At the moment, I’m working on an Arduino compatible clock. Like most of my Arduino compatible boards, this one uses an Atmega32U4, with USB built in. With the Atmega32U4, I sacrifice a little in program memory and SRAM, but gain a bit in reduced parts count.
A USB capable Arduino-compatible is, of course, programmed via USB, and can be powered by the USB port. Most Arduino boards also have a 5V regulator to be used when being powered by a wall-bug power supply. Naturally, I put the USB connector on the clock board, as well as the 5V regulator. With the two different supplies, I also put in circuitry to auto switch sources and protect the USB host when both supplies are connected at the same time.
My first PCB revision required a few hand-mods, but not many. Still, I decided to respin the board and remove the two mod wires. While doing so, it suddenly occurred to me — a blinding flash of the obvious — that most cellphones and other small devices are charged with a USB-connector 5V wall-bug power supply. Why then, would I also need a separate power supply and on-board 5V regulator?
By pulling the regulator off of the board, I could eliminate a few capacitors and the supply auto-select / protection circuitry. Not only did I save in component cost, but I was able to reduce the PCB size, and thus cost, by about a third.
- I had the 5V regulator in the design because Arduinos can be powered by either USB or a non-regulated power supply.
- The reverse power protection is necessary to prevent damage to the USB host if the other power is also connected.
- The auto-power switching circuit is necessary so that a user doesn’t need to flip a switch or change a jumper when changing power sources.
- I had two extra LEDs to indicate which supply was powering the clock.
I questioned my original assumptions, found a “because it’s always done that way” and eliminated it. Assumptions are meant to be challenged.
Duane Benson
Question authority!
And then get squashed
(or, squash extra space out of your PCB)