Another month, another board version. This time the updated edition of the USB current meter. And not just one, but 20 boards; kindly made by Seeed Studios. White (of course!), with rounded corners, about the size of a USB drive. Once assembled, it is pleasingly solid to hold. These boards are meant to serve as demonstration samples: I can populate them, show them to people, and mail them all over the world. In addition, building more than one unit adds confidence that the circuit functions as expected.
The only major change on this revision is the use of an ZXCT1010 to measure the output current – my previous LM358-based effort struggled at low currents, and required a larger sense voltage than I felt comfortable with. The ZXCT1010 is suitably sensitive, highly linear and requires very little additional components. (I did, however, spend a day debugging an unexpectedly high transimpedance, before I realised that I had been referring to the datasheet of the ZXCT1110.)
In addition to the change in current sensing, I moved the components closer together and shortened the board to be less than 50mm long. The LED also moved to be next to the switch. Although the footprint for the programming header is still present, I plan on leaving it unpopulated, and just connecting using pogo pins.