* Power jack
I saw a DC jack on one of your prototypes, this is definitely a good idea.
If you're out-and-about and need more voltage/current then finding a 9-15v wallwart on some nearby device is pretty easy.
Coupling the dc jack with a rectifier deals with the polarity and ac/dc issue quite nicely.
In terms of display, if it's LED then 7-seg will probably draw less power than matrix - you can get some pretty low power ones nowadays. Individual LEDs on the PCB might be able to squeeeze a little more efficiency at a slight reduciton in readability.
For isolated USB you could consider one of these ICs:
http://www.analog.com/en/interface/digital-isolators/adum3160/products/product.html
$3.61 in 100qty might be a killer though - so again, it could be an optional feature. And this of course opens up the avenue for computer control, datalogging etc. It would be very useful to log the power consumption of a widget against time. If you break out some GPIOs on your micro to an unsoldered header (don't need any extra components, just bare GPIOs, let users install divider resistors if they need), you've just created a product which could measure power consumption or efficiency with temperature, humidity, processing load...
Absolutely - must have 4mm sockets and binding posts - a possible compromise if space is short is 4mm sockets plus some sort of spring terminal - like the sort used for speakers on some amps.
I have considered the dual 4mm socket and spring terminal idea, as it does save protrusion space. But ultimately 4mm binding posts are just too irresistible.
Dave.
Dave, seeing this project, this comes to mind
A micro for LCD that still had some ADCs free.
hi, just some thoughts.
It's not that mobile if you need to connect it to a laptop to make it work or did i miss something?, why not add a rechargeable battery and make it charge-up via a usb, in-car, solar or even a novel toothbrush type charger. If it was portable and I could use in the field (literally) now that would be different.
Dave's other portable power supply will have a rechargeable battery and all the other stuff (USB control etc). No point duplicating it in this, which is supposed to be cheap, simple and small. In fact, I would drop the secondary display and just illuminate an LED when it was in current mode.
With all the talk above about power negotiation, I'm surprised nobody has mentioned the USB Battery Charging Specification. This allows compliant devices to supply and draw up to 1.5 A by simple hardware negotiation, with no USB protocol negotiation necessary. Some summary details are quoted below, extracted from a Maxim data sheet.
Dave, I wonder would your power supply be able to take advantage of this capability in devices that support it?
"Cheating"—Noncompliant USB Charging
A bolder noncompliant scheme assumes that 500mA will be available and instructs users to plug only into powered ports and hubs that are capable of 500mA. Again, since most USB ports do not disconnect power, this approach can work in most cases. When such a device is plugged into a port that cannot support 500mA, the port is supposed to shut down. However, the overload behavior of a USB port is not always well defined and can lead to system reset or damage. Fortunately, this level of desperation is no longer required since battery charging is now an active part of the USB specification.
I think this is a fantastic idea! The only feature I want is a low, low price. I'm looking forward to the kit, and I hope you do a video explaining how it works too! Keep up the great work.
binding posts are overkill.I deem them to be essential.
Dave.