswaluw about 4 years ago

For the record, I am NOT stating that the UF1 or USB-BUB II has non-genuine FTDI-chips. Just making a remark, for anyone who might have a cheap Ebay FDTI-converter they want to use. Be carefull…

http://www.eevblog.com/2014/10/27/eevblog-676-rant-ftdi-bricking-counterfeit-chips/

RE: Getting started, episode 2 - Added by JohnO about 4 years ago

mvdswaluw wrote:
> Quote: “The worst that can happen is a “bricked” FTDI board – take your time!"
>
> Or insert him in to a windows PC with the new “ftdigate” drivers. And safe yourself some time if you have a fake FTDI chip. ;-)

Ah, I misunderstood, I thought you had a driver alternative to a soldering iron when using a fake FTDI chip.

RE: Getting started, episode 2 - Added by Mars about 4 years ago

>> We can send an autonomous vehicle to Mars using a mighty/majestic Saturn V rocket, or we can launch a fragile/fickle weather balloon and enjoy the view on earth from above.

O, Yes you can… But please don’t litter my planet… ;-)

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