I drive everything off the one power rail, nominally 5V. This
works fine. It's just that I wish I'd given the LEDs their own
supply rail.
At design time I had to choose a value for the current limiting
resistors in series with each LED. For a 5V supply, for
roughly 10mA it was 470 Ω. After building the first few boards
that seemed ok so I stuck with it and that value is now used on
all boards. But it means I have no control over the LED
brightness. It's nailed to the logic power supply.
There are times when it would be nice to turn them off
(debugging a board with a scope probe is not helped by having
several dozen very bright lights 6 inches from your face).
There are times when it would be nice to have them a bit dimmer,
it would make photography easier.
And if I decide that I need to change the supply voltage to help
the logic work better then the LED brightness will change.
It would have meant that I'd need to run an extra power cable to
each board which would actually be quite a lot of work. But
there are occasions when I find myself thinking I should have
done it. If I had then I might have considered using a USB
connector for the board power connections. The connectors and
cables are quite cheap and they'd make life a lot easier than
running 3 cables everywhere.
In fact I didn't think through the LEDs properly at all despite
them being quite important to the whole thing. I was quite sure
I wanted a fair bit of brightness and was initially just happy
that my first LEDs worked as I wanted. But with a bit more
investigation and calculation I could have had as much
brightness as I wanted for less current. Whilst power
consumption isn't really a priority for this project things
would be at least cheaper and possibly easier with smaller/fewer
power supplies
© 2014-2016 James Newman.