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
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.